Impact of meltwater flow intensity on the spatiotemporal heterogeneity of microbial mats in the McMurdo Dry Valleys, Antarctica.

The meltwater streams of the McMurdo Dry Valleys are hot spots of biological diversity in the climate-sensitive polar desert landscape. Microbial mats, largely comprised of cyanobacteria, dominate the streams which flow for a brief window of time (~10 weeks) over the austral summer. These communities, critical to nutrient and carbon cycling, display previously uncharacterized patterns of rapid destabilization and recovery upon exposure to variable and physiologically detrimental conditions. Here, we characterize changes in biodiversity, transcriptional responses and activity of microbial mats in response to hydrological disturbance over spatiotemporal gradients. While diverse metabolic strategies persist between marginal mats and main channel mats, data collected from 4 time points during the austral summer revealed a homogenization of the mat communities during the mid-season peak meltwater flow, directly influencing the biogeochemical roles of this stream ecosystem. Gene expression pattern analyses identified strong functional sensitivities of nitrogen-fixing marginal mats to changes in hydrological activities. Stress response markers detailed the environmental challenges of each microhabitat and the molecular mechanisms underpinning survival in a polar desert ecosystem at the forefront of climate change. At mid and end points in the flow cycle, mobile genetic elements were upregulated across all mat types indicating high degrees of genome evolvability and transcriptional synchronies. Additionally, we identified novel antifreeze activity in the stream microbial mats indicating the presence of ice-binding proteins (IBPs). Cumulatively, these data provide a new view of active intra-stream diversity, biotic interactions and alterations in ecosystem function over a high-flow hydrological regime.

Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters.

To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice ( L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Relationship between dissolved organic matter quality and microbial community composition across polar glacial environments.

Vast expanses of Earth's surface are covered by ice, with microorganisms in these systems affecting local and global biogeochemical cycles. We examined microbial assemblages from habitats fed by glacial meltwater within the McMurdo Dry Valleys, Antarctica and on the west Greenland Ice Sheet (GrIS), evaluating potential physicochemical factors explaining trends in community structure. Microbial assemblages present in the different Antarctic dry valley habitats were dominated by Sphingobacteria andFlavobacteria, while Gammaproteobacteria and Sphingobacteria prevailed in west GrIS supraglacial environments. Microbial assemblages clustered by location (Canada Glacier, Cotton Glacier and west GrIS) and were separated by habitat type (i.e. ice, cryoconite holes, supraglacial lakes, sediment and stream water). Community dissimilarities were strongly correlated with dissolved organic matter (DOM) quality. Microbial meltwater assemblages were most closely associated with different protein-like components of the DOM pool. Microbes in environments with mineral particles (i.e. stream sediments and cryoconite holes) were linked to DOM containing more humic-like fluorescence. Our results demonstrate the establishment of distinct microbial communities within ephemeral glacial meltwater habitats, with DOM-microbe interactions playing an integral role in shaping communities on local and polar spatial scales.

Iron photoreduction and oxidation in an acidic mountain stream.

In a small mountain stream in Colorado that receives acidic mine drainage, photoreduction of ferric iron results in a well-defined increase in dissolved ferrous iron during the day. To quantify this process, an instream injection of a conservative tracer was used to measure discharge at the time that each sample was collected. Daytime production of ferrous iron by photoreduction was almost four times as great as nighttime oxidation of ferrous iron. The photoreduction process probably involves dissolved or colloidal ferric iron species and limited interaction with organic species because concentrations of organic carbon are low in this stream.