Description: The Arctic Ocean is considerably affected by the consequences of global warming, including more extreme seasonal fluctuations in the physical environment. So far, little is known about seasonality in Arctic marine ecosystems in particular microbial dynamics and cycling of organic matter. The limited characterization can be partially attributed to logistic difficulties of sampling in the Arctic Ocean beyond the summer season. Here, we investigated the distribution and composition of dissolved organic matter (DOM), gel particles and heterotrophic bacterial activity in the Fram Strait during summer and autumn. Our results revealed that phytoplankton biomass influenced the concentration and composition of semi-labile dissolved organic carbon (DOC), which strongly decreased from summer to autumn. The seasonal decrease in bioavailability of DOM appeared to be the dominant control on bacterial abundance and activity, while no temperature effect was determined. Additionally, there were clear differences in transparent exopolymer particles (TEP) and Coomassie Blue stainable particles (CSP) dynamics. The amount of TEP and CSP decreased from summer to autumn, but CSP was relatively enriched in both seasons. Our study therewith indicates clear seasonal differences in the microbial cycling of organic matter in the Fram Strait. Our data may help to establish baseline knowledge about seasonal changes in microbial ecosystem dynamics to better assess the impact of environmental change in the warming Arctic Ocean.

Description: Seasonal variations in day length and temperature, in combination with dynamic factors such as advection from the North Atlantic, influence primary production and the microbial loop in the Fram Strait. Here, we investigated the seasonal variability of biopolymers, microbial abundance, and microbial composition within the upper 100 m during summer and fall. Flow cytometry revealed a shift in the autotrophic community from picoeukaryotes dominating in summer to a 34-fold increase of Synechococcus by fall. Furthermore, a significant decline in biopolymers concentrations covaried with increasing microbial diversity based on 16S rRNA gene sequencing along with a community shift towards fewer polymer-degrading genera in fall. The seasonal succession in the biopolymer pool and microbes indicates distinct metabolic regimes, with a higher relative abundance of polysaccharide-degrading genera in summer and a higher relative abundance of common taxa in fall. The parallel analysis of DOM and microbial diversity provides an important baseline for microbe-substrate relationships over the seasonal cycle in the Arctic Ocean.

Description: Seamounts are abundant features on the seafloor that serve as hotspots and barriers for the dispersal of benthic organisms. The primary focus of seamount ecology has typically been on the composition and distribution of faunal communities, with far less attention given to microbial communities. Here, we investigated the microbial communities in the water column (0-3400 m depth) and sediments (619-3883 m depth, 0-16 cm below seafloor) along the ice-covered Arctic ridge system called the Langseth Ridge. We contextualized the microbial community composition with data on the benthic trophic state (i.e., organic matter, chlorophyll- a content, and porewater geochemistry) and substrate type (i.e., sponge mats, sediments, basaltic pebbles). Our results showed slow current velocities throughout the water column, a shift in the pelagic microbial community from a dominance of Bacteroidia in the 0-10 m depth towards Proteobacteria and Nitrososphaeria below the epipelagic zone. In general, the pelagic microbial communities showed a high degree of similarity between the Langseth Ridge seamounts to a northern reference site. The only notable differences were decreases in richness between ~600 m and the bottom waters (~10 m above the seafloor) that suggest a pelagic-benthic coupling mediated by filter feeding of sponges living on the seamount summits. On the seafloor, the sponge spicule mats, and polychaete worms were the principal source of variation in sedimentary biogeochemistry and the benthic microbial community structure. The porewater signature suggested that low organic matter degradation rates are accompanied by a microbial community typical of deep-sea oligotrophic environments, such as Proteobacteria, Acidimicrobiia, Dehalococcoidia, Nitrospira, and archaeal Nitrososphaeria. The combined analysis of biogeochemical parameters and the microbial community suggests that the sponges play a significant role for pelagic-benthic coupling and acted as ecosystem engineers on the seafloor of ice-covered seamounts in the oligotrophic central Arctic Ocean.

Description: The long-term dynamics of microbial communities across geographic, hydrographic, and biogeochemical gradients in the Arctic Ocean are largely unknown. To address this, we annually sampled polar, mixed, and Atlantic water masses of the Fram Strait (2015–2019; 5–100 m depth) to assess microbiome composition, substrate concentrations, and oceanographic parameters. Longitude and water depth were the major determinants (~30%) of microbial community variability. Bacterial alpha diversity was highest in lower-photic polar waters. Community composition shifted from west to east, with the prevalence of, for example, Dadabacteriales and Thiotrichales in Arctic- and Atlantic-influenced waters, respectively. Concentrations of dissolved organic carbon peaked in the western, compared to carbohydrates in the chlorophyll-maximum of eastern Fram Strait. Interannual differences due to the time of sampling, which varied between early (June 2016/2018) and late (September 2019) phytoplankton bloom stages, illustrated that phytoplankton composition and resulting availability of labile substrates influence bacterial dynamics. We identified 10 species clusters with stable environmental correlations, representing signature populations of distinct ecosystem states. In context with published metagenomic evidence, our microbial-biogeochemical inventory of a key Arctic region establishes a benchmark to assess ecosystem dynamics and the imprint of climate change.

Description: Sea spray aerosols (SSA) greatly affect the climate system by scattering solar radiation and acting as seeds for cloud droplet formation. The ecosystems in the Arctic Ocean are rapidly changing due to global warming, and the effects these changes have on the generation of SSA, and thereby clouds and fog formation in this region, are unknown. During the ship-based Arctic Century Expedition, we examined the dependency of forced SSA production on the biogeochemical characteristics of seawater using an on-board temperature-controlled aerosol generation chamber with a plunging jet system. Our results indicate that mainly seawater salinity and organic content influence the production and size distribution of SSA. However, we observed a 2-fold higher SSA production from waters with similar salinity collected north of 81°N compared to samples collected south of this latitude. This variability was not explained by phytoplankton and bacterial abundances or Chlorophyll-a concentration but by the presence of glucose in seawater. The synergic action of sea salt (essential component) and glucose or glucose-rich saccharides (enhancer) accounts for 〉80% of SSA predictability throughout the cruise. Our results suggest that besides wind speed and salinity, SSA production in Arctic waters is also affected by specific organics released by the microbiota.

Description: The discrete biogeochemical measurements from RV Maria S. Merian MSM77 sampled for organic matter, primary production and phytoplankton abundances. The parameters include POC, phytoplankton abundances and production. The samples were acquired within 0-100 m of the Greenland Sea between 78°N to 80°N and 3°E to 10°E. The data was collected daily on board MSM77 from 16th September 2018-4th October 2018. The water collection occurred using a CTD and laboratory methods varied by parameter. POC was analyzed using a EURO EA CHNS-O Elemental Analyzer, carbohydrates were analyzed using High performance anion exchange chromatography with pulsed amperometric detection, amino acids were analyzed using High Performance Liquid Chromatography (HPLC) ortho-phthaldialdehyde derivatization, rates of primary production (PP) were measured in situ using the 14C method and subsampled as total PP (PP-TOC), particulate PP (PP-POC), and dissolved PP (PP-DOC), cell abundances were measured using orange and red autofluorescence detected by flow cytometry. The purpose of data collection was to understand seasonal cycling of organic mater.

Description: The discrete biogeochemical measurements from RV Polarstern PS114 sampled for organic matter, primary production and phytoplankton abundances. The parameters include POC, phytoplankton abundances and production. The samples were acquired within 0-100 m of the Greenland Sea between 78°N to 80°N and 3°E to 10°E. The data was collected daily on board PS114 from 16th July 2018-23rd July 2018 . The water collection occurred using a CTD and laboratory methods varied by parameter. POC was analyzed using a EURO EA CHNS-O Elemental Analyzer, carbohydrates were analyzed using High performance anion exchange chromatography with pulsed amperometric detection, amino acids were analyzed using High Performance Liquid Chromatography (HPLC) ortho-phthaldialdehyde derivatization, rates of primary production (PP) were measured in situ using the 14C method and subsampled as total PP (PP-TOC), particulate PP (PP-POC), and dissolved PP (PP-DOC), cell abundances were measured using orange and red autofluorescence detected by flow cytometry. The purpose of data collection was to understand seasonal cycling of organic mater.