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We describe Picocystis salinarum R.A. Lewin, gen. et sp. nov., a hitherto unrecognized kind of green picoplanktonic alga from a saline pond, now isolated in pure culture. The cells are normally spherical or oval, 2-3 Jm in diameter, but tend towards a trilobed shape under conditions of nutrient depletion. The ultrastructure is typical for green algae. The pigment complement comprises chlorophylls a and b and the carotenoids neoxanthin, violaxanthin, alloxanthin, monadoxanthin, diatoxanthin, lutein and zeaxanthin. The major component of the cell wall is polyarabinose. Such a unique combination of features suggests that this alga might be assigned to a new lineage within the Chlorophyta.
INTRODUCTION
Although autotrophic picoplankton can contribute up to 90% of the primary production and autotrophic biomass of aquatic ecosystems (Li et al. 1983; Joint et al. 1986; Stockner 1988) and are thus of great ecological importance, little is known about their taxonomy.
Among the smallest eukaryonts are various autotrophic picoplanktonic chlorophytes, variously assigned to the Prasinophyceae [e.g. Bathycoccus Eikrem & Throndsen, Ostreococcus Courties & Chretiennot-Dinet in Chretiennot-Dinet et al., Pycnococcus Guillard in Guillard et al. (Eikrem & Throndsen 1990; Guillard et al. 1991; Courties et al. 1994, 1998; Daugbjerg et al. 1995)], the Trebouxiophyceae [e.g. Chlorella Beijerinck, Choricystis (Skuja) Fott, Nanochlorum C. Wilhelm, Eisenbeis, A. Wild & R. Zahn (Andreoli et al. 1978; Huss & Sogin 1990; Krienitz et al. 1996; Huss et al. 1999)], or the Chlorophyceae [e.g. Mychonastes PD. Simpson & Van Valkenburg (Kalina & Puncochgova 1987; Hanagata 1998; Krienitz et al. 1999)]. Such tiny round cells presumably evolved by convergent evolution from larger ancestors (Courties et al. 1994; Potter et al. 1997), possibly because selection favoured smaller cells with less requirements per cell for N, P and other elements (greater surface-to-volume ratios facilitating the exchange of dissolved gases and nutrient solutes) and perhaps a reduced tendency to be captured by filter-feeding predators (Raven 1999).
MATERIAL AND METHODS
Source of the organism
The strain (SFBB) was isolated from a water sample collected in December 1991 from a saline pond (c. 10% salt) at the San Francisco Salt Works, California, where it was associated with Dunaliella spp.
Clonal cultures were established by streaking on 1% agar medium prepared with seawater enriched with nitrate (1.0 mM), phosphate (0.1 mM), iron...