Marine Biology

Other Phytoplankton

              Constituents of the nanoplankton and picoplankton size classes (sometimes collectively called nanoplankton) include a number of photosynthetic microalgal organisms.  These organisms are important in primary productivity as well as  in oceanic food webs, which has only been recently realized. The important groups among these organisms are the prochlorophytes, the haptophytes (Coccolithophoridae, Haptophyceae) and the blue-green algae, also called the cyanobacteria (Cyanophyceae) .  The cyanobacteria are prokaryotic cells that possess chlorophyll-a, but this is not in plastids and occurs in single cells, filaments, or chains.  Cyanobacteria are abundant in the tropics, where they occasionally form dense mats of filaments and discolour the water (red tide)  brownish or saw-dust coloured by Trichodesmium erythraeum.   The abundant haptophytes are the coccolithophores, easily distinguished by the tiny calcareous plates (coccoliths) on their outer surface. They have complex life histories, with several morphologically different cells present in the same species, and several modes of reproduction.  Coccolithophores are now recognized as a major source of primary production in many ocean areas.  Other less abundant microalgae include the silicoflagellates (Chrysophyceae), the cryptomonads (Cryptophyceae), and certain motile green algae (Chlorophyceae).

           Pelagic bacteria or bacterioplankton, are also found in all oceans.  They are most abundant near the sea surface and are now thought to equal or exceed the total biomass of phytoplankton.  They are usually found in association with organic particles in the water column, collectively called Particulate Organic Carbon (POC), or on various gelatinous zooplankton pieces known as marine snow. They decrease markedly with depth, and their role in the microbial loop of the oceanic food web is now clearly recognised.