Peura, S. | bioRxiv Science

bioRxiv2018

Decreased snow cover stimulates under ice primary producers, but impairs methanotrophic capacity

Climate change scenarios anticipate decrease of spring snow cover in boreal and subarctic regions. Forest lakes are abundant in these regions and substantial contributors of methane emissions. We performed an experiment on an anoxic frozen lake and observed that the removal of snow increased light penetration through the ice into the water modifying the microbial composition across depths. A shift in photosynthetic primary production was reflected by the increase of chlorophyll a and b concentrations in the upper depths of the water column, while Chlorobia, one of the key photosynthetic bacteria in anoxic lakes, shifted to lower depths. Moreover, a decrease in abundance of methanotrophs, such as Methylococcaceae, was noted concurrently to an increase in methane concentration in the water column. These results indicate that decrease of snow cover impacts both primary production and methane production/consumption, ultimately leading to increased methane emissions after spring ice off.

ecology↗

bioRxiv2018

Novel autotrophic organisms contribute significantly to the internal carbon cycling potential of a boreal lake

The authors declare no conflict of interest\n\nFunding sources: the Academy of Finland, Science for Life Laboratories, Tryggers Foundation, the Swedish Research Council VR and the Swedish Foundation for strategic research\n\nAbstractOxygen stratified lakes are typical for the boreal zone, and also a major source of greenhouse gas emissions in the region. Due to shallow light penetration, restricting the growth of phototrophic organisms, and large allochthonous organic carbon inputs from the catchment area, the lake metabolism is expected to be dominated by heterotrophic organisms. In this study we test this assumption and show that the potential for autotrophic carbon fixation and internal carbon cycling is high throughout the water column. Further, we show that during the summer stratification carbon fixation can exceed respiration in a boreal lake even below the euphotic zone. Metagenome assembled genomes and 16S profiling of a vertical transect of the lake revealed multiple organisms in oxygen depleted compartment belonging to novel or poorly characterized phyla. Many of these organisms were chemolithotrophic, deriving their energy from reactions related to sulfur, iron and nitrogen transformations. The community as well as the functions were stratified following the redox potentials. The autotrophic potential in the lake metagenome below the oxygenic zone was high, pointing towards a need for revising our concepts of internal carbon cycling in boreal lakes. Further, the importance of chemolithoautotrophy for the internal carbon cycling suggests that many predicted climate change associated changes in the physical properties of the lake, such as altered mixing patterns, likely have consequences for the whole lake metabolism even beyond the impact to the phototrophic community.\n\nImportanceAutotrophic organisms at the base of the food web are the only life form capable of turning inorganic carbon into organic form, facilitating the survival of all other organisms. In certain environments the autotrophic production is limited by environmental conditions and the food web is supported by carbon coming from outside the ecosystem. One such environment is stratified boreal lakes, which are one of the biggest sources of greenhouse gas emissions in the boreal region. Thus, carbon cycling in these habitats is of outmost importance for the future climate. Here we demonstrate a high potential for internal carbon cycling via phototrophic and novel chemolithotrophic organisms in the dark and anoxic layers of a boreal lake. Our results significantly increase our knowledge on the microbial communities and their metabolic potential in oxygen depleted freshwaters and help to understand and predict how climate change induced alterations could impact the lake carbon dynamics.

ecology↗

bioRxiv2017

Novel Chemolithotrophic And Anoxygenic Phototrophic Genomes Extracted From Ice-Covered Boreal Lakes

Although an important fraction of the worlds lakes remains ice-covered during a large proportion of the year, little is known about the microorganisms that govern the biogeochemical processes occurring under-ice along the stratigraphic redox gradients. Reconstructed genomes provide evidence for anoxygenic photosynthesis involving fixation of carbon using reduced sulphur and iron as an electron donor in the anoxic zone of the sampled lake systems. In addition to anoxygenic photosynthesis, our molecular data reveals novel chemolithoautotrophic organisms and supports the existence of methanotrophs in bottom anoxic waters. Reconstructed genomes matched methanotrophs related to Methylobacter tundripaludum, phototrophic Chloroflexi and Chlorobia, as well as lithoautotrophic genomes affiliated to the Betaproteobacteria class and Planctomycetes phylum. Based on our in-depth characterization, complex metabolic interactomes emerge unique to each lakes redox tower and with sulfur, iron and carbon cycling tightly intertwined through chemolithotrophy and anoxygenic photosynthesis.

ecology↗