Cabrol, L. | bioRxiv Science

bioRxiv2021

Temperature increase affects acetate-derived methane production in Alaskan lake sediments and wetland soils

Under climate change framework, methanogens activity is expected to be strongly affected, eventually resulting in positive feedback on global climate, with higher greenhouse gas (GHG) emissions in the Arctic. This work aimed to evaluate the effect of increasing temperature on methane production rate and archaeal community of lake sediments and wetland soils from Denali to Toolik regions in Alaska (USA). For that, anaerobic acetate-amended microcosms were incubated at 5, 10, 15 and 20 {degrees}C. The acetate-derived methanogenic rate was determined and the methanogenic communities were analyzed by qPCR and 16S rRNA sequencing. Warmer temperatures yielded 4-6 times higher methane production rates and organic matter content (OM) showed significant positive correlation to methane production. Different patterns were observed in the archaeal communities after incubation at higher temperatures, with an increase in Methanosarcina abundance for most of the samples and Methanosaeta in one of the lakes tested, showing the adaptation of key acetoclastic groups among different temperatures. Our results demonstrate the impact of increasing temperature on methane production, bringing insights on key drivers involved in the process of acetoclastic methanogenic potential occurring in these ecosystems in Alaska.

microbiology↗

bioRxiv2021

Exploring the microdiversity within marine bacterial taxa: Towards an integrated biogeography in the Southern Ocean

The phylogeography traditionally correlates the genetic relationships among individuals within a macroorganism species, to their spatial distribution. Most microbial phylogeographic studies so far have been restricted to narrow geographical regions, mainly focusing on isolated strains, either obtained by culture or single-strain natural enrichments. However, the laborious culture-based methodology imposes a low number of studied individuals, leading to poor resolution of haplotype frequency estimation, making difficult a realistic evaluation of the genetic structure of natural microbial populations in the environment. To tackle this limitation, we present a new approach to unravel the phylogeographic patterns of bacteria combining (i) community-wide survey by 16S rRNA gene metabarcoding, (ii) intra-species resolution through the oligotyping method, and (iii) genetic and phylogeographic indices, as well as migration parameters, estimated from populational molecular data as traditionally developed for macroorganisms as models. As a proof-of-concept, we applied this methodology to the bacterial genus Spirochaeta, classically reported as a gut endosymbiont of various invertebrates inhabiting the Southern Ocean (SO), but also described in marine sediment and in open waters. For this purpose, we centered our sampling into three biogeographic provinces of the SO; maritime Antarctica (King George Island), sub-Antarctic Islands (Kerguelen archipelago) and Patagonia in southern South America. Each targeted OTU was chaLRracterized by substantial intrapopulation microdiversity, a significant genetic differentiation and a robust phylogeographic structure among the three distant biogeographic provinces. Patterns of gene flow in Spirochaeta populations support the role of the Antarctic Polar Front (APF) as a biogeographic barrier to bacterial dispersal between Antarctic and sub-Antarctic provinces. Conversely, the Antarctic Circumpolar Current (ACC) appears as the main driver of connectivity between geographically distant sub-Antarctic areas such as Patagonia and Kerguelen archipelago, and between Kerguelen archipelago and maritime Antarctica. Additionnally, we found that historical processes (drift and dispersal limitation) together govern up to 86% of the spatial turnover among Spirochaeta populations. Overall, our approach represents a substantial first attempt to bridge the gap between microbial and macrobial ecology by unifying the way to study phylogeography. We revealed that strong congruency with macroorganisms patterns at the populational level shaped by the same oceanographic structures and ecological processes.

microbiology↗

bioRxiv2019

Widespread microbial mercury methylation genes in the global ocean

Methylmercury is a neurotoxin that bioaccumulates from seawater to high concentrations in marine fish, putting human and ecosystem health at risk. High methylmercury levels have been found in the oxic subsurface waters of all oceans, yet only anaerobic microorganisms have been identified so far as efficient methylmercury producers in anoxic environments. The microaerophilic nitrite oxidizing bacteria Nitrospina has been previously suggested as a possible mercury methylator in Antarctic sea ice. However, the microorganisms processing inorganic mercury into methylmercury in oxic seawater remain unknown. Here we show metagenomic evidence from open ocean for widespread microbial methylmercury production in oxic subsurface waters. We find high abundances of the key mercury methylating genes hgcAB across all oceans corresponding to taxonomic relatives of known mercury methylators from Deltaproteobacteria, Firmicutes and Chloroflexi. Our results identify Nitrospina as the predominant and widespread key player for methylmercury production in the oxic subsurface waters of the global ocean.

genomics↗