Beam, J. P. | bioRxiv Science

bioRxiv2020

Mud, microbes, and macrofauna: seasonal dynamics of the iron biogeochemical cycle in an intertidal mudflat

Microorganisms and burrowing animals exert a pronounced impact on the cycling of redox sensitive metals in coastal sediments. Sedimentary metal cycling is likely controlled by seasonal processes including changes in temperature, animal feeding behavior due to food availability, and availability of organic matter in sediments. We hypothesized that the iron biogeochemical cycle and associated sedimentary microbial community will respond to seasonal changes in a bioturbated intertidal mudflat. In this study, we monitored the spatiotemporal dynamics of porewater and highly reactive solid phase iron with the corresponding prokaryotic and eukaryotic sedimentary microbial communities over one annual cycle from November 2015 to November 2016. Continuous and seasonally variable pools of both porewater Fe(II) and highly reactive iron (FeHR) were observed throughout the season with significant increases of Fe(II) and FeHR in response to increased sediment temperature in summer months. Maximum concentrations of Fe(II) and FeHR were predominantly confined to the upper 5 cm of sediment throughout the season. Iron-oxidizing and -reducing microorganisms were present and stable throughout the season, and exhibited strong depth-dependent stratification likely due to availability of Fe(II) and FeHR pools, respectively. Otherwise, the community was dominated by Deltaproteobacteria, which are involved in sulfur and potentially iron cycling, as well as Gammaproteobacteria and Bacteroidetes. The microbial community was relatively stable throughout the seasonal cycle, but showed strong separation with depth, probably driven by changes in oxygen availability and organic matter. The relative abundance of diatoms revealed a noticeable seasonal signature, which we attribute to spring and fall blooms recorded in the sediments. Macro-, meio, and microfauna were detected throughout the season with some seasonal variations that may influence sedimentary iron transformations by active microbial grazing. The seasonal dynamics of the sedimentary iron cycle are controlled by numerous, interdependent processes, with macrobiota-microbiota relationships and depth stratification comprising primary components. Deciphering these processes in natural ecosystems is essential to understand how they might respond to future environmental perturbations, such as anthropogenic nutrient release to coastal systems.

microbiology↗

bioRxiv2020

Ancestral absence of electron transport chains in Patescibacteria and DPANN

Recent discoveries suggest that the candidate superphyla Patescibacteria and DPANN constitute a large fraction of the phylogenetic diversity of Bacteria and Archaea. Their small genomes and limited coding potential have been hypothesized to be ancestral adaptations to obligate symbiotic lifestyles. To test this hypothesis, we performed cell-cell association, genomic, and phylogenetic analyses on 4,829 individual cells of Bacteria and Archaea from 46 globally distributed surface and subsurface field samples. This confirmed the ubiquity and abundance of Patescibacteria and DPANN in subsurface environments, the small size of their genomes and cells, and the divergence of their gene content from other Bacteria and Archaea. Our analyses suggest that most Patescibacteria and DPANN in the studied subsurface environments do not form specific physical associations with other microorganisms. These data also suggest that their unusual genomic features and prevalent auxotrophies may be a result of minimal cellular energy transduction mechanisms that potentially precede the evolution of respiration, thus relying solely on fermentation for energy conservation.

evolutionary biology↗

bioRxiv2017

A biological source of marine sedimentary iron oxides

The biogeochemical cycle of iron is intricately linked to numerous element cycles. Although reductive biological processes that bridge the iron cycle to other element cycles are established, little is known about microbial oxidative processes on iron cycling in sedimentary environments--resulting in the formation of iron oxides. Here, we show that a major source of sedimentary iron oxides originates from the metabolic activity of iron-oxidizing bacteria from the class Zetaproteobacteria, stimulated by burrowing animals in coastal sediments. Zetaproteobacteria were estimated to be a global total of 1026 cells in coastal, bioturbated sediments and would equate to an annual production of approximately 7.9 x 1015 grams of sedimentary iron oxides--twenty-five times larger than the annual flux of iron oxides by rivers. These data suggest that iron-oxidizing Zetaproteobacteria are keystone organisms in marine sedimentary environments given their low numerical abundance; yet exert a profound impact via the production of iron oxides.

microbiology↗