Thaumarchaeota and Bacteria in the deep hypolimnion of subalpine lakes: abundance and in situ activity

In recent years the thermal features of the hypolimnion of deep large lakes have attracted the interest of limnologists due to its dependence on global warming. Comparatively few studies have been published on the microorganisms inhabiting the hypolimnion and on their role in the biogeochemical cycles of lakes. In this study the prokaryotic community inhabiting the water column of five large and deep lakes was investigated and the abundance of the Thaumarchaeota versus Bacteria was measured with CARD-FISH technique. The percentage of Thaumarchaeota increased with depth in all lakes reaching a maximum of 19% of total DAPI counts and the maximum of Thaumarchaeota abundance was 188 x 103 cells ml-1, higher than in bathypelagic marine waters. In addition, the activities of Bacteria and Archaea as dark [14C] bicarbonate uptake were measured in situ in the hypolimnion of the deep oligotrophic subalpine Lake Maggiore (Northern Italy). The use of a custom device designed for this research allowed sample collection, addition of tracers and inhibitors and incubation at depth, thus avoiding pressure, temperature and light stress to microorganisms. The dark [14C]HCO3 incorporation in the hypolimnion of Lake Maggiore was 187.7±15 ?gC m-3 d-1, higher than in Tyrrhenian Deep Water, and Archaea were responsible for 28% of total dark [14C]HCO3 uptake (52.7±0.4 ?gC 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 mgC 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.