Seaweed (or macroalgae) are a diverse group of mostly photosynthetic algae found in marine and freshwater environments. They are eukaryotic organisms and lack any vascular tissue (for the transport of water and other compounds such as sugars) or any organised tissue. The macroalgae are extremely diverse and have evolved in three different divisions within the algae clade; the Rhodophyta or ‘red seaweeds’, Phaeophyta or ‘brown seaweeds’ and Chlorophyta or ‘green seaweeds’.
As seaweeds are mostly photosynthetic, they play a similar role to plants in terrestrial ecosystems. Being able to create sugars from carbon dioxide means seaweeds provide the basis of many food chains in both marine and freshwater ecosystems and can sometimes be a significant source of food in terrestrial environments. Along with the production of food, macroalgae can also provide shelter and habitat, as plants do, but for small fish and invertebrates in aquatic environments and are important buffers against erosion as they help to stabilize sediments.
There are a large number of physical differences between the seaweed and plants which identifies their distinct evolutionary paths. A big generalisation, but nonetheless useful, is that macroalgae are far more structurally simple than plants. Instead of the typical root, stem and leaf structure that plants have, seaweeds have no roots but have a holdfast that secures them to the ground, and have a non-vascular stype and blades instead of a stem and leaves. They lack a waxy cuticle and multi-cellular sex organs and instead of storing their energy in starch, as plants typically do, they use other compounds such as mannitol and floridean.
Chlorophyll a is what makes plants green and is what absorbs light to kick off the process of photosynthesis. In water, much of light wavelengths that cholorophyll a rely on are absorbed rapidly through the water column making chlorophyll a much less efficient and making other pigments much more advantageous than they are on land. This allows for different colored seaweed to be far more common than different photosynthetic material in land plants.
The difference in the composition of pigments between different seaweeds is the easiest way to distinguish between the three different seaweed divisions. All macroalgae have chlorophyll a but they may also have other compounds that contribute to the overall colour of the seaweed. The Rhodophyta or ‘red seaweeds’ for example have a high concentration of the phycobilin pigments while the brown seaweeds will often have a high concentration of fucoxanthan, which makes them brown.
The abundance of seaweed is determined by a number of biotic and abiotic factors. Light limits the lower distribution of seaweed as light availability decreases rapidly with depth, keeping the seaweed communities within a maximum depth of around 40 m. Desiccation (or drying out) prevent seaweeds from living above the low tide mark. Seaweeds are therefore generally restricted the 0-40 m depth range. Grazing from invertebrates such as urchins can completely remove forests of seaweed. This can often be caused by the removal of predatory fish through over-fishing which would normally keep grazing populations at bay at control the grazing pressure on macroalgae stands.
The Chlorophyta or ‘green seaweeds’ are very diverse themselves. They can be filamentous algae, sheets or unicellular. The unicellular chlorophyta make up a large proportion of the global phytoplankton populations, the microscopic organisms that float with the currents throughout the sea. Chlorophyta are believed to be the ancestor of the terrestrial plants and are classed within the Kingdom Plantae. They exhibit many similar characteristics such as using starch for food storage, cellulose cell walls and having chlorophyll a as their primary photosynthetic pigment.
Rhodophyta or ‘red seaweeds’ are multi cellular and can come as filamentous algae, in sheets or as calcareous algae. The red seaweeds are often common at very shallow depths and even in the intertidal zone but can also be found at depth, below that of the green and brown seaweeds. They store food as floridean, a branched starch-like compound, and are built around a range of cell wall compounds that vary from species to species.
The ‘brown algae’ or Phaeophyta is the most complex group of macroalgae. They include the kelp species that dominate many sub-tidal zones in lower latitudes and can form dense kelp forest canopies that provide a drastically different environment than the otherwise barren rocky sub-tidal zone. The Phaeophyta also have a number of species that are able to survive permanently adrift such as Sargassum spp. in the North Atlantic Ocean which provdes shelter and habitat for many fish and invertebrate communities.
Economically, the production of seaweed is a multi-billion dollar industry as they are used in a huge variety of industries. The most obvious example is the importance of seaweed in the Japanese food industry, but compounds derived from macroalgae are also important as thickening agents, emulsifiers, adhesives, in agar plates, cosmetics, pharmaceutical products, paper, welding and plastics.
Last edited: 24 May 2015
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