Seasonal and stable heterotrophic guilds drive Arctic benthic microbiome functioning across polar day and night

Abstract

The remineralization of organic matter by benthic bacteria is an essential process in the marine carbon cycle. In polar regions, strong variation in daylength causes pronounced seasonality in primary productivity, but the responses of sedimentary bacteria to these fluctuations are not well understood. We investigated the seasonal dynamics of benthic bacterial communities from an Arctic fjord and found a partitioning of the communities into seasonally responsive and stable guilds. We separately analyzed the fractions of cells in the porewater and those loosely and firmly attached to sand grains through 16S ribosomal RNA gene sequencing, cell counting, rate measurements, and geochemical analyses. The porewater and loosely attached bacterial communities showed a dynamic response in composition and activity, suggesting that they play a central role in benthic-pelagic coupling by responding rapidly to seasonal fluctuations in organic matter availability. In contrast, the majority of the firmly attached cells showed a more buffered response, as reflected, e.g. in the consistently high cell numbers of Woeseiaceae. This fraction is potentially key to maintaining baseline remineralization processes throughout the year, independent of fresh organic matter input. These findings provide a new mechanistic understanding of carbon cycling in Arctic surface sediments that may also apply beyond polar regions.

Keywords: fraction of dividing cells; hydrolysis rates; microbial community; microbial diversity; oxygen consumption; porewater; sandy sediments.

Figure 1

Bulk sediment parameters across seasons. (A) Pigment content (in μg g⁻¹ sediment dry weight). Each bar represents one replicate sediment grab. X-axis labels indicate the sediment grab number. (B) Total organic carbon (TOC, in % dry weight). (C) Total organic nitrogen content (TON, in % dry weight). Each dot represents one replicate sediment grab. For (B) and (C), the solid horizontal bar represents the mean.

Figure 2

Diversity in surface (SSW) and overlying seawater (OSW), sediment fractions (PW = porewater; LA = loosely attached; FA = firmly attached), and bulk sediment (Bulk). (A) Richness based on the observed number of amplicon sequence variants (ASVs). (B) Evenness based on the Simpson’s evenness index. Sequences were subsampled to the minimum number of sequences (10 113 sequences) in the entire dataset. Each dot represents one replicate sediment grab and is colored according to the sampling date.

Figure 3

Community composition of fractions across seasons based on 16S rRNA amplicon sequencing. (A) Nonmetric multi-dimensional scaling (NMDS) plot of sediment fractions. Shapes represent the fractions; colors represent the sampling date. An NMDS plot including bulk sediment samples and with sample labels is shown in Fig. S3. (B) Pairwise PERMANOVA P-values (Bonferroni corrected) within fractions and across sampling dates. P-values from the ANOVA of betadisper results (test for homogeneity of within-group dispersion), as well as the PERMANOVA for each fraction are also reported. (C) The 20 most abundant genera per fraction and sampling date. The relative abundance of each genus for a specific date and fraction (mean of 4–8 replicate grabs collected on two sampling days) was used to select the most abundant genera.

Figure 4

ASV relative abundances of selected genera for 16S rRNA datasets across fractions and seasons. The five most abundant ASVs across fractions and seasons are shown. Each bar represents one replicate grab sample; x-axis labels indicate the sediment grab number. Note the different y-axis scales for each taxon.

Figure 5

In situ cell abundances and fraction of dividing cells of major taxa. (A) Abundance of major taxa shown relative to total cell counts, determined by CARD-FISH. (B) Absolute abundance of major taxa in cells ml⁻¹ sediment. Percentages and absolute abundances obtained from CARD-FISH are detailed in Table S7. (C) Fraction of dividing cells for selected taxonomic groups. Data from April 2022 have been described previously in [25]. Each dot represents one sediment grab. The solid horizontal bars represent the mean. Within one fraction, statistically significant differences between seasons were determined by one-way ANOVA followed by pairwise t-tests with Bonferroni correction for normally distributed data and Kruskal–Wallis followed by Dunn’s test with Bonferroni correction for non-normally distributed data. P-value: 0–.001 “^***”, .001–.01 “^**”, .01–.05 “^*.” January and April 2023 fractions were not processed for CARD-FISH and fraction of dividing cells measurements.

Figure 6

Mean extracellular hydrolysis rates of FLA-laminarin in the overlying seawater, sediment fractions, and bulk sediment from the January and April 2023 incubations. The final concentrations of added FLA-laminarin are indicated in parentheses. The error bars show the standard deviation of triplicates. The asterisk indicates a very high standard deviation for the triplicates (bulk 100 μM from January 2023; mean: 3118.5 nmol monomer l⁻¹ h⁻¹; SD: 3133.4 nmol monomer l⁻¹ h⁻¹). Note the different y-axis scales. April 2023 data have been described previously in [25]. Note that the measured rates are considered potential rates because the presence of laminarin in the environmental samples could compete with the added fluorescently labeled polysaccharides for enzyme active sites. Rate data for December 2021, April 2022, and June 2022 are shown in Fig. S10.

Figure 7

Per-cell oxygen consumption in sediment fractions across sampling dates. The horizontal bars represent the mean of the replicates. Data from April 2023 have been described previously in [25]. For June 2022, each dot represents measurements from one sediment grab. For January and April 2023, each dot represents the mean of 1–3 technical replicates from one sediment grab. Asterisk indicates P-value <.05. Within one fraction, statistical significance was determined by one-way ANOVA followed by pairwise t-tests between seasons with Bonferroni correction. Oxygen consumption in the fractions was not measured in December 2021 and April 2022.

Figure 8

Seasonal responses of bacterial communities in sediment fractions. (A) Community changes in the PW-LA during polar day and (B) polar night. (C) Community changes in the FA during polar day and (D) polar night. (E) The preferred electron acceptors and potential substrates between fractions and seasonal extremes. Asterisk indicates processes inferred from the presence of taxa. Supporting data and literature on the potential substrates are detailed in Table S8.