New and poorly known coccoid species (Cyanoprokaryota) from the mid-depth and deep epilithon of a carbonate mountain lake

The deep littoral zone hosts a unique but understudied subset of lake periphyton. We investigated epilithic cyanoprokaryotes in a carbonate meromictic lake (Lake Tovel, southeastern Alps) to obtain information on 2 poorly known, deep-dwelling coccoid species (one new to science) and to analyze their distribution in light of the current understanding of the depth distribution of lake periphyton. Cyanoprokaryotes in Lake Tovel have distinct depth distributions among 3 zones: shallow (0-4 m), mid-depth (6-12 m), and deep (15-24 m). The mid-depth and deep zones are characterized by high stability with favorable growth conditions and by severe light limitation, respectively. Both zones have unique taxa that frequently possess colorless sheaths or pink-redviolet cell contents. The 2 most poorly known cyanoprokaryotes found in Lake Tovel (both pink-red-violet) were a species of Geitleribactron new to science (Geitleribactron purpureum sp. nov. Cantonati et Komarek) and the rare Chlorogloea purpurea. We described and documented, respectively, these 2 species by means of light microscopy, careful comparison with the most-similar morphospecies, transmission electron microscopy (TEM), and ecological characteristics. We are the first to describe the TEM ultrastructure of a Chlorogloea species and to characterize the autecology and distribution of C. purpurea, which, in spite of its very specific habitat requirements (deep waters of carbonate oligotrophic mountain lakes), has been reported in several studies from disparate habitats and geographic locations.