Depth-related patterns in microbial community responses to complex organic matter in the western North Atlantic Ocean

Oceanic bacterial communities process a major fraction of marine organic carbon. A substantial portion of this carbon transformation occurs in the mesopelagic zone, and a further fraction fuels bacteria in the bathypelagic zone. However, the capabilities and limitations of the diverse microbial communities at these depths to degrade high-molecular-weight (HMW) organic matter are not well constrained. Here, we compared the responses of distinct microbial communities from North Atlantic epipelagic (0–200 m), mesopelagic (200–1000 m), and bathypelagic (1000–4000 m) waters at two open-ocean stations to the same input of diatom-derived HMW particulate and dissolved organic matter. Microbial community composition and functional responses to the input of HMW organic matter – as measured by polysaccharide hydrolase, glucosidase, and peptidase activities – were very similar between the stations, which were separated by 1370 km but showed distinct patterns with depth. Changes in microbial community composition coincided with changes in enzymatic activities: as bacterial community composition changed in response to the addition of HMW organic matter, the rate and spectrum of enzymatic activities increased. In epipelagic mesocosms, the spectrum of peptidase activities became especially broad and glucosidase activities were very high, a pattern not seen at other depths, which, in contrast, were dominated by leucine aminopeptidase and had much lower peptidase and glucosidase rates in general. The spectrum of polysaccharide hydrolase activities was enhanced particularly in epipelagic and mesopelagic mesocosms, with fewer enhancements in rates or spectrum in bathypelagic waters. The timing and magnitude of these distinct functional responses to the same HMW organic matter varied with depth. Our results highlight the importance of residence times at specific depths in determining the nature and quantity of organic matter reaching the deep sea.</p

Multiomics in the central Arctic Ocean for benchmarking biodiversity change.

Multiomics approaches need to be applied in the central Arctic Ocean to benchmark biodiversity change and to identify novel species and their genes. As part of MOSAiC, EcoOmics will therefore be essential for conservation and sustainable bioprospecting in one of the least explored ecosystems on Earth

Impacts of sea ice melting procedures on measurements of microbial community structure

Microorganisms play critical roles in sea ice biogeochemical processes. However, microbes living within sea ice can be challenging to sample for scientific study. Because most techniques for microbial analysis are optimized for liquid samples, sea ice samples are typically melted first, often applying a buffering method to mitigate osmotic lysis. Here, we tested commonly used melting procedures on three different ice horizons of springtime, first year, land-fast Arctic sea ice to investigate potential methodological impacts on resulting measurements of cell abundance, photophysiology, and microbial community structure as determined by 16S and 18S rRNA gene amplicon sequencing. Specifically, we compared two buffering methods using NaCl solutions (???seawater,??? melting the ice in an equal volume of 35-ppt solution, and ???isohaline,??? melting with a small volume of 250-ppt solution calculated to yield meltwater at estimated in situ brine salinity) to direct ice melting (no buffer addition) on both mechanically ???shaved??? and ???non-shaved??? samples. Shaving the ice shortened the melting process, with no significant impacts on the resulting measurements. The seawater buffer was best at minimizing cell lysis for this ice type, retaining the highest number of cells and chlorophyll a concentration. Comparative measurements of bacterial (16S) community structure highlighted ecologically relevant subsets of the community that were significantly more abundant in the buffered samples. The results for eukaryotic (18S) community structure were less conclusive. Taken together, our results suggest that an equivalent-volume seawater-salinity buffered melt is best at minimizing cell loss due to osmotic stress for springtime Arctic sea ice, but that either buffer will reduce bias in community composition when compared to direct melting. Overall, these findings indicate potential methodological biases that should be considered before developing a sea ice melting protocol for microbiological studies and afterwards, when interpreting biogeochemical or ecological meaning of the results

Multiomics in the central Arctic Ocean for benchmarking biodiversity change

Multiomics approaches need to be applied in the central Arctic Ocean to benchmark biodiversity change and to identify novel species and their genes. As part of MOSAiC, EcoOmics will therefore be essential for conservation and sustainable bioprospecting in one of the least explored ecosystems on Earth