Microbial biogeography is a subset of
biogeography
Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, ...
, a field that concerns the distribution of organisms across space and time.
Although biogeography traditionally focused on plants and larger animals, recent studies have broadened this field to include distribution patterns of
microorganisms
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 ...
. This extension of biogeography to smaller scales—known as "microbial biogeography"—is enabled by ongoing advances in genetic technologies.
The aim of microbial biogeography is to reveal where microorganisms live, at what abundance, and why. Microbial biogeography can therefore provide insight into the underlying mechanisms that generate and hinder
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
.
Microbial biogeography also enables predictions of where certain organisms can survive and how they respond to changing environments, making it applicable to several other fields such as climate change research.
History
Schewiakoff (1893) theorized about the cosmopolitan habitat of free-living protozoans. In 1934,
Lourens Baas Becking
Lourens Gerhard Marinus Baas Becking (4 January 1895 in Deventer – 6 January 1963 in Canberra, Australia) was a Dutch botanist and microbiologist. He is known for the Baas Becking hypothesis, which he originally formulated as ''"Everything ...
, based on his own research in California's salt lakes, as well as work by others on salt lakes worldwide,
[Baas-Becking, L.G.M. (1934). ''Geobiologie of inleiding tot de milieukunde''. The Hague, the Netherlands: W.P. Van Stockum & Zoon]
English translation, 2015
concluded that "everything is everywhere, but the environment selects". Baas Becking attributed the first half of this hypothesis to his colleague
Martinus Beijerinck
Martinus Willem Beijerinck (, 16 March 1851 – 1 January 1931) was a Dutch microbiologist and botanist who was one of the founders of virology and environmental microbiology. He is credited with the discovery of viruses, which he called "''c ...
(1913).
Baas Becking hypothesis of cosmopolitan microbial distribution would later be challenged by other works.
Microbial vs macro-organism biogeography
The biogeography of macro-organisms (i.e., plants and animals that can be seen with the naked eye) has been studied since the eighteenth century. For macro-organisms, biogeographical patterns (i.e., which organism assemblages appear in specific places and times) appear to arise from both past and current environments. For example,
polar bears
The polar bear (''Ursus maritimus'') is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the largest extant bear species ...
live in the
Arctic
The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
but not the
Antarctic
The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...
, while the reverse is true for
penguins
Penguins ( order Sphenisciformes , family Spheniscidae ) are a group of aquatic flightless birds. They live almost exclusively in the Southern Hemisphere: only one species, the Galápagos penguin, is found north of the Equator. Highly adapt ...
; although both polar bears and penguins have adapted to cold climates over many generations (the result of past environments), the distance and warmer climates between the north and south poles prevent these species from spreading to the opposite hemisphere (the result of current environments). This demonstrates the biogeographical pattern known as "isolation with geographic distance" by which the limited ability of a species to physically disperse across space (rather than any
selective genetic reasons) restricts the geographical range over which it can be found.
The biogeography of
microorganism
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 ...
s (i.e., organisms that cannot be seen with the naked eye, such as fungi and bacteria) is an emerging field enabled by ongoing advancements in genetic technologies, in particular cheaper
DNA sequencing
DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...
with higher throughput that now allows analysis of global datasets on microbial biology at the molecular level. When scientists began studying microbial biogeography, they anticipated a lack of biogeographic patterns due to the high dispersibility and large population sizes of microbes, which were expected to ultimately render geographical distance irrelevant. Indeed, in microbial ecology the oft-repeated saying by
Lourens Baas Becking
Lourens Gerhard Marinus Baas Becking (4 January 1895 in Deventer – 6 January 1963 in Canberra, Australia) was a Dutch botanist and microbiologist. He is known for the Baas Becking hypothesis, which he originally formulated as ''"Everything ...
that “everything is everywhere, but the environment selects” has come to mean that as long as the environment is
ecologically
Ecology () is the study of the relationships between living organisms, including humans, and their biophysical environment, physical environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosy ...
appropriate,
geological
Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...
barriers are irrelevant.
However, recent studies show clear evidence for biogeographical patterns in microbial life, which challenge this common interpretation: the existence of microbial biogeographic patterns disputes the idea that “everything is everywhere” while also supporting the idea that environmental selection includes geography as well as historical events that can leave lasting signatures on microbial communities.
Microbial biogeographic patterns are often similar to those of macro-organisms. Microbes generally follow well-known patterns such as the
distance decay Distance decay is a geographical term which describes the effect of distance on cultural or spatial interactions. The distance decay effect states that the interaction between two locales declines as the distance between them increases. Once the d ...
relationship, the
abundance-range relationship, and
Rapoport's rule
Rapoport's rule is an ecogeographical rule that states that latitudinal ranges of plants and animals are generally smaller at lower latitudes than at higher latitudes.
Background
Stevens (1989) named the rule after Eduardo H. Rapoport, who had e ...
.
This is surprising given the many disparities between microorganisms and macro-organisms, in particular their size (
micrometers
The micrometre (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
vs. meters), time between generations (minutes vs. years), and dispersibility (global vs. local). However, important differences between the biogeographical patterns of microorganism and macro-organism do exist, and likely result from differences in their underlying biogeographic processes (e.g., drift,
dispersal, selection, and
mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
). For example, dispersal is an important biogeographical process for both microbes and larger organisms, but small microbes can disperse across much greater ranges and at much greater speeds by traveling through 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 s ...
(for larger animals dispersal is much more constrained due to their size).
As a result, many microbial species can be found in both northern and southern hemispheres, while larger animals are typically found only at one pole rather than both.
Distinct patterns
Reversed latitudinal diversity gradient
Larger organisms tend to exhibit
latitudinal gradients in species diversity
Species richness, or biodiversity, increases from the poles to the tropics for a wide variety of terrestrial and marine organisms, often referred to as the latitudinal diversity gradient. The latitudinal diversity gradient is one of the most widel ...
, with larger biodiversity existing in the tropics and decreasing toward more temperate polar regions. In contrast, a study on indoor fungal communities
found microbial biodiversity to be significantly higher in temperate zones than in the tropics. The same study found that drastically different buildings exhibited the same indoor fungal composition in any given location, where similarity increased with proximity. Thus despite human efforts to control indoor climates, outside environments appear to be the strongest determinant of indoor fungal composition.
Bipolar latitude distributions
Certain microbial populations exist in opposite hemispheres and at complementary latitudes. These ‘bipolar’ (or ‘antitropical’) distributions are much rarer in macro-organisms; although macro-organisms exhibit latitude gradients, ‘isolation by geographic distance’ prevents bipolar distributions (e.g., polar bears are not found at both poles). In contrast, a study on marine surface bacteria
showed not only a latitude gradient, but also complementarity distributions with similar populations at both poles, suggesting no "isolation by geographic distance". This is likely due to differences in the underlying biogeographic process, dispersal, as microbes tend to disperse at high rates and far distances by traveling through the atmosphere.
Seasonal variations
Microbial diversity can exhibit striking seasonal patterns at a single geographical location. This is largely due to dormancy, a microbial feature not seen in larger animals that allows microbial community composition to fluctuate in relative abundance of persistent species (rather than actual species present). This is known as the "seed-bank hypothesis" and has implications for our understanding of
ecological resilience
In ecology, resilience is the capacity of an ecosystem to respond to a perturbation or disturbance by resisting damage and recovering quickly. Such perturbations and disturbances can include stochastic events such as fires, flooding, windstorms ...
and thresholds to change.
Applications
Directed panspermia
Panspermia
Panspermia () is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraf ...
suggests that life can be distributed throughout
outer space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
via
comets
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...
,
asteroids
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
, and
meteoroids
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as micr ...
. Panspermia assumes that life can survive the harsh space environment, which features vacuum conditions, intense radiation, extreme temperatures, and a dearth of available nutrients. Many microorganisms are able to evade such stressors by forming
spores
In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...
or entering a state of low-metabolic dormancy. Studies in microbial biogeography have even shown that the ability of microbes to enter and successfully emerge from dormancy when their respective environmental conditions are favorable contributes to the high levels of microbial biodiversity observed in almost all
ecosystems
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
.
Thus microbial biogeography can be applied to panspermia as it predicts that microbes are able to protect themselves from the harsh space environment, know to emerge when conditions are safe, and also take advantage of their dormancy capability to enhance biodiversity wherever they may land.
Directed panspermia
Directed panspermia is the deliberate transport of microorganisms into space to be used as introduced species on lifeless but habitable astronomical objects.
Historically, Shklovskii and Sagan (1966) and Crick and Orgel (1973) hypothesized that li ...
is the deliberate transport of microorganisms to colonize another
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
. If aiming to colonize an Earth-like environment, microbial biogeography can inform decisions on the biological payload of such a mission. In particular, microbes exhibit latitudinal ranges according to
Rapoport's rule
Rapoport's rule is an ecogeographical rule that states that latitudinal ranges of plants and animals are generally smaller at lower latitudes than at higher latitudes.
Background
Stevens (1989) named the rule after Eduardo H. Rapoport, who had e ...
, which states that organisms living at lower
latitudes
In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...
(near the
equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...
) are found within smaller latitude ranges than those living at higher
latitudes
In geography, latitude is a coordinate that specifies the north–south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pole ...
(near the poles). Thus the ideal biological payload would include widespread, higher-latitude microorganisms that can tolerate of a wider range of climates. This is not necessarily the obvious choice, as these widespread organisms are also rare in microbial communities and tend to be weaker competitors when faced with
endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
organisms. Still, they can survive in a range of climates and thus would be ideal for inhabiting otherwise lifeless Earth-like planets with uncertain environmental conditions.
Extremophiles
An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme temper ...
, although tough enough to withstand the space environment, may not be ideal for directed panspermia as any given extremophile species requires a very specific climate to survive. However, if the target was closer to Earth, such as a planet or moon in our
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, it may be possible to select a specific extremophile species for the well-defined target environment.
See also
*
Microbiomes of the built environment
Microbiomes of the built environment is a field of inquiry into the communities of microorganisms that live in human constructed environments like houses, cars and water pipes. It is also sometimes referred to as ''microbiology of the built envi ...
*
Microbial ecology
Microbial ecology (or environmental microbiology) is the ecology of microorganisms: their relationship with one another and with their environment. It concerns the three major domains of life—Eukaryota, Archaea, and Bacteria—as well as viru ...
References
{{microorganisms
Biogeography
Microorganisms