Microorganisms play an important role in the ecological balancing of extreme ecosystems. Over the last few years, polar regions have been affected by global warming and extinction of native species. The archaeal and bacterial communities have a very significant and interchangeable role in the nitrogen and sulfur cycling as they perform biological oxidation of ammonia and sulfur. A study was conducted to understand the microbial communities present at different oceanic depths of Krossfjorden with the help of high throughput sequencing methods. The aim of the study was also to decipher the role of microbial communities in the oceanic biogeochemical cycling with focus on ammonia and sulfur cycling.
The sediment samples were collected in triplicates from Krossfjorden (Norway) during the arctic summer. DNA extraction and further amplicon sequencing and analysis was done. The data suggested that bacterial communities are prevalent at the middle and lower sediments while archaeal communities are mainly present at the middle sediment. Thaumarchaeota population was predominant followed by Crenarchaeota, Euryarchaeota, Woesearchaeota and Marine Hydrothermal Vent Group. The study indicated major microbial biomass comprising of Proteobacteria, Bacteroidetes, Verrucomicrobia, Actinobacteria, Chloroflexi and Lentisphaerae, along with Marinicella, Desulfobulbus, Lutimonas, Sulfurovum and clade SEEP-SRB4 as major members of surface sediments. Interestingly, Bacteroidetes, Firmicutes, Verrucomicrobia and Lentisphaerae were found to be dominant members at lower depth (~180 m), while Proteobacteria, Actinobacteria and Planctomycetes showed more profusion at depth of ~250 m which can be related to its maximum activity at relatively higher depths in the Arctic. Similarly, Fusobacteria, Chloroflexi and Acidobacteria outnumbered other bacterial phyla at the depth of ~300 m. The genera Psychrilyobacter, Psychromonas, Marinifilum were observed in this study are likely to be involved in the hydrolysis and fermentation of spirulina forming volatile fatty acids, mainly acetate which is later utilized by sulfate-reducing bacteria. Sequences related to sulfate-reducing bacteria like Desulfobacteraceae and Desulfobulbaceae were detected in this study which are known for the acetate mineralization. Besides that, the abundant proportion of sulfur oxidizers Sulfurovum, Sulfurimonas from Epsilonproteobacteria was observed which could grow chemolithoautotrophically, which implies its ability to survive in nutrient-deprived conditions.
The study also indicated that archaeal communities across all depths of the fjord were found to engage in ammonia cycling. The bacterial communities showed divergence in the gene abundance of ammonia and sulfur cycling along the different depths. Members of Thaumarchaeota from the domain archaea have the ability to oxidize ammonia and are present ubiquitously in soil, ocean and extreme environments. Members
of Desulfobulbus that reduce both iron and sulfur were observed in this study known for the potential to reduce iron oxide. Sulfurovum and Sulfurimonas belong to the Epsilonproteobacteria and are known for their important role in sulfur cycling in marine and other aquatic environments . The predominance of Sulfurovum with a significant proportion of Sulfurimonas at this site may play the crucial role in the sulfur cycle as Sulfurovum is known to grow chemolithoautotrophically using hydrogen, sulfur, and thiosulfate as an electron donor while oxygen, nitrate, thiosulfate, and sulfur as an electron acceptor.
The study provided a detailed insight into the microbial community composition at Krossfjorden and understanding their metabolic fate. The study also tried to understand the potential of the microbial community to oxidize ammonia and Sulfur at different sites of Arctic fjord by targeted metagenomics.