Discrimination between bacteria, picocyanobacteria, sulfur-bacteria and eukaryotes by flow cytometry in five volcanic lakes from the Azores Archipelago (Portugal)

São Miguel Island (Azores Archipelago, Portugal) hosts five volcanic lakes: Sete Cidades, Santiago, Fogo, Congro and Furnas. Due to their peculiar thermal characteristics, they can be defined as atelomictic with partial stratification and destratification on a daily basis. This thermal regime is peculiar of tropical lakes and can influence lake functioning from both the chemical and biological point of view, as the extension of the epilimnion and metalimnion may vary. In summer, the lakes are stratified with a significant concentration of dissolved gas in the anoxic hypolimnion. These multilayer lakes have a dynamic structure and the lacustrine microbial communities are not easily distinguished. We measured different chemical and physical parameters and sampled top and bottom water from the five lakes to carry out a first survey, characterizing the microbial population by flow cytometer. We found marked differences of the microbial populations of the five lakes. Their composition revealed interesting autotrophic microorganisms and generally abundant bacterial population (around or above 5 x106 cells mL-1). Picocyanobacteria were present with phycoerythrin-rich (PE) or phycocyanin-rich (PC) cells depending on the prevalent phycobiliproteins. PEs were abundant in Lake Sete Cidades, whereas PCs and eukaryotes prevailed in Lake Congro. Lake Furnas, Congro and Sete Cidades were characterized by the presence of Purple Sulfur Bacteria (PSB) in the anoxic bottom, visible in the cytograms as a cloud, with a strong signal in FL4 due to their pigments. The five lakes had low, though significant CH4 concentrations at lake bottom, gradually declining in the upper layer. We hypothesized that the methane can likely be metabolized by sulfate reducing bacteria to produce H2S, successively used by PSB as electron donor to perform photosynthesis in the anoxic illuminated layer. They can be purple or green sulfur bacteria and differ in size and in S storage, as well as in pigment composition. These findings indicate that biogeochemical processes can significantly develop in volcanic tropical lakes and create microenvironments harboring different microbial community.