Opportunistic Cyanobacteria in benthic microbial mats of a tropical lagoon, Tikehau Atoll, Tuamotu Archipelago: Minor in natural populations, major in cultures

Studies in microbial ecology focus on identifying field dominant microbial populations using culture-independent tools, whereas minor populations are often ignored. We characterized the cyanobacterial populations from the Tikehau Atoll lagoon, Tuamotu Archipelago, which responded to standard culturing media. The cultivation approach recruited cryptic cyanobacterial taxa, which were not observed in the studied mats, as revealed by microscopic comparison. Twelve strains belonging to the unicellular genera (Aphanothece Nägeli, Chlorogloea Wille and Cyanocystis Borzi) and the filamentous cyanobacteria with narrow filaments (Leptolyngbya Anagnostidis et Komárek, Phormidium Küzing ex Gomont and Pseudanabaena Lauterborn) dominated the cultures. The genera Cyanocystis and Chlorogloea were genotypically characterized for the first time. Four strains assigned to Phormidium showed 100% identity using 16S rRNA sequences but additional gene analyses as well as phenotypic distinctions suggest finer differentiation within this group. Eight cyanobacterial strains out of twelve fixed dinitrogen with rates ranging from 3.3 up to 15.8 nmoles N₂ μg^-1 Chl-a h^-1 and their nitrogen fixation ability was further confirmed by molecular detection of nifH gene. Nine strains possessed phycoerythrin, and two performed complementary chromatic adaptation. The present study documented the existence of an autochthonous subdominant cyanobacterial assemblage with variable physiological features that may enable them to replace dominant species in the microbial mats of Tikehau Atoll lagoon. These minor populations may be ecologically important, particularily in case of environmental disturbances.