Fadeev, E. | Search bioRxiv

bioRxiv2020

Submesoscale dynamics directly shape bacterioplankton community structure in space and time

Submesoscale eddies and fronts are recognized as important components of oceanic mixing and energy fluxes. These submesoscale phenomena occur in the surface ocean for a period of a few days on scales between several hundred meters and a few tens of kilometers. Remote sensing and modeling suggest that they may influence marine ecosystem dynamics, but their limited temporal and spatial scales make them challenging for observation and in situ sampling. Here, the study of a submesoscale filament in summerly Arctic waters (depth 0 - 400 m) revealed enhanced vertical mixing of Polar and Atlantic water masses, resulting in a ca. 4 km wide and ca. 50 km long filament with distinct physical and biogeochemical conditions. Compared to the surrounding waters the filament was characterized by a distinct phytoplankton bloom dominated by diatoms and two-fold higher bacterioplankton cell densities. High-throughput 16S rRNA gene sequencing of both bacterioplankton communities revealed 3-4 orders of magnitude higher sequence abundance of Synechococcus inside the filament, as well as tenfold higher sequence abundance of taxonomic groups typically found during summertime in aging phytoplankton blooms (e.g., Flavobacteriales). In contrast, the surrounding waters contained severalfold higher sequence abundance of winterly taxonomic groups that are also associated with polar water masses (e.g., SAR202 clade). Altogether, our results show that physical submesoscale processes in the ocean can shape distinct biogeochemical conditions and microbial communities within a few kilometers. Furthermore, our results underline the importance of such submesoscale features for our understanding of surface ocean diversity and biogeochemical processes.

ecology↗

bioRxiv2020

Spatial dynamics in Arctic bacterioplankton community densities are strongly linked to distinct physical and biological processes (Fram Strait, 79°N)

The Arctic is impacted by climate warming faster than any other oceanic region on Earth. Assessing the baseline of microbial communities in this rapidly changing ecosystem is vital for understanding the implications of ocean warming and sea ice retreat on ecosystem functioning. Using CARD-FISH and semi-automated counting, we quantified 14 ecologically relevant taxonomic groups of bacterioplankton (Bacteria and Archaea) from surface (0-30 m) down to deep waters (2500 m) in summerly ice-covered and ice-free regions of the Fram Strait, the main gateway for Atlantic inflow into the Arctic Ocean. Cell abundances of the bacterioplankton communities in surface waters varied from 105 cells mL-1 in ice-covered regions to 106 cells mL-1 in the ice-free regions, and were overall driven by variations in phytoplankton bloom conditions across the Strait. The bacterial classes Bacteroidia and Gammaproteobacteria showed several-fold higher cell abundances under late phytoplankton bloom conditions of the ice-free regions. Other taxonomic groups, such as the Rhodobacteraceae, revealed a distinct association of cell abundances with the surface Atlantic waters. With increasing depth (>500 m), the total cell abundances of the bacterioplankton communities decreased by up to two orders of magnitude, while largely unknown taxonomic groups (e.g., SAR324 and SAR202 clades) maintained constant cell abundances throughout the entire water column (ca. 103 cells mL-1). This suggests that these enigmatic groups may occupy a specific ecological niche in the entire water column. Our results provide the first quantitative spatial variations assessment of bacterioplankton in the summerly ice-covered and ice-free Arctic water column, and suggest that further shift towards ice-free Arctic summers with longer phytoplankton blooms can lead to major changes in the associated standing stock of the bacterioplankton communities.

microbiology↗