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The Synurophyceae were separated from the Chrysophyceae based on the absence of chlorophyll c2, parallel basal body of the flagella (instead at ~90^o angle), more gyres in the transitional helix of the flagella, absence of phagotrophy, and silica scale formation associated with the plastid (not in the cytoplasm) (Anderson 1987). They are photosynthetic flagellated protists containing chlorophylls a and c (c1 only, unique to this class), and b-linked storage reserve (chrysolaminarin). Protective pigments include fucoxanthin and violaxanthin (Graham and Wilcox 2000). Cells deposit silica scales on the outside of their outer membrane. The class is similar to Bacillariophyceae, Chrysophyceae, Dictyochophyceae, Eustigmatophyceae, Raphidophyceae, Silicoflagellates, Tribophyceae, and several additional smaller classes, and has been grouped with them in Division Ochrophyta, Kingdom Chromista (Cavalier-Smith 1986), or Stramenopiles (Brown and Sorhannus 2010). Class Synurophyceae was recognized by Andersen (1987) who moved Mallomonas and Synura from the Chrysophyceae. Subsequently several authors have added and removed genera -- currently four genera (Chrysodidymus, Mallomonas, Synura, and Tessellaria) are recognized in the class (Siver 2003). Currently Heterogloea, Spiniferomonas and Tessellaria are also included in PhycoKey.
References:
Anderson, R.A. 1987. Synurophyceae classic nov., a new class of algae. American J. Bot. 74:337-353. Brown, J.W., and U. Sorhannus 2010. A molecular genetic timescale for the diversification of autotrophic Stramenopiles (Ochrophyta): Substantive underestimation of putative fossil ages. PLoS One 5(9): e12759. doi:10.137/journal.pone.0012759. Cavalier-Smith, 1986. The Kingdom Chromista: origin and systematics. In: Round, F.E., and D.J. Chapman, editors. Progress in Phycological Research. Bristol: Biopress. pp. 309-347. Kristiansen, Jorgen 2002. Phylum Chrysophyta (Golden Algae). In: The freshwater algal flora of the British Isles (pp. 214-244). Siver, P.A. 2003. Synurophyte algae. In: Wehr, J.D., and R.G. Sheath (eds.). Freshwater Algae of North America (pp. 523-557).