The polar night shift: seasonal dynamics and drivers of Arctic Ocean microbiomes revealed by autonomous sampling

Abstract

The Arctic Ocean features extreme seasonal differences in daylight, temperature, ice cover, and mixed layer depth. However, the diversity and ecology of microbes across these contrasting environmental conditions remain enigmatic. Here, using autonomous samplers and sensors deployed at two mooring sites, we portray an annual cycle of microbial diversity, nutrient concentrations and physical oceanography in the major hydrographic regimes of the Fram Strait. The ice-free West Spitsbergen Current displayed a marked separation into a productive summer (dominated by diatoms and carbohydrate-degrading bacteria) and regenerative winter state (dominated by heterotrophic Syndiniales, radiolarians, chemoautotrophic bacteria, and archaea). The autumn post-bloom with maximal nutrient depletion featured Coscinodiscophyceae, Rhodobacteraceae (e.g. Amylibacter) and the SAR116 clade. Winter replenishment of nitrate, silicate and phosphate, linked to vertical mixing and a unique microbiome that included Magnetospiraceae and Dadabacteriales, fueled the following phytoplankton bloom. The spring-summer succession of Phaeocystis, Grammonema and Thalassiosira coincided with ephemeral peaks of Aurantivirga, Formosa, Polaribacter and NS lineages, indicating metabolic relationships. In the East Greenland Current, deeper sampling depth, ice cover and polar water masses concurred with weaker seasonality and a stronger heterotrophic signature. The ice-related winter microbiome comprised Bacillaria, Naviculales, Polarella, Chrysophyceae and Flavobacterium ASVs. Low ice cover and advection of Atlantic Water coincided with diminished abundances of chemoautotrophic bacteria while others such as Phaeocystis increased, suggesting that Atlantification alters microbiome structure and eventually the biological carbon pump. These insights promote the understanding of microbial seasonality and polar night ecology in the Arctic Ocean, a region severely affected by climate change.

Variations of microbial communities and substrate regimes in the eastern Fram Strait between summer and fall

“…Around 81% ASVs were shared between summer and fall, illustrating a considerable number of taxa that do not respond to the changing biopolymer pool and other seasonal changes (Wietz et al ., 2021). Common taxa were defined as ASVs with >3 counts, >3% sequence abundance, and occurred in all the samples.…”

“…As the productive season progresses, phytoplankton release biopolymers that promote the growth of microbial communities, which re‐mineralize this organic matter (Piontek et al ., 2014; von Jackowski et al ., 2020). To continue understanding the substrate requirements of microbes, this study expands on demonstrated seasonal changes in biopolymer concentrations and microbial composition in the Fram Strait as the primary inflow to the Arctic Ocean (von Jackowski et al ., 2020; Wietz et al ., 2021). We hypothesized that the pronounced seasonal change in labile biopolymers would considerably shift the relative abundances of biopolymer‐degrading microbes between summer and fall.…”

Variations in Atlantic water influx and sea-ice cover drive taxonomic and functional shifts in Arctic marine bacterial communities

“…Autonomous sample collection and subsequent processing of samples proceeded according to Wietz et al 23 . Briefly, Remote Access Samplers (RAS; McLane) were deployed over four consecutive annual cycles between 2016-2020, with deployments and recovery occurring each summer (recover of 2019/2020 mooring occurred in 2021).…”

“…and Polar Water (PW) as described previously by Wietz et al 23 . Physical sensors were manufacturer-calibrated and processed in accordance with https://epic.awi.de/id/eprint/43137.…”

Phytoplankton Surveys in the Arctic Fram Strait Demonstrate the Tiny Eukaryotic Alga Micromonas and Other Picoprasinophytes Contribute to Deep Sea Export

“…The flux of the Micromonas , based on the quantitative analysis of the trap data, exhibited temporal variation, with peaks in the middle of the summer (second part of July; Figure 3 c). Strong seasonality in eukaryotic microbial community composition and in the export related to meltwater-derived stratification, is reported for the Fram Strait [ 20 , 59 ], and other means of export exist through mesoscale features [ 15 ] in the eastern North Atlantic. Likewise, the influence of seasonality on both biological composition and carbon export appears to be very strong at more temperate and subtropical sites, where sediment traps have been monitored using molecular approaches in the North Pacific gyre (HOT [ 10 ]) or by comparing two seasons in the North Atlantic gyre (BATS; [ 60 ]).…”