Archaeal blooms and busts in an estuarine time series.

Coastal bays, such as Delaware Bay, are highly productive, ecologically important transitions between rivers and the coastal ocean. They offer opportunities to investigate archaeal assemblages across seasons, with the exchange of water masses that occurs with tidal cycles, and in the context of variable organic matter quality. For a year-long estuarine, size-fractionated time series, we used amplicon sequencing, chemical measurements, and qPCR to follow archaeal groups through the seasons. We detected seasonally high abundances of Marine Group II archaea in summer months which correlate with indicators of phytoplankton production, although not phytoplankton biomass. Although previous studies have reported associations between Marine Group II archaea and particles, here they are almost entirely found in very small particles (0.22-0.7 µm), suggesting they are free-living cells. Populations of Nitrososphaeria did not vary with particle size or environmental conditions. Methanogens were significant fractions of archaeal sequences in large particles at low tide during winter months. Contrary to expectations, Nanoarchaeia were found predominantly in the free-living fraction despite the previous observation that they require an association with hosts. These results underscore the utility of time series studies in shallow, tidally mixed estuarine environments that capture variable conditions for understanding the ecology and biogeochemistry of planktic archaea.

Viral infections likely mediate microbial controls on ecosystem responses to global warming.

Climate change is affecting how energy and matter flow through ecosystems, thereby altering global carbon and nutrient cycles. Microorganisms play a fundamental role in carbon and nutrient cycling and are thus an integral link between ecosystems and climate. Here, we highlight a major black box hindering our ability to anticipate ecosystem climate responses: viral infections within complex microbial food webs. We show how understanding and predicting ecosystem responses to warming could be challenging-if not impossible-without accounting for the direct and indirect effects of viral infections on different microbes (bacteria, archaea, fungi, protists) that together perform diverse ecosystem functions. Importantly, understanding how rising temperatures associated with climate change influence viruses and virus-host dynamics is crucial to this task, yet is severely understudied. In this perspective, we (i) synthesize existing knowledge about virus-microbe-temperature interactions and (ii) identify important gaps to guide future investigations regarding how climate change might alter microbial food web effects on ecosystem functioning. To provide real-world context, we consider how these processes may operate in peatlands-globally significant carbon sinks that are threatened by climate change. We stress that understanding how warming affects biogeochemical cycles in any ecosystem hinges on disentangling complex interactions and temperature responses within microbial food webs.

Metagenomic views of microbial dynamics influenced by hydrocarbon seepage in sediments of the Gulf of Mexico.

Microbial cells in the seabed are thought to persist by slow population turnover rates and extremely low energy requirements. External stimulations such as seafloor hydrocarbon seeps have been demonstrated to significantly boost microbial growth; however, the microbial community response has not been fully understood. Here we report a comparative metagenomic study of microbial response to natural hydrocarbon seeps in the Gulf of Mexico. Subsurface sediments (10-15 cm below seafloor) were collected from five natural seep sites and two reference sites. The resulting metagenome sequencing datasets were analyzed with both gene-based and genome-based approaches. 16S rRNA gene-based analyses suggest that the seep samples are distinct from the references by both 16S rRNA fractional content and phylogeny, with the former dominated by ANME-1 archaea (~50% of total) and Desulfobacterales, and the latter dominated by the Deltaproteobacteria, Planctomycetes, and Chloroflexi phyla. Sulfate-reducing bacteria (SRB) are present in both types of samples, with higher relative abundances in seep samples than the references. Genes for nitrogen fixation were predominantly found in the seep sites, whereas the reference sites showed a dominant signal for anaerobic ammonium oxidation (anammox). We recovered 49 metagenome-assembled genomes and assessed the microbial functional potentials in both types of samples. By this genome-based analysis, the seep samples were dominated by ANME-1 archaea and SRB, with the capacity for methane oxidation coupled to sulfate reduction, which is consistent with the 16S rRNA-gene based characterization. Although ANME-1 archaea and SRB are present in low relative abundances, genome bins from the reference sites are dominated by uncultured members of NC10 and anammox Scalindua, suggesting a prevalence of nitrogen transformations for energy in non-seep pelagic sediments. This study suggests that hydrocarbon seeps can greatly change the microbial community structure by stimulating nitrogen fixation, inherently shifting the nitrogen metabolism compared to those of the reference sediments.

Mercury geochemistry and microbial diversity in meromictic Glacier Lake, Jamesville, NY.

Meromictic lakes are stratified lakes that typically stimulate phototrophic anoxic microbial metabolism, including the transformation of sulphur. Less studied are the transformations of mercury in these environments, and the microorganisms, which mediate these reactions. In order to further an understanding of redox species, mercury and microbial populations in meromictic lakes, we examined the geochemistry and microbiology of Glacier Lake in Jamesville, NY. We found an anoxic transition at a depth of 6 m, followed by active nitrate and sulphate utilization. A chlorophyll a maximum was located at 11 m, coinciding with peaks of several photoautotrophic microbial lineages and total mercury and methyl mercury. Via amplicon sequencing, the microbial population showed pronounced peaks of cyanobacteria at 10 m, Chlorobi at 12 m and Chloroflexi at 14 m. Sulphate-reducing bacteria were also most abundant between 10 and 14 m depth. A functional gene indicating the potential for the production of methyl mercury, hgcA, was detected at several depths in the lake. Our work suggests that in addition to the sulphur cycle, the cycling of mercury may be indirectly coupled with phototrophic processes in Glacier Lake.