Temperature Driven Changes in Benthic Bacterial Diversity Influences Biogeochemical Cycling in Coastal Sediments

Abstract

Marine sediments are important sites for global biogeochemical cycling, mediated by macrofauna and microalgae. However, it is the microorganisms that drive these key processes. There is strong evidence that coastal benthic habitats will be affected by changing environmental variables (rising temperature, elevated CO₂), and research has generally focused on the impact on macrofaunal biodiversity and ecosystem services. Despite their importance, there is less understanding of how microbial community assemblages will respond to environmental changes. In this study, a manipulative mesocosm experiment was employed, using next-generation sequencing to assess changes in microbial communities under future environmental change scenarios. Illumina sequencing generated over 11 million 16S rRNA gene sequences (using a primer set biased toward bacteria) and revealed Bacteroidetes and Proteobacteria dominated the total bacterial community of sediment samples. In this study, the sequencing coverage and depth revealed clear changes in species abundance within some phyla. Bacterial community composition was correlated with simulated environmental conditions, and species level community composition was significantly influenced by the mean temperature of the environmental regime (p = 0.002), but not by variation in CO₂ or diurnal temperature variation. Species level changes with increasing mean temperature corresponded with changes in NH₄ concentration, suggesting there is no functional redundancy in microbial communities for nitrogen cycling. Marine coastal biogeochemical cycling under future environmental conditions is likely to be driven by changes in nutrient availability as a direct result of microbial activity.

Keywords: benthic biogeochemistry; benthic microbial ecology; biogeochemical cycles; environmental change; marine sediments; microbial communities.

Figure 1

Relative abundance of bacterial community compositions for 52 sediment samples at species level, including taxonomic identification for only sequences that comprised >1% of the total bacterial community for each sample. “Minor” species are all species that comprise < 1% of the total bacterial community for each sample, which have been artificially clustered together into a “minor” species group, depicted by the gray coloring.

Figure 2

UPGMA tree with Jacknife support using weighted Unifac distance. Nodes with >0.8 Jacknife support are labeled. Branches are color coded to reflect the mean temperature of the experimental regime: 6°C (blue), 12°C (red) and 18°C (green). The labels of each branch correspond to environmental conditions.

Figure 3

Non-metric multidimensional scaling plot of bacterial community composition color coded according to mean temperature: 6°C (blue), 12°C (red), and 18°C (green). The labels correspond to environmental conditions.

Figure 4

Canonical Correspondence Analysis (CCA) plot for major bacterial species (comprising >1% of the total bacterial community composition of each sample) at species level (OTU) resolution. The blue lines and labels correspond to the environmental conditions and nutrient concentrations, and the black labels represent the individual treatments.

Figure 5

Boxplots showing the raw data for each nutrient concentration (A–I) and microphytobenthos (MPB) biomass (J–L) against each environmental: NH₄ concentration (A,E,I); NO_x concentration (B,F,J); PO₄ concentration (C,G,K) and MPB biomass (D,H,L) against mean temperature (top plots); CO₂ regime (middle plots); and temperature fluctuation (bottom plots). Colors indicate mean temperature treatments of 6°C (blue), 12°C (green), and 18°C (red) in the top three graphs; represent CO₂ levels of 380 ppmv (blue) and 1,000 ppmv (red) in the middle plots; and temperature fluctuation of 1°C (blue), 3°C (green), and 6°C (red).

Figure 6

Canonical Correspondence Analysis (CCA) plot for major bacterial species (comprising >1% of the total bacterial community composition of each sample) at class/order resolution. The blue lines and labels correspond to the environmental conditions and nutrient concentrations, and the black labels represent the individual treatments.