The sea surface microlayer (SML) is the boundary interface between the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
and
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wor ...
, covering about 70% of
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's surface. With an operationally defined thickness between 1 and , the SML has physicochemical and biological properties that are measurably distinct from underlying waters. Recent studies now indicate that the SML covers the ocean to a significant extent, and evidence shows that it is an aggregate-enriched
biofilm
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular po ...
environment with distinct
microbial communities Microbial population biology is the application of the principles of population biology to microorganisms.
Distinguishing from other biological disciplines
Microbial population biology, in practice, is the application of population ecology and popu ...
. Because of its unique position at the air-sea interface, the SML is central to a range of global
marine biogeochemical and climate-related processes.
The sea surface microlayer is the boundary layer where all
exchange
Exchange may refer to:
Physics
*Gas exchange is the movement of oxygen and carbon dioxide molecules from a region of higher concentration to a region of lower concentration. Places United States
* Exchange, Indiana, an unincorporated community
* ...
occurs between the atmosphere and the ocean.
The chemical, physical, and biological properties of the SML differ greatly from the sub-surface water just a few centimeters beneath.
[Zhang, Zhengbin et al. (2003). Studies on the sea surface microlayer II. The layer of sudden change of physical and chemical properties. Journal of Colloid and Interface Science. 264, 148-159.]
Despite the huge extent of the ocean's surface, until now relatively little attention has been paid to the sea surface microlayer (SML) as the ultimate interface where
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
,
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...
and
mass exchange between the ocean and the atmosphere takes place. Via the SML, large-scale environmental changes in the ocean such as
warming,
acidification,
deoxygenation
Deoxygenation is a chemical reaction involving the removal of oxygen atoms from a molecule. The term also refers to the removal of molecular oxygen (O2) from gases and solvents, a step in air-free technique and gas purifiers. As applied to orga ...
, and
eutrophication
Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phyt ...
potentially influence
cloud formation,
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
, and the
global radiation balance. Due to the deep connectivity between biological, chemical, and physical processes, studies of the SML may reveal multiple sensitivities to global and regional changes.
Understanding the processes at the ocean's surface, in particular involving the SML as an important and determinant interface, could provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. As of 2017, processes occurring within the SML, as well as the associated rates of material exchange through the SML, remained poorly understood and were rarely represented in marine and atmospheric numerical models.
Overview
The sea surface microlayer (SML) is the boundary interface between the atmosphere and ocean, covering about 70% of the Earth's surface. The SML has physicochemical and biological properties that are measurably distinct from underlying waters. Because of its unique position at the air-sea interface, the SML is central to a range of global biogeochemical and climate-related processes. Although known for the last six decades, the SML often has remained in a distinct research niche, primarily as it was not thought to exist under typical oceanic conditions. Recent studies now indicate that the SML covers the ocean to a significant extent,
highlighting its global relevance as the boundary layer linking two major components of the Earth system – the ocean and the atmosphere.
In 1983, Sieburth hypothesised that the SML was a
hydrate
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
d
gel-like layer formed by a complex mixture of
carbohydrate
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
s,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s, and
lipid
Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids in ...
s.
In recent years, his hypothesis has been confirmed, and scientific evidence indicates that the SML is an aggregate-enriched biofilm environment with distinct microbial communities.
In 1999 Ellison et al. estimated that 200
Tg C yr
−1 (200 million tonnes of carbon per year) accumulates in the SML, similar to
sedimentation rates of carbon to the ocean's seabed, though the accumulated carbon in the SML probably has a very short
residence time
The residence time of a fluid parcel is the total time that the parcel has spent inside a control volume (e.g.: a chemical reactor, a lake, a human body). The residence time of a set of parcels is quantified in terms of the frequency distribu ...
. Although the total volume of the microlayer is very small compared to the ocean's volume, Carlson suggested in his seminal 1993 paper that unique interfacial reactions may occur in the SML that may not occur in the underlying water or at a much slower rate there.
He therefore hypothesised that the SML plays an important role in the diagenesis of carbon in the upper ocean.
Biofilm-like properties and highest possible exposure to solar radiation leads to an intuitive assumption that the SML is a biochemical microreactor.
File:Sea surface microlayer as a biochemical microreactor.png, Sea surface microlayer as a biochemical microreactor
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(I) Unique chemical orientation, reaction and aggregation
(II) Distinct microbial communities processing dissolved and
particulate organic matter
(III) Highest exposure of solar radiation drives photochemical
reactions and formation of radicals
Historically, the SML has been summarized as being a microhabitat composed of several layers distinguished by their ecological, chemical and physical properties with an operational total thickness of between 1 and 1000 µm. In 2005 Hunter defined the SML as a "microscopic portion of the surface ocean which is in contact with the atmosphere and which may have physical, chemical or biological properties that are measurably different from those of adjacent sub-surface waters". He avoids a definite range of thickness as it depends strongly on the feature of interest. A thickness of 60 µm has been measured based on sudden changes of the pH, and could be meaningfully used for studying the physicochemical properties of the SML. At such thickness, the SML represents a laminar layer, free of turbulence, and greatly affecting the exchange of gases between the ocean and atmosphere. As a habitat for neuston (surface-dwelling organisms ranging from bacteria to larger siphonophores), the thickness of the SML in some ways depends on the organism or ecological feature of interest. In 2005, Zaitsev described the SML and associated near-
surface layer (down to 5 cm) as an incubator or nursery for eggs and larvae for a wide range of aquatic organisms.
Hunter's definition includes all interlinked layers from the laminar layer to the nursery without explicit reference to defined depths. In 2017, Wurl ''et al.'' proposed Hunter's definition be validated with a redeveloped SML paradigm that includes its global presence, biofilm-like properties and role as a nursery. The new paradigm pushes the SML into a new and wider context relevant to many ocean and climate sciences.
According to Wurl ''et al.'', the SML can never be devoid of organics due to the abundance of surface-active substances (e.g., surfactants) in the upper ocean
and the phenomenon of surface tension at air-liquid interfaces. The SML is analogous to the thermal boundary layer, and remote sensing of the sea surface temperature shows ubiquitous anomalies between the sea surface skin and bulk temperature. Even so, the differences in both are driven by different processes. Enrichment, defined as concentration ratios of an analyte in the SML to the underlying bulk water, has been used for decades as evidence for the existence of the SML. Consequently, depletions of organics in the SML are debatable; however, the question of enrichment or depletion is likely to be a function of the thickness of the SML (which varies with sea state; including losses via sea spray, the concentrations of organics in the bulk water,
and the limitations of sampling techniques to collect thin layers . Enrichment of surfactants, and changes in the sea surface temperature and salinity, serve as universal indicators for the presence of the SML. Organisms are perhaps less suitable as indicators of the SML because they can actively avoid the SML and/or the harsh conditions in the SML may reduce their populations. However, the thickness of the SML remains "operational" in field experiments because the thickness of the collected layer is governed by the sampling method. Advances in SML sampling technology are needed to improve our understanding of how the SML influences air-sea interactions.
Marine surface habitats sit at the interface between the atmosphere and the ocean. The
biofilm-like habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to as
neuston
Neuston, also known as pleuston, are organisms that live at the surface of the ocean or an estuary, or at the surface of a lake, river or pond. Neuston can live on top of the water surface or may be attached to the underside of the water surface. ...
.
The sea surface microlayer (SML) constitutes the uppermost layer of the ocean, only 1–1000 μm thick, with unique chemical and biological properties that distinguish it from the underlying water (ULW).
Due to the location at the air-sea interface, the SML can influence exchange processes across this boundary layer, such as air-sea gas exchange and the formation of sea spray aerosols.
[ ]
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Due to its exclusive position between the atmosphere and the hydrosphere and by spanning about 70% of the Earth's surface, the sea-surface microlayer (sea-SML) is regarded as a fundamental component in air–sea exchange processes and in biogeochemical cycling.
Although having a minor thickness of <1000 µm,
the elusive SML is long known for its distinct physicochemical characteristics compared to the underlying water,
e.g., by featuring the accumulation of dissolved and particulate organic matter,
transparent exopolymer particles (TEP), and surface-active molecules.
Therefore, the SML is a gelatinous biofilm, maintaining physical stability through surface tension forces.
It also forms a vast habitat for different organisms, collectively termed as neuston
with a recent global estimate of 2 × 1023 microbial cells for the sea-SML.
Life at air–water interfaces has never been considered easy, mainly because of the harsh environmental conditions that influence the SML. However, high abundances of microorganisms, especially of bacteria and picophytoplankton, accumulating in the SML compared to the underlying water were frequently reported,
accompanied by a predominant heterotrophic activity. This is because primary production at the immediate air–water interface is often hindered by photoinhibition. However, some exceptions of photosynthetic organisms, e.g., Trichodesmium, Synechococcus, or Sargassum, show more tolerance towards high light intensities and, hence, can become enriched in the SML.
Previous research has provided evidence that neustonic organisms can cope with wind and wave energy,
solar and ultraviolet (UV) radiation,
fluctuations in temperature and salinity, and a higher potential predation risk by the zooneuston. Furthermore, wind action promoting sea spray formation and bubbles rising from deeper water and bursting at the surface release SML-associated microbes into the atmosphere. In addition to being more concentrated compared to planktonic counterparts, the bacterioneuston, algae, and protists display distinctive community compositions compared to the underlying water, in both marine
and freshwater habitats. Furthermore, the bacterial community composition was often dependent on the SML sampling device being used.
While being well defined with respect to bacterial community composition, little is known about viruses in the SML, i.e., the virioneuston. This review has its focus on virus–bacterium dynamics at air–water interfaces, even if viruses likely interact with other SML microbes, including archaea and the phytoneuston, as can be deduced from viral interference with their planktonic counterparts. Although viruses were briefly mentioned as pivotal SML components in a recent review on this unique habitat,
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Creative Commons Attribution 4.0 International License
a synopsis of the emerging knowledge and the major research gaps regarding bacteriophages at air–water interfaces is still missing in the literature.
Properties
Organic compounds such as
amino acids
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
,
carbohydrates
In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or m ...
,
fatty acids
In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated and unsaturated compounds#Organic chemistry, saturated or unsaturated. Most naturally occurring fatty acids have an B ...
, and
phenols
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (— O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds are ...
are highly enriched in the SML interface. Most of these come from
biota in the sub-surface waters, which decay and become transported to the surface,
[Aller, J., Kuznetsova, M., Jahns, C., Kemp, P. (2005) The sea surface microlayer as a source of viral and bacterial enrichment in marine aerosols. Journal of aerosol science. Vol. 36, pp. 801-812.][Carlson, David J. (1983). Dissolved Organic Materials in Surface Microlayers: Temporal and Spatial Variability and Relation to Sea State. Limnology and Oceanography, 28.3. 415-431] though other sources exist also such as
atmospheric deposition, coastal
runoff
Runoff, run-off or RUNOFF may refer to:
* RUNOFF, the first computer text-formatting program
* Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed
* Runoff or run-off, a stock marke ...
, and anthropogenic nutrification.
The relative concentration of these compounds is dependent on the nutrient sources as well as
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
conditions such as
wind speed
In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed ...
and
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
.
These
organic compounds
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
on the surface create a "film," referred to as a "slick" when visible,
which affects the physical and optical properties of the interface. These films occur because of the
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
tendencies of many organic compounds, which causes them to protrude into the air-interface.
[Carlson, David J. (1982). Surface microlayer phenolic enrichments indicate sea surface slicks. Nature. 296.1. 426-429.] The existence of organic
surfactants
Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming ...
on the ocean surface impedes wave formation for low wind speeds. For increasing concentrations of surfactant there is an increasing critical wind speed necessary to create ocean waves.
Increased levels of organic compounds at the surface also hinders air-sea gas exchange at low wind speeds.
[Woodcock, A. (1953). Salt nuclei in marine air as a function of altitude and wind force. Journal of Meteorology, 10, 362–371.] One way in which particulates and organic compounds on the surface are transported into the atmosphere is the process called "bubble bursting".
[Wallace Jr., G.T., Duce, R.A., 1978. Transport of particulate organic matter by bubbles in marine waters. Limnol. Oceanogr. 23 Ž6., 1155–1167.] Bubbles generate the major portion of marine
aerosols.
[Gustafsson, M. E. R., & Franzen, L. G. (2000). Inland transport of marine aerosols in southern Sweden. Atmospheric Environments, 34, 313–325.][Grammatika, M., & Zimmerman, W. B. (2001). Microhydrodynamics offloatation process in the sea surface layer. Dynamics of Atmospheres and Ocean, 34, 327–348.] They can be dispersed to heights of several meters, picking up whatever particles latch on to their surface. However, the major supplier of materials comes from the SML.
Processes
Surfaces and interfaces are critical zones where major physical, chemical, and biological exchanges occur. As the ocean covers 362 million km
2, about 71% of the Earth's surface, the ocean-atmosphere interface is plausibly one of the largest and most important interfaces on the planet. Every substance entering or leaving the ocean from or to the atmosphere passes through this interface, which on the water-side -and to a lesser extent on the air-side- shows distinct physical, chemical, and biological properties. On the water side the uppermost 1 to 1000 μm of this interface are referred to as the sea surface microlayer (SML).
Like a skin, the SML is expected to control the rates of exchange of energy and matter between air and sea, thereby potentially exerting both short-term and long-term impacts on various Earth system processes, including biogeochemical cycling, production and uptake of radiately active gases like CO2 or DMS, thus ultimately climate regulation.
As of 2017, processes occurring within the SML, as well as the associated rates of material exchange through the SML, remained poorly understood and were rarely represented in marine and atmospheric numerical models.
File:Transport processes across the sea surface microlayer.jpg, Transport processes across the sea surface microlayer
An improved understanding of the biological, chemical, and physical processes at the ocean's upper surface could provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. Due to its positioning between atmosphere and ocean, the SML is the first to be exposed to climate changes including temperature, climate relevant trace gases, wind speed, and precipitation as well as to pollution by human waste, including nutrients, toxins, nanomaterials, and plastic debris.
Bacterioneuston
The term
neuston
Neuston, also known as pleuston, are organisms that live at the surface of the ocean or an estuary, or at the surface of a lake, river or pond. Neuston can live on top of the water surface or may be attached to the underside of the water surface. ...
describes the organisms in the SML and was first suggested by Naumann in 1917. As in other marine ecosystems, bacterioneuston communities have important roles in SML functioning.
Bacterioneuston community composition of the SML has been analysed and compared to the underlying water in different habitats with varying results, and has primarily focused on coastal waters and shelf seas, with limited study of the open ocean .
In the North Sea, a distinct bacterial community was found in the SML with Vibrio spp. and Pseudoalteromonas spp. dominating the bacterioneuston. During an artificially induced phytoplankton bloom in a fjord mesocosm thumb , Diagram of a small form closed system mesocosm.
A mesocosm (''meso-'' or 'medium' and ''-cosm'' 'world') is any outdoor experimental system that examines the natural environment under controlled conditions. In this way mesocosm studie ...
experiment, the most dominant denaturing gradient gel electrophoresis (DGGE) bands of the bacterioneuston consisted of two bacterial families: Flavobacteriaceae and Alteromonadaceae
The Alteromonadaceae are a family of Pseudomonadota.George M. Garrity: '' Bergey's Manual of Systematic Bacteriology''. 2. Auflage. Springer, New York, 2005, Volume 2: ''The Proteobacteria, Part B: The Gammaproteobacteria'' They are now one of ...
. Other studies have however, found little or no differences in the bacterial community composition of the SML and the ULW. Difficulties in direct comparisons between studies can arise because of the different methods used to sample the SML, which result in varied sampling depths.
Even less is known about the community control mechanisms in the SML and how the bacterial community assembles at the air-sea interface. The bacterioneuston community could be altered by differing wind conditions and radiation levels, with high wind speeds inhibiting the formation of a distinct bacterioneuston community. Wind speed and radiation levels refer to external controls, however, bacterioneuston community composition might also be influenced by internal factors such as nutrient availability and organic matter (OM) produced either in the SML or in the ULW.
One of the principal OM components consistently enriched in the SML are transparent exopolymer particles (TEP), which are rich in carbohydrates and form by the aggregation of dissolved precursors excreted by phytoplankton in the euphotic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
. Higher TEP formation rates in the SML, facilitated through wind shear and dilation of the surface water, have been proposed as one explanation for the observed enrichment in TEP. Also, due to their natural positive buoyancy, when not ballasted by other particles sticking to them, TEP ascend through the water column and ultimately end up at the SML . A second possible pathway of TEP from the water column to the SML is by bubble scavenging.
Next to rising bubbles, another potential transport mechanism for bacteria from the ULW to the SML could be ascending particles or more specifically TEP. Bacteria readily attach to TEP in the water column
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
. TEP can serve as microbial hotspots and can be used directly as a substrate for bacterial degradation, and as grazing protection for attached bacteria, e.g., by acting as an alternate food source for zooplankton. TEP have also been suggested to serve as light protection for microorganisms in environments with high irradiation.
Virioneuston
File:Viral–bacterial dynamics in the ocean surface.png, Viral–bacterial dynamics in the surface microlayer (SML) of the ocean and beyond. DOM = dissolved organic matter
Dissolved organic carbon (DOC) is the fraction of organic carbon operationally defined as that which can pass through a filter with a pore size typically between 0.22 and 0.7 micrometers. The fraction remaining on the filter is called part ...
, UV = ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
.[. ]
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Viruses in the sea surface microlayer, the so-called ''virioneuston'', have recently become of interest to researchers as enigmatic biological entities in the boundary surface layers with potentially important ecological impacts. Given this vast air–water interface sits at the intersection of major air–water exchange processes spanning more than 70% of the global surface area, it is likely to have profound implications for marine biogeochemical cycles
Marine biogeochemical cycles are biogeochemical cycles that occur within marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. These biogeochemical cycles are the pathways chemical substanc ...
, on the microbial loop
The microbial loop describes a trophic pathway where, in aquatic systems, dissolved organic carbon (DOC) is returned to higher trophic levels via its incorporation into bacterial biomass, and then coupled with the classic food chain formed by phy ...
and gas exchange, as well as the marine food web
Compared to terrestrial environments, marine environments have biomass pyramids which are inverted at the base. In particular, the biomass of consumers (copepods, krill, shrimp, forage fish) is larger than the biomass of primary producers. Th ...
structure, the global dispersal of airborne viruses originating from the sea surface microlayer, and human health.
Viruses are the most abundant biological entities in the water column of the world's oceans. In the free water column, the virioplankton typically outnumbers the bacterioplankton by one order of magnitude reaching typical bulk water concentrations of 107 viruses mL−1. Moreover, they are known as integral parts of global biogeochemical cycles to shape and drive microbial diversity and to structure trophic networks. Like other neuston members, the virioneuston likely originates from the bulk seawater. For instance, in 1977 Baylor et al. postulated adsorption of viruses onto air bubbles as they rise to the surface, or viruses can stick to organic particles also being transported to the SML via bubble scavenging.
Within the SML, viruses interacting with the bacterioneuston will probably induce the viral shunt, a phenomenon that is well known for marine pelagic systems. The term viral shunt describes the release of organic carbon and other nutritious compounds from the virus-mediated lysis of host cells, and its addition to the local dissolved organic matter (DOM) pool. The enriched and densely packed bacterioneuston forms an excellent target for viruses compared to the bacterioplankton populating the subsurface. This is because high host-cell numbers will increase the probability of host–virus encounters. The viral shunt might effectively contribute to the SML's already high DOM content enhancing bacterial production as previously suggested for pelagic ecosystems and in turn replenishing host cells for viral infections. By affecting the DOM pool, viruses in the SML might directly interfere with the microbial loop being initiated when DOM is microbially recycled, converted into biomass, and passed along the food web. In addition, the release of DOM from lysed host cells by viruses contributes to organic particle generation. However, the role of the virioneuston for the microbial loop has never been investigated.
Measurement
Devices used to sample the concentrations of particulates and compounds of the SML include a glass fabric, metal mesh screens, and other hydrophobic surfaces. These are placed on a rotating cylinder which collects surface samples as it rotates on top of the ocean surface.[Harvey, George W. (1966). Microlayer Collection from the Sea Surface: A New Method and Initial Results. Limnology and Oceanography, 11.4. 608-613]
The glass plate sampler is commonly used.[ ]
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It was first described in 1972 by Harvey and Burzell as a simple but effective method of collecting small sea surface microlayer samples. A clean glass plate is immersed vertically into the water and then withdrawn in a controlled manner. Harvey and Burzell used a plate which was 20 cm square and 4 mm thick. They withdrew it from the sea at the rate of 20 cm per second. Typically the uppermost 20–150 µm of the surface microlayer adheres to the plate as it is withdrawn. The sample is then wiped from both sides of the plate into a sampling vial.[Cunliffe, M. and Wurl, O. (2014) ''Guide to best practices to study the ocean's surface''. Occasional publication of the ''Marine Biological Association of the United Kingdom''. ]
File:Glass plate sampling of the sea surface microlayer.webp
File:Sampling the sea surface microlayer 2.jpg
For a plate of the size used by Harvey and Burzel, the resulting sample volumes are between about 3 and 12 cubic centimetres. The sampled SML thickness ''h'' in micrometres is given by:
:
where ''V'' is the sample volume in cm3, ''A'' is the total immersed plate area of both sides in cm2, and ''N'' is the number of times the sample was dipped.
Remote sensing
File:Bacteria, sea slicks and satellite remote sensing.webp, Bacteria, sea slicks and satellite remote sensing
Surfactants are capable of dampening the short capillary ocean surface waves and smoothing the sea surface. Synthetic aperture radar
Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide fine ...
(SAR) satellite remote sensing can detect areas with concentrated surfactants or sea slicks, which appear as dark areas on the SAR images.[Kurata, N., Vella, K., Hamilton, B., Shivji, M., Soloviev, A., Matt, S., Tartar, A. and Perrie, W. (2016) "Surfactant-associated bacteria in the near-surface layer of the ocean". ''Nature: Scientific Reports'', 6(1): 1–8. . ]
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Creative Commons Attribution 4.0 International License
Ocean surface habitats sit at the interface between the ocean and the atmosphere. The biofilm-like habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to as neuston
Neuston, also known as pleuston, are organisms that live at the surface of the ocean or an estuary, or at the surface of a lake, river or pond. Neuston can live on top of the water surface or may be attached to the underside of the water surface. ...
. This vast air–water interface sits at the intersection of major air–water exchange processes spanning more than 70% of the global surface area . Bacteria in the surface microlayer of the ocean, called ''bacterioneuston'', are of interest due to practical applications such as air-sea gas exchange of greenhouse gases, production of climate-active marine aerosols, and remote sensing of the ocean. Of specific interest is the production and degradation of surfactants (surface active materials) via microbial biochemical processes. Major sources of surfactants in the open ocean include phytoplankton, terrestrial runoff, and deposition from the atmosphere.
Unlike coloured algal blooms, surfactant-associated bacteria may not be visible in ocean colour imagery. Having the ability to detect these "invisible" surfactant-associated bacteria using synthetic aperture radar
Synthetic-aperture radar (SAR) is a form of radar that is used to create two-dimensional images or three-dimensional reconstructions of objects, such as landscapes. SAR uses the motion of the radar antenna over a target region to provide fine ...
has immense benefits in all-weather conditions, regardless of cloud, fog, or daylight. This is particularly important in very high winds, because these are the conditions when the most intense air-sea gas exchanges and marine aerosol production take place. Therefore, in addition to colour satellite imagery, SAR satellite imagery may provide additional insights into a global picture of biophysical processes at the boundary between the ocean and atmosphere, air-sea greenhouse gas exchanges and production of climate-active marine aerosols.
Aeroplankton
File:Ocean mist and spray 2.jpg, Sea spray
Sea spray are aerosol particles formed from the ocean, mostly by ejection into Earth's atmosphere by bursting bubbles at the air-sea interface. Sea spray contains both organic matter and inorganic salts that form sea salt aerosol (SSA). SSA ha ...
containing marine microorganisms
Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe) is any microscopic livin ...
can be swept high into the atmosphere and may travel the globe as aeroplankton
Aeroplankton (or aerial plankton) are tiny lifeforms that float and drift in the air, carried by wind. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of ...
before falling back to earth.
A stream of airborne microorganisms, including marine viruses, bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and protists, circles the planet above weather systems but below commercial air lanes. Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray
Sea spray are aerosol particles formed from the ocean, mostly by ejection into Earth's atmosphere by bursting bubbles at the air-sea interface. Sea spray contains both organic matter and inorganic salts that form sea salt aerosol (SSA). SSA ha ...
. In 2018, scientists reported that hundreds of millions of these viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.
Compared to the sub-surface waters, the sea surface microlayer contains elevated concentration of bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
and viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's ...
, as well as toxic metals
Metal toxicity or metal poisoning is the toxic effect of certain metals in certain forms and doses on life. Some metals are toxic when they form poisonous soluble compounds. Certain metals have no biological role, i.e. are not essential minerals, ...
and organic pollutants.[Blanchard, D.C., 1983. The production, distribution and bacterial enrichment of the sea-salt aerosol. In: Liss, P.S., Slinn, W.G.N. ŽEds.., Air–Sea Exchange of Gases and Particles. D. Reidel Publishing Co., Dordrecht, Netherlands, pp. 407-444.][Hoffmann, G.L., Duce, R.A., Walsh, P.R., Hoffmann, E.J., Ray, B.J., 1974. Residence time of some particulate trace metals in the oceanic surface microlayer: significance of atmospheric deposition. J. Rech. Atmos. 8, 745–759.][Hunter, K.A., 1980. Process affecting particulate trace metals in the sea surface microlayer. Mar. Chem. 9, 49–70.][Hardy, J.T., Word, J., 1986. Contamination of the water surface of Puget Sound. Puget Sound Notes, U.S. EPA. Region 10 Seattle, WA, pp. 3–6.] These materials can be transferred from the sea-surface to the atmosphere in the form of wind-generated aqueous aerosols due to their high vapor tension and a process known as volatilisation
Volatilization is the process whereby a dissolved sample is vaporised. In atomic spectroscopy this is usually a two-step process. The analyte is turned into small droplets in a nebuliser which are entrained in a gas flow which is in turn volatilis ...
. When airborne, these microbes
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
can be transported long distances to coastal regions. If they hit land they can have detrimental effects on animals, vegetation and human health.[WHO, 1998. Draft guidelines for safe recreational water environments: coastal and fresh waters, draft for consultation. World Health Organization, Geneva, EOSrDRAFTr98 14, pp. 207–299.] Marine aerosols that contain viruses can travel hundreds of kilometers from their source and remain in liquid form as long as the humidity is high enough (over 70%).[Klassen, R. D., & Roberge, P. R. (1999). Aerosol transport modeling as an aid to understanding atmospheric corrosivity patterns. Materials & Design, 20, 159–168.][Moorthy, K. K., Satheesh, S. K., & Krishna Murthy, B.V. (1998). Characteristics ofspectral optical depths and size distributions of aerosols over tropical oceanic regions. Journal of Atmospheric and Solar–Terrestrial Physics, 60, 981–992.
][Chow, J. C., Watson, J. G., Green, M. C., Lowenthal, D. H., Bates, B., Oslund, W., & Torre, G. (2000). Cross-border transport and spatial variability of suspended particles in Mexicali and California's Imperial Valley. Atmospheric Environment, 34, 1833–1843.] These aerosols are able to remain suspended in the atmosphere for about 31 days. Evidence suggests that bacteria can remain viable after being transported inland through aerosols. Some reached as far as 200 meters at 30 meters above sea level. It was also noted that the process which transfers this material to the atmosphere causes further enrichment in both bacteria and viruses in comparison to either the SML or sub-surface waters (up to three orders of magnitude in some locations).[Cincinelli A.; Stortini A.M.; Perugini M.; Checchini L.; Lepri L., 2001. Organic Pollutants in sea-surface microlayer and aerosol in the coastal environment Of Leghorn- (Tyrrhenian Sea). Marine Chemistry, Volume 76, Number 1, pp. 77-98(22)][Marks, R., Kruczalak, K., Jankowska, K., & Michalska, M. (2001). Bacteria and fungi in air over the GulfofGdansk and Baltic sea. Journal of Aerosol Science, 32, 237–250.]
See also
* Ocean surface topography
Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map.
These variations are exp ...
* Surface layer
References
{{physical oceanography, expanded=other
Surface science
Marine biology