Microbial Communities in the East and West Fram Strait During Sea Ice Melting Season

Abstract

Climate models project that the Arctic Ocean may experience ice-free summers by the second half of this century. This may have severe repercussions on phytoplankton bloom dynamics and the associated cycling of carbon in surface waters. We currently lack baseline knowledge of the seasonal dynamics of Arctic microbial communities, which is needed in order to better estimate the effects of such changes on ecosystem functioning. Here we present a comparative study of polar summer microbial communities in the ice-free (eastern) and ice-covered (western) hydrographic regimes at the LTER HAUSGARTEN in Fram Strait, the main gateway between the Arctic and North Atlantic Oceans. Based on measured and modeled biogeochemical parameters, we tentatively identified two different ecosystem states (i.e., different phytoplankton bloom stages) in the distinct regions. Using Illumina tag-sequencing, we determined the community composition of both free-living and particle-associated bacteria as well as microbial eukaryotes in the photic layer. Despite substantial horizontal mixing by eddies in Fram Strait, pelagic microbial communities showed distinct differences between the two regimes, with a proposed early spring (pre-bloom) community in the ice-covered western regime (with higher representation of SAR11, SAR202, SAR406 and eukaryotic MALVs) and a community indicative of late summer conditions (post-bloom) in the icefree eastern regime (with higher representation of Flavobacteria, Gammaproteobacteria and eukaryotic heterotrophs). Co-occurrence networks revealed specific taxon-taxon associations between bacterial and eukaryotic taxa in the two regions. Our results suggest that the predicted changes in sea ice cover and phytoplankton bloom dynamics will have a strong impact on bacterial community dynamics and potentially on biogeochemical cycles in this region.

Spatial distribution of Arctic bacterioplankton abundance is linked to distinct water masses and summertime phytoplankton bloom dynamics (Fram Strait, 79°N)

“…This pattern fits the known global trend of archaeal enrichment in epipelagic waters (Karner et al, 2001;Herndl et al, 2005;Kirchman et al, 2007;Varela et al, 2008;Schattenhofer et al, 2009) and was also observed in other regions of the Arctic Ocean (Amano-Sato et al, 2013). Altogether, the observed cell numbers in the surface waters were well within the range of previous observations in the Fram Strait waters (Piontek et al, 2014;Fadeev et al, 2018;Engel et al, 2019). Below the upper 100 m the observed bacterioplankton cell abundances were lower than the ones previously reported from the eastern Fram Strait by Quero et al (2019).…”

“…At the time of sampling in June-July 2016, chlorophyll a concentrations showed a developed phytoplankton bloom across the Strait (Table S1). Thus, fitting the interannual trend of summerly phytoplankton bloom conditions in the western icecovered EGC and the eastern ice-free WSC (Nöthig et al, 2015;Fadeev et al, 2018).…”

“…Phytoplankton blooms in surface waters lead to an increasing cell abundance of heterotrophic bacteria that are specialized on degradation of organic matter from algal exudates and phytodetritus (Buchan et al, 2014;Teeling et al, 2016). Previous observations in the Fram Strait, acquired using high-throughput sequencing of the 16S rRNA gene, revealed a strong influence of the summerly phytoplankton bloom conditions on the bacterioplankton communities (Wilson et al, 2017;Müller et al, 2018a), differing between the ice-covered and ice-free regions of the Strait (Fadeev et al, 2018).During the sampling period, distinct phytoplankton bloom communities in surface waters across the Strait were observed, with a Phaeocystis-dominated bloom in the ice-free HG stations, a diatom-dominated bloom in the ice-covered EG stations, and a mixed diatoms and Phaeocystis populations bloom in the ice-margin stations N (data not included). Surface water total bacterioplankton communities showed significantly higher cell abundances in the HG and N stations (13-21 ×10 5 cells mL -1 ), compared to the EG stations (0.5-2 ×10 5 cells mL -1 ; Kruskal-Wallis test; Chi square = 81.85, df = 2, p-value < 0.01).…”

“…Within the bacterial communities, a group of bacterial classes that are functionally associated with phytoplankton blooms in the region (Bacteroidia, Gammaproteobacteria, and the phylum Verrucomicrobia) (Fadeev et al, 2018) showed together several-fold higher cell abundances in the HG and N stations (2.3-10 × 10 5 cells mL -1 ), compared to the EG stations (0.5-1.5 × 10 5 cells mL -1 ). Jointly comprising up to 50% of the bacterioplankton communities (Table 1).…”

“…The main objective of this study was to assess the standing stocks of key taxonomic groups in summerly Arctic bacterioplankton across the Strait. Using the high-throughput data of bacterioplankton cell abundances we tested previously established hypotheses that: 1) in surface waters the abundance of various taxonomic groups is associated with phytoplankton bloom conditions and is linked to the abundance of specific phytoplankton populations (Fadeev et al, 2018); 2) with an increasing depth the differences between bacterioplankton communities in ice-covered and ice-free regions decrease, and various taxonomic groups that are associated with winterly bacterioplankton dominate the deep sea communities.…”