Changes in Marine Prokaryote Composition with Season and Depth Over an Arctic Polar Year

Abstract

As the global climate changes, the higher latitudes are seen to be warming significantly faster. It is likely that the Arctic biome will experience considerable shifts in ice melt season length, leading to changes in photoirradiance and in the freshwater inputs to the marine environment. The exchange of nutrients between Arctic surface and deep waters and their cycling throughout the water column is driven by seasonal change. The impacts, however, of the current global climate transition period on the biodiversity of the Arctic Ocean and its activity are not yet known. To determine seasonal variation in the microbial communities in the deep water column, samples were collected from a profile (1-1000 m depth) in the waters around the Svalbard archipelago throughout an annual cycle encompassing both the polar night and day. High-throughput sequencing of 16S rRNA gene amplicons was used to monitor prokaryote diversity. In epipelagic surface waters (<200 m depth), seasonal diversity varied significantly, with light and the corresponding annual phytoplankton bloom pattern being the primary drivers of change during the late spring and summer months. In the permanently dark mesopelagic ocean depths (>200 m), seasonality subsequently had much less effect on community composition. In summer, phytoplankton-associated Gammaproteobacteria and Flavobacteriia dominated surface waters, whilst in low light conditions (surface waters in winter months and deeper waters all year round), the Thaumarchaeota and Chloroflexi-type SAR202 predominated. Alpha-diversity generally increased in epipelagic waters as seasonal light availability decreased; OTU richness also consistently increased down through the water column, with the deepest darkest waters containing the greatest diversity. Beta-diversity analyses confirmed that seasonality and depth also primarily drove community composition. The relative abundance of the eleven predominant taxa showed significant changes in surface waters in summer months and varied with season depending on the phytoplankton bloom stage; corresponding populations in deeper waters however, remained relatively unchanged. Given the significance of the annual phytoplankton bloom pattern on prokaryote diversity in Arctic waters, any changes to bloom dynamics resulting from accelerated global warming will likely have major impacts on surface marine microbial communities, those impacts inevitably trickling down into deeper waters.

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

“…Several cryptic taxonomic groups, such as Chloroflexi (SAR202), Marinimicrobia (SAR406) and various members of Deltaproteobacteria, were significantly enriched in EGC (Figure 5), and also consisted of a large number of unique OTUs in this region. These enriched taxonomic groups in the icecovered EGC were previously reported from surface waters in the western Svalbard region (WSC) during the Arctic winter (Wilson et al, 2017). Therefore, our results support and strengthen the hypothesis of Wilson et al (2017) that bacterial community dynamics in Fram Strait are to a large extent affected by seasonal variability (e.g., availability of light under changing sea ice conditions), rather than hydrographic differences between water masses.…”

“…However, monitoring the dynamics of heterotrophic microorganisms requires physical sampling. Wilson et al (2017) were the first to describe changes of bacterial community composition in the eastern Fram Strait throughout a polar year. In accordance with observations from other polar regions (Alonso-Sáez et al, 2008;Iversen and Seuthe, 2011;Ghiglione and Murray, 2012;Williams et al, 2012), their results showed that the extreme seasonality of polar marine ecosystems, with ice-covered dark winter conditions and extended irradiance in summer, leads to pronounced seasonal differences in heterotrophic bacterial communities.…”

Bacterial Response to Permafrost Derived Organic Matter Input in an Arctic Fjord

“…Environmental conditions in the Arctic are highly affected by seasonality. In accordance, seasonal changes in microbial community composition have been reported for different parts of the Arctic Oceans, i.e., the increase of Gammaproteobacteria, in association to phytoplankton bloom dynamics and increased concentrations of dissolved organic matter in the summer months (Alonso-Sáez et al, 2008;Buchan et al, 2014;El-Swais et al, 2015;Wilson et al, 2017). The fjord water used for our incubation experiments, taken in June, was indicative for a post-bloom situation, with high relative abundance of the phylum Proteobacteria (96.5%) and in particular the class Gammaproteobacteria, with up to 76.1% of all proteobacterial reads (Figure 4), similar to reports from other studies (Piquet et al, 2010;Zeng et al, 2013).…”

“…The increase of Gammaproteobacteria, in particular of taxa belonging to the order Alteromonadales including Glaciecola and Colwellia, has also been observed during marine phytoplankton spring blooms in lower latitudes (Tada et al, 2011;Teeling et al, 2012) and in the Arctic Ocean (Bano and Hollibaugh, 2002;Wilson et al, 2017). This suggests that they can rapidly proliferate in response to new carbon sources, including phytoplankton-derived organic carbon or tDOM as indicated in our study.…”

Biological transformation of Arctic dissolved organic matter in a NE Greenland fjord

“…Amplification of complementary DNA (cDNA) and DNA was performed using a two‐step nested PCR approach with primers 519F and 806R targeting both the archaeal and the bacterial 16S rRNA gene V4 hypervariable region. Details regarding extraction, amplification and amplicon library preparation can be found in Wilson et al (). Libraries were sequenced at the Norwegian Sequencing Centre (Oslo, Norway) using their MiSeq platform (MiSeq Reagent Kit v2, Illumina, California, U.S.A.).…”