Phaeocystis

, important phytoplankters of the Ross Sea that dominate early season blooms after the sea ice retreats and export significant carbon. of photosymbiotic acantharian hosting Phaeocystis symbionts. Red fluorescence is chlorophyll autofluorescence and allows observation of the altered morphology of Phaeocystis chloroplasts. Green fluorescence corresponds to LysoTracker dye, which stains digestive compartments. Symbionts are not being digested. Free-living forms of Phaeocystis are globally distributed and occur in a variety of marine habitats, including coastal oceans, open oceans, polar seas and sea ice. Seven species are currently assigned to the genus: P. antarctica, P. jahnii, P. globosa, P. pouchetti, P. scrobiculata (not in culture), P. cordata, and P. rex. Three species (P. globosa, P. pouchetii, and P. antarctica) are associated with bloom formation in nutrient-rich areas, which can occur either naturally (e.g. in the Ross Sea, Greenland Sea or the Barents Sea) or due to anthropogenic inputs (e.g. in the Southern Bight of the North Sea or the Persian Gulf). Generally, P. globosa blooms in temperate and tropical waters, whereas P. pouchetii and P. antarctica are better adjusted to the cold temperatures prevailing in Arctic and Antarctic waters, respectively. However, P. pouchetii also tolerates warmer temperatures and has been seen in temperate waters. Genome comparison has shown that the RUBISCO spacer region (located in the plastid DNA, between two subunits of the enzyme 1,5 -bisphosphate carboxylase) is highly conserved among closely related colonial Phaeocystis species and identical in P. antarctica, P. pouchetii and two warm-temperate strains of P. globosa, with a single base substitution in two cold-temperate strains of P. globosa. Phaeocystis can exist as either free-living cells or colonies. Free-living cells can show a variety of morphologies, depending on the species. All species can exist as scaled flagellates, and this is the only form that has been observed for P. scrobiculata and P. cordata. Three species have been observed as colonies (P. globosa, P. pouchetii and P. antarctica) and these can also exist as a flagellate devoid of scales and filaments. While suspected in other species (P. pouchetii and P. antarctica), a haploid-diploid life cycle has only been observed in P. globosa. In this cycle, sexual reproduction is dominant in colony bloom formation/termination, and two types of vegetative reproduction exist. == Impacts on global ocean ==

The genus Phaeocystis is a major producer of 3-dimethylsulphoniopropionate (DMSP), the precursor of dimethyl sulfide (DMS). Biogenic DMS contributes approximately 1.5×1013 g sulfur to the atmosphere annually and plays a major part in the global sulfur cycle, which can affect cloud formation and, potentially, climate regulation. == Symbiosis ==

Phaeocystis species are endosymbionts to acantharian radiolarians. Acantharians collected in different ocean basins host different species of Phaeocystis as their dominant symbionts: P. antarctica is found as the primary symbiont to acantharians in the Southern Ocean and P. cordata and P. jahnii are among the dominant symbionts found in acantharians collected in warm oligotrophic regions of the Indian and Pacific oceans. Extreme cellular remodeling is observed in symbiotic Phaeocystis, including a drastic increase in chloroplast number and an enlarged central vacuole. but it remains to be determined how the symbiosis has affected Phaeocystis evolution. == References ==