Sponges are permanent benthos residents which establish complex associations with a variety of microorganisms that raise interest in the nature of sponge-symbionts interactions. A molecular approach, based on the identification of the 16S rRNA and ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit genes, was applied to investigate diversity and phylogeny of bacterial phototrophs associated with four species of Lubomirskiidae in Lake Baikal. The phylogeny inferred from both genes showed three main clusters of Synechococcus associated with Baikalian sponges. One of the clusters belonged to the cosmopolitan Synechococcus rubescens group and the two other were not related to any of the assigned phylogenetic groups but placed as sister clusters to S. rubescens. These results expanded the understanding of freshwater sponge-associated photoautotroph diversity and suggested that the three phylogenetic groups of Synechococcus are common photosynthetic symbionts in Lubomirskiidae sponges. 1. Introduction Sponges are an important component of the marine and freshwater benthos ecosystems that establish associations with a great diversity of unicellular and multicellular organisms [1]. At the photosynthetic zone, sponges can benefit from phototrophic symbionts which fix carbon using the Calvin-Benson cycle and provide products of photosynthesis to the host [2–4]. Photosynthetic symbionts are prevalent in marine sponges of coastal regions worldwide where they contribute significantly to net primary production [5, 6]. From one-third to more than half of the sponges of tropical and temperate regions harbor a high level of photosynthetic symbionts [7, 8]. In Lake Baikal, sponges are necessary components of the benthos and ubiquitous on rocky grounds in the littoral zone. Sponges from the endemic family Lubomirskiidae are widely distributed in Lake Baikal and often harbor photosynthetic symbionts. From 14 described species of Lubomirskiidae [9], there are three common species among which photosynthetic Lubomirskia baicalensis (L. baicalensis) and Baikalospongia bacillifera (B. bacillifera) are widely distributed in the photic zone of Lake Baikal. In contrast to marine sponges, there is not a lot of data on photosynthetic symbionts of freshwater sponges, although associations with unicellular green algae, including Chlorella spp., Choricystis minor, yellow-green algae, and Chloroflexi have been shown in cosmopolitan sponges [4, 10–13] and cyanobacterial sequences detected in L. baicalensis [14]. Nevertheless, the identification and diversity of
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