Reinhard, C. T. | bioRxiv Science

bioRxiv2020

Microbial helpers allow cyanobacteria to thrive in ferruginous waters

The Great Oxidation Event (GOE) was a rapid accumulation of oxygen in the atmosphere as a result of the photosynthetic activity of cyanobacteria. This accumulation reflected the pervasiveness of O2 on the planets surface, indicating that cyanobacteria had become ecologically successful in Archean oceans. Micromolar concentrations of Fe2+ in Archean oceans would have reacted with hydrogen peroxide, a byproduct of oxygenic photosynthesis, to produce hydroxyl radicals, which cause cellular damage. Yet cyanobacteria colonized Archean oceans extensively enough to oxygenate the atmosphere, which likely required protection mechanisms against the negative impacts of hydroxyl radical production in Fe2+-rich seas. We identify several factors that could have acted to protect early cyanobacteria from the impacts of hydroxyl radical production and hypothesize that microbial cooperation may have played an important role in protecting cyanobacteria from Fe2+ toxicity before the GOE. We found that several strains of facultative anaerobic heterotrophic bacteria (Shewanella) with ROS defense mechanisms increase the fitness of cyanobacteria (Synechococcus) in ferruginous waters. Shewanella species with manganese transporters provided the most protection. Our results suggest that a tightly regulated response to prevent Fe2+ toxicity could have been important for the colonization of ancient ferruginous oceans, particularly in the presence of high manganese concentrations, and may expand the upper bound for tolerable Fe2+ concentrations for cyanobacteria.

microbiology↗

bioRxiv2019

Expression of extracellular multiheme cytochromes discovered in a betaproteobacterium during Mn(III) reduction

Soluble ligand-bound Mn(III) can support anaerobic microbial respiration in diverse aquatic environments. Thus far, Mn(III) reduction has only been associated with certain Gammaproteobacteria. Here, we characterized microbial communities enriched from Mn-replete sediments of Lake Matano, Indonesia. Our results provide the first evidence for biological reduction of soluble Mn(III) outside the Gammaproteobacteria. Metagenome assembly and binning revealed a novel betaproteobacterium, which we designate "Candidatus Dechloromonas occultata." This organism dominated the enrichment and expressed a porin-cytochrome c complex typically associated with iron-oxidizing Betaproteobacteria and a novel cytochrome c-rich protein cluster (Occ), including an undecaheme putatively involved in extracellular electron transfer. This occ gene cluster was also detected in diverse aquatic bacteria, including uncultivated Betaproteobacteria from the deep subsurface. These observations provide new insight into the taxonomic and functional diversity of microbially-driven Mn(III) reduction in natural environments. Originality-Significance StatementRecent observations suggest that Mn(III)-ligand complexes are geochemically important in diverse aquatic environments. Thus far, microbially-driven Mn(III) reduction has only been associated with Gammaproteobacteria encoding three-component outer-membrane porin-cytochrome c conduits. Here, we demonstrate that Betaproteobacteria dominate in abundance and with respect to protein expression during biologically-mediated Mn(III) reduction in an enrichment culture from an anoxic lacustrine system. Using metaproteomics, we detect for the first time that Betaproteobacteria express a two-component porin-cytochrome c conduit, and an uncharacterized extracellular undecaheme (11-heme) c-type cytochrome. Although this is the first definitive report of an undecaheme within the Betaproteobacteria, we find evidence that they are widespread in uncultivated strains. These results widen the phylogenetic diversity of Mn(III)-reducing bacteria, and provide new insights into potential molecular mechanisms for soluble Mn(III) reduction

microbiology↗