Seaweeds

Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and ''Chlorophyta'' (green) macroalgae. Seaweed species such as kelps provide essential nursery habitat for fisheries and other marine species and thus protect food sources; other species, such as planktonic algae, play a vital role in capturing carbon, producing at least 50% of Earth's oxygen.

Natural seaweed ecosystems are sometimes under threat from human activity. For example, mechanical dredging of kelp destroys the resource and dependent fisheries. Other forces also threaten some seaweed ecosystems; a wasting disease in predators of purple urchins has led to a urchin population surge which destroyed large kelp forest regions off the coast of California.

Humans have a long history of cultivating seaweeds for their uses. In recent years, seaweed farming has become a global agricultural practice, providing food, source material for various chemical uses (such as carrageenan), cattle feeds and fertilizers. Because of their importance in marine ecologies and for absorbing carbon dioxide, recent attention has been on cultivating seaweeds as a potential climate change mitigation strategy for biosequestration of carbon dioxide, alongside other benefits like nutrient pollution reduction, increased habitat for coastal aquatic species, and reducing local ocean acidification. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" as a mitigation tactic.

Taxonomy

"Seaweed" lacks a formal definition, but seaweed generally lives in the ocean and is visible to the naked eye. The term refers to both flowering plants submerged in the ocean, like eelgrass, as well as larger marine algae. Generally it is one of several groups of multicellular algae: red, green and brown. They lack one common multicellular ancestor, forming a polyphyletic group. In addition, bluegreen algae ( Cyanobacteria) are occasionally considered in seaweed literature.

The number of seaweed species is still discussed among scientists, but most likely there are several thousand species of seaweed.

Genera

The following table lists a very few example genera of seaweed.

Anatomy

Seaweed's appearance resembles non-woody terrestrial plants. Its anatomy includes:
* Thallus: algal body
** Lamina or blade: flattened structure that is somewhat leaf-like
*** Sorus: spore cluster
*** ''pneumatocyst'', air bladder: a flotation-assisting organ on the blade
*** Kelp, float: a flotation-assisting organ between the lamina and stipe
** Stipe: stem-like structure, may be absent
** Holdfast: basal structure providing attachment to a substrate
*** Haptera: finger-like extension of the holdfast that anchors to a benthic substrate
The stipe and blade are collectively known as the frond.

Ecology

Two environmental requirements dominate seaweed ecology. These are seawater (or at least brackish water) and light sufficient to support photosynthesis. Another common requirement is an attachment point, and therefore seaweed most commonly inhabits the littoral zone (nearshore waters) and within that zone, on rocky shores more than on sand or shingle. In addition, there are few genera (e.g., ''Sargassum'' and ''Gracilaria'') which do not live attached to the sea floor, but float freely.

Seaweed occupies various ecological niches. At the surface, they are only wetted by the tops of sea spray, while some species may attach to a substrate several meters deep. In some areas, littoral seaweed colonies can extend miles out to sea. The deepest living seaweed are some species of red algae. Others have adapted to live in tidal rock pools. In this habitat, seaweed must withstand rapidly changing temperature and salinity and occasional drying.

Macroalgae and macroalgal detritus have also been shown to be an important food source for benthic organisms, because macroalgae shed old fronds..
These macroalgal fronds tend to be utilized by benthos