Eustigmatophyceae

Abstract

Eustigmatophyceae (eustigmatophytes) are a distinct lineage of ochrophyte (stramenopile) algae with a relatively small number (~30) of described species, but with evidence for a substantial taxonomic diversity yet to be explored. Eustigmatophytes are all unicellular coccoid algae, usually spherical or ovoid, but sometimes with a more distinctive shape (e.g., stipitate, tetrahedral, or with branched projections). Most eustigmatophytes live in freshwater, but some are common in terrestrial habitats and one subgroup is mostly marine. Reproduction occurs primarily via autosporogenesis, but many members of this class form zoospores with an anterior mastigoneme-bearing flagellum and a (sometimes missing) posterior bare flagellum. Sexual reproduction has not been directly observed, but genomic evidence suggests its presence in some species. Eustigma-tophytes are distinguished from other ochrophytes by a suite of cytological features (not all are necessarily present in all taxa): A pigmented lipidic body (reddish globule), a swelling at the base of the anterior flagellum associated with an extraplastidial stigma (eyespot), lamellate vesicles (with a putative reserve product), and plastids without a girdle lamella and lacking continuity with the nuclear envelope. Also characteristic is the lack of chlorophyll c and violaxanthin as the dominant xanthophyll. Because of their tendency to accumulate large amounts of lipids, including polyunsaturated fatty acids, eustigmatophytes are extensively used for biotechnology applications. The potential for commercial use has sparked a renewed interest in the basic biology of Eustigmatophyceae, including initiation of genome sequencing projects, although attention remains highly biased toward a single lineage comprising the genera Nannochloropsis and Microchloropsis.