Thaumarcheota e Bacteria nell'ipolimnio profondo dei laghi subalpini

Little is known about microorganisms inhabiting the hypolimnion and about their role in the biogeochemical cycles of lakes. We studied the abundances of Thaumarchaeota and Bacteria throughout the water column of five large and deep subalpine lakes of different trophic conditions. The percentage of Thaumarchaeota increased with depth in all lakes reaching a maximum of 19% of total DAPI counts, corresponding to abundances of 188 x 103 cells mL-1, i.e., higher than maxima reported from bathypelagic marine waters. In addition, we measured bacterial and archaeal dark [14C]-bicarbonate uptake, using metabolic inhibitors, in the hypolimnion of the deep oligotrophic subalpine Lake Maggiore. We incubated in situ, with a custom-made automatic apparatus which takes samples, adds tracers, and makes incubations at depth, thus avoiding pressure, temperature, and light stress on microorganisms. The dark [14C]HCO3 incorporation in the hypolimnion of Lake Maggiore was 187.7±15 µg C m-3 d-1, higher than in Tyrrhenian Deep Waters, and Archaea were responsible for 28% of total dark [14C]HCO3 uptake (52.7±0.4 µg C m-3 d-1). Dark [14C]HCO3 incorporation integrated along the aphotic water column (20-370 m) of Lake Maggiore was 65.8±5.2 mg C m-2 d-1. This corresponds to 14% of the total primary production and 87% of picophytoplanktonic production taking place in the photic zone. This study demonstrates that niche partitioning between prokaryotes in the vertical gradient of deep lakes is a general pattern, irrespective of the trophic condition, and provides the first evidence of prokaryotic dark CO2 fixation in the deep hypolimnion of a subalpine lake.