First record of Nannochloropsis limnetica (Eustigmatophyceae) in the autotrophic picoplankton from Lake Baikal

Three new strains of eukaryotic picoplankton, isolated from Lake Baikal, were identified as Nannochloropsis limnetica Krienitz, Hepperle, Stich & Weiler. To date, N. limnetica had only been detected in small German and North American inland water bodies. On determination of the 18S rDNA sequence, the three new strains were found to be identical to each other and to the type strain KR 1998/3 (GenBank accession no. AF251496). Rapid analysis of polymorphic DNA-PCR revealed that the genotypes were different, although the Baikalian eustigmatophycean strains were more similar to each other compared with the type strain KR 1998/3 from Germany. Ecophysiological differences were also evident between the new strains from Lake Baikal and the type strain from growth rate determinations. The morphological characteristics were similar to that of a previous description of the species. However, although the cells of Eustigmatophyceae usually propagate by autosporulation, in these newly detected species germination of single daughter cells from thick-walled cells was observed for the first time. Based on pigment analysis, the occurrence of Eustigmatophyceae in Lake Baikal was estimated. Eustigmatophyceae were established to be common members of the phytoplankton community of this large oligotrophic Siberian lake and occurred throughout the year, even under the ice cover during winter. Moreover, they peaked during early summer and in the South Basin. Hence, the widely accepted opinion that Chlorophyceae solely comprise the eukaryotic picoplankton should be changed and the Eustigmatophyceae considered.