Page 189 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Interestingly, the MAGs showed unique responses to the temperature scenarios (Table 1). At 4°C Methanosarcinaceae MAG 4 dominated, whereas at 10°C all three Methanosarcinaceae MAGs were present, with a dominance of Methanosarcinaceae MAG 5. Rapid and efficient substrate conversions indicated their adaptation to low temperatures. Several Methanosarcina species have been isolated from cold habitats (Simankova et al. 2003; Morozova et al. 2015). Despite their growth at low temperatures (1-5°C), most isolates are psychrotolerant and show optimum growth at moderate temperatures (25-35°C) (Simankova et al. 2003; Wagner and Liebner 2010). Methanosarcinaceae MAG 5 and 9 follow this trend on TMA with increases at 10°C, indicating a preference for higher temperatures. We performed a closer investigation of the three Methanosarcinaceae MAGs in an attempt to gain insights into their differences. None of the methanogen MAGs contained a 16S rRNA gene. Therefore, the complete methyl- coenzyme M reductase gene cluster that was present in all three MAGs was used for species identification.
Methanosarcinaceae MAG 4 showed a strong enrichment on TMA at 4°C and was nearly
absent in the acetate-amended sediments. In MAG 4 a complete mcrAGCDB gene cluster was
found with 91.8-99.5% average amino acid identity (AAI) to Methanosarcina sp. sequences.
McrABG is most identical to Methanosarcina sp. 2.H.A.1B.4 that was obtained from a metagenome sequencing experiment on Columbia River sediment culture grown on acetate 8 (Youngblut et al. 2015). Nearest cultured representatives are M. lacustris (94.0-99.0% AAI for McrAGCB) and M. acetivorans (86.5% AAI to McrD). M. lacustris is a psychrotolerant methanogen isolated from anoxic lake sediments in Switzerland (Simankova et al. 2001). MAG
4 contains acetate kinase (ack), phosphate acetyltransferase (pta) and acetyl-CoA synthetase (ACS) and CODH/ACS gene cluster encoding CdhABCDE for carbon fixation through the Wood-Ljungdahl pathway.
Methanosarcinaceae MAG 5 showed strict enrichment on TMA and was not observed in the sediments amended with acetate. Its response showed a high temperature specificity with a high enrichment at 10°C and low coverage at 4°C (Table 2). The mcrAGCDB gene cluster was most identical to Methanosarcina sp (91.8-99.0% AAI). Nearest cultured representatives are M. spelaei (93.7% AAI for McrA), M. lacustris (34.9-98.5% AAI for McrGCB) and M. acetivorans (85.9% AAI for McrD). M. spelaei is isolated from a floating biofilm in mesothermal water of a subsurface lake (Ganzert et al. 2014). Interestingly, the optimal growth
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