Quality of phytoplankton deposition structures bacterial communities at the water-sediment interface

Transition of organic matter from allochthonous to autochthonous alters benthic nutrient dynamics via microbial food webs

The impoundment of rivers by dams has significantly modified sedimentation patterns and trophic structures. As a result, the algal-derived organic matter (OM), as opposed to terrestrial-derived OM, plays an increasingly important role along the river-reservoir gradient. This study utilized water-sediment microcosms to explore the impacts of allochthonous and autochthonous OM deposition on benthic nutrient dynamics mediated by microbial food webs. Our results revealed that OM addition led to increased fluxes of NH4+ and CO2, with the highest flux induced by cyanobacteria OM, followed by diatom and allochthonous OM. N2 release flux was promoted by allochthonous and diatom OM deposition but inhibited by cyanobacteria OM deposition. The amendment of autochthonous OM increased the activity of dehydrogenase and urease, while allochthonous OM with a higher C/N ratio enhanced the catalytic abilities of polyphenol oxidase and β-glucosidase. Furthermore, OM deposition significantly reduced microbial community richness and diversity, except for eukaryotic richness, and induced pronounced changes in bacterial and eukaryotic community structures. Allochthonous OM deposition stimulated the utilization of bacteria and protozoan on native OM, resulting in a positive priming effect of 26.78 %. In contrast, diatom and cyanobacteria OM additions exerted negative priming effects of -44.53 % and -29.76 %, respectively. Bayesian stable isotope mixing models showed that diatom OM was primarily absorbed by protozoan and metazoan, while cyanobacteria OM was more easily decomposed by bacteria and transferred to higher trophic levels through microbial food webs. In addition, bacterial ammonification accounted for 74.5 % of NH4+ release in the allochthonous OM deposition treatment, whereas eukaryotic excretion contributed separately 83.3 % and 83.1 % to NH4+ release in the diatom and cyanobacteria OM addition treatments. These findings highlight the significance of accounting for the regulatory capacity of OM deposition when studying benthic metabolism within river-reservoir systems.

Microbial functional genes are driven by gradients in sediment stoichiometry, oxygen, and salinity across the Baltic benthic ecosystem

BACKGROUND

Microorganisms in the seafloor use a wide range of metabolic processes, which are coupled to the presence of functional genes within their genomes. Aquatic environments are heterogenous and often characterized by natural physiochemical gradients that structure these microbial communities potentially changing the diversity of functional genes and its associated metabolic processes. In this study, we investigated spatial variability and how environmental variables structure the diversity and composition of benthic functional genes and metabolic pathways across various fundamental environmental gradients. We analyzed metagenomic data from sediment samples, measured related abiotic data (e.g., salinity, oxygen and carbon content), covering 59 stations spanning 1,145 km across the Baltic Sea.

RESULTS

The composition of genes and microbial communities were mainly structured by salinity plus oxygen, and the carbon to nitrogen (C:N) ratio for specific metabolic pathways related to nutrient transport and carbon metabolism. Multivariate analyses indicated that the compositional change in functional genes was more prominent across environmental gradients compared to changes in microbial taxonomy even at genus level, and indicate functional diversity adaptation to local environments. Oxygen deficient areas (i.e., dead zones) were more different in gene composition when compared to oxic sediments.

CONCLUSIONS

This study highlights how benthic functional genes are structured over spatial distances and by environmental gradients and resource availability, and suggests that changes in, e.g., oxygenation, salinity, and carbon plus nitrogen content will influence functional metabolic pathways in benthic habitats. Video Abstract.

Phytoplankton settling quality has a subtle but significant effect on sediment microeukaryotic and bacterial communities

In coastal aphotic sediments, organic matter (OM) input from phytoplankton is the primary food resource for benthic organisms. Current observations from temperate ecosystems like the Baltic Sea report a decline in spring bloom diatoms, while summer cyanobacteria blooms are becoming more frequent and intense. These climate-driven changes in phytoplankton communities may in turn have important consequences for benthic biodiversity and ecosystem functions, but such questions are not yet sufficiently explored experimentally. Here, in a 4-week experiment, we investigated the response of microeukaryotic and bacterial communities to different types of OM inputs comprising five ratios of two common phytoplankton species in the Baltic Sea, the diatom Skeletonema marinoi and filamentous cyanobacterium Nodularia spumigena. Metabarcoding analyses on 16S and 18S ribosomal RNA (rRNA) at the experiment termination revealed subtle but significant changes in diversity and community composition of microeukaryotes in response to settling OM quality. Sediment bacteria were less affected, although we observed a clear effect on denitrification gene expression (nirS and nosZ), which was positively correlated with increasing proportions of cyanobacteria. Altogether, these results suggest that future changes in OM input to the seafloor may have important effects on both the composition and function of microbenthic communities.