Thaumarcheota and Bacteria in the deep hypolimnion of subalpine lakes: in situ dark [14C] bicarbonate uptake and cell abundance

We studied the spatial abundance of Thaumarchaeota (Thau) and Bacteria, using CARD-FISH, in five deep lakes, located North and South of the Alps. We found that the % of Thau increased with depth in all lakes (max: 188 cells x 103 ml-1). To clarify the role partitioning between the two domains we measured bacterial and archaeal dark CO2 uptake, using metabolic inhibitors, in the hypolimnion of L.Maggiore. We incubated in situ, with an automatic apparatus, avoiding stress to microorganisms. The dark CO2 uptake was 187.7±15 µgC m-3 d-1, and Thau were responsible for 28% of total dark uptake. The dark CO2 uptake in 20-370m was 66±5 mgC m-2 d-1, that is 14% of the total primary production and 87% of picoplanktonic production, taking place in the photic zone. This study demonstrates that niche partitioning among prokaryotes in the vertical gradient of deep lakes has a general pattern, and provides the first evidence of prokaryote dark CO2 fixation in the deep hypolimnion of a subalpine lake.