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

Abstract

36Oxygen stratified lakes are typical for the boreal zone, and also a major source of 37 greenhouse gas emissions in the region. Due to shallow light penetration, restricting 38 the growth of phototrophic organisms, and large allochthonous organic carbon 39 inputs from the catchment area, the lake metabolism is expected to be dominated by 40 heterotrophic organisms. In this study we test this assumption and show that the 41 potential for autotrophic carbon fixation and internal carbon cycling is high 42 throughout the water column. Further, we show that during the summer 43 stratification carbon fixation can exceed respiration in a boreal lake even below the 44 euphotic zone. Metagenome assembled genomes and 16S profiling of a vertical 45 transect of the lake revealed multiple organisms in oxygen depleted compartment 46 belonging to novel or poorly characterized phyla. Many of these organisms were 47Importance 58Autotrophic organisms at the base of the food web are the only life form capable of 59 turning inorganic carbon into organic form, facilitating the survival of all other 60 organisms. In certain environments the autotrophic production is limited by 61 environmental conditions and the food web is supported by carbon coming from 62 outside the ecosystem. One such environment is stratified boreal lakes, which are 63 one of the biggest sources of greenhouse gas emissions in the boreal region. Thus, 64 carbon cycling in these habitats is of outmost importance for the future climate. 65Here we demonstrate a high potential for internal carbon cycling via phototrophic 66 and novel chemolithotrophic organisms in the dark and anoxic layers of a boreal 67 lake. Our results significantly increase our knowledge on the microbial communities 68 and their metabolic potential in oxygen depleted freshwaters and help to 69 understand and predict how climate change induced alterations could impact the 70 lake carbon dynamics. 71 72 73 74 75

Variable impact of geochemical gradients on the functional potential of bacteria, archaea, and phages from the permanently stratified Lac Pavin

“…Our analyses show that oxygen impacts metabolic potential, in part by selecting for bacteria and archaea with certain C1 and carbon fixation pathways in different zones of the lake. In particular, the Wood-Ljungdahl and reverse TCA cycle showed strong signals of stratification and were essentially restricted to the lake’s anoxic zone, consistent with findings from another stratified lake [36], although that lake is ∼15 times shallower. Thus, potential for autotrophy via these pathways may vary predictably with geochemical conditions in a scale-independent manner.…”

Archaea in boreal Swedish lakes are diverse, dominated by Woesearchaeota and follow deterministic community assembly

“…Many dystrophic lakes are characterized by anoxic bottom waters (hypolimnion) where electron acceptors for respiration are highly depleted. Accordingly, the genomes of hypolimnion microbes show the potential for fermentation and methanogenesis (Peura et al 2015;Peura et al 2018). A high prevalence of archaea also in the surface waters (epilimnion) of these net-heterotrophic, greenhouse-gasemitting systems can be speculated to be the result of anoxic microenvironments (Grossart et al, 2011) and their transitory occurrence in oxygenated waters.…”

Fourteen new Polynucleobacter species: P. brandtiae sp. nov., P. kasalickyi sp. nov., P. antarcticus sp. nov., P. arcticus sp. nov., P. tropicus sp. nov., P. bastaniensis sp. nov., P. corsicus sp. nov., P. finlandensis sp. nov., P. ibericus sp. nov., P. hallstattensis sp. nov., P. alcilacus sp. nov., P. nymphae sp. nov., P. paludilacus sp. nov. and P. parvulilacunae sp. nov.

“…6), but with lower percentages of recruited reads compared to strain MWH-Mekk-B1 T , suggesting that P. paneuropaeus possessed lower abundances than P. finlandensis sp. nov. Interestingly, analyses of metagenome samples from Lake Alinen Mustajärvi [52,55], which is located only a few kilometres away from Lake Mekkojärvi and shares with that lake very similar water chemistry, revealed that P. paneuropaeus is more abundant than P. finlandensis sp. nov. in the oxygenated epilimnion of that lake but in the anoxic hypolimnion the abundances were reversed (Fig.…”