Salt marshes sequester carbon at rates more than an order of magnitude greater than their terrestrial counterparts, helping to mitigate climate change. As nitrogen loading to coastal waters continues, primarily in the form of nitrate, it is unclear what effect it will have on carbon storage capacity of these highly productive systems. This uncertainty is largely driven by the dual role nitrate can play in biological processes, where it can serve as a nutrient-stimulating primary production or a thermodynamically favorable electron acceptor fueling heterotrophic metabolism. Here, we used a controlled flow-through reactor experiment to test the role of nitrate as an electron acceptor, and its effect on organic matter decomposition and the associated microbial community in salt marsh sediments. Organic matter decomposition significantly increased in response to nitrate, even at sediment depths typically considered resistant to decomposition. The use of isotope tracers suggests that this pattern was largely driven by stimulated denitrification. Nitrate addition also significantly altered the microbial community and decreased alpha diversity, selecting for taxa belonging to groups known to reduce nitrate and oxidize more complex forms of organic matter. Fourier Transform-Infrared Spectroscopy further supported these results, suggesting that nitrate facilitated decomposition of complex organic matter compounds into more bioavailable forms. Taken together, these results suggest the existence of organic matter pools that only become accessible with nitrate and would otherwise remain stabilized in the sediment. The existence of such pools could have important implications for carbon storage, since greater decomposition rates as N loading increases may result in less overall burial of organic-rich sediment. Given the extent of nitrogen loading along our coastlines, it is imperative that we better understand the resilience of salt marsh systems to nutrient enrichment, especially if we hope to rely on salt marshes, and other blue carbon systems, for long-term carbon storage. K E Y W O R D S16S rRNA gene, anaerobic respiration, decomposition, flow-through reactor, microbes, nitrate, organic matter, salt marsh | 3225 BULSECO Et aL.
The balance between nitrate respiration pathways, denitrification and dissimilatory nitrate (NO 3 − ) reduction to ammonium (DNRA), determines whether bioavailable nitrogen is removed as N 2 gas or recycled as ammonium. Saltwater intrusion and organic matter enrichment may increase sulphate reduction leading to sulphide accumulation. We investigated the effects of sulphide on the partitioning of NO 3 − between complete denitrification and DNRA and the microbial communities in salt marsh sediments. Complete denitrification significantly decreased with increasing sulphide, resulting in an increase in the contribution of DNRA to NO 3 − respiration. Alternative fates of NO 3 − became increasingly important at higher sulphide treatments, which could include N 2 O production and/or transport into intracellular vacuoles. Higher 16S transcript diversity was observed in the high sulphide treatment, with clear shifts in composition. Generally, low and no sulphide, coupled with high NO 3 − , favoured the activity of Campylobacterales, Oceanospirillales and Altermonadales, all of which include opportunistic denitrifiers. High P sulphide conditions promoted the activity of potential sulphide oxidizing nitrate reducers (Desulfobulbaceae, Acidiferrobacteraceae and Xanthomonadales) and sulphate reducers (Desulfomonadaceae, Desulfobacteraceae). Our study highlights the tight coupling between N and S cycling, and the implications of these dynamics on the fate of bioavailable N in coastal environments susceptible to intermittent saltwater inundation and organic matter enrichment.Received
We aimed to determine if chronic endurance-exercise habits affected redox status and paracrine function of CD34(+) and CD34(-)/CD31(+) circulating angiogenic cells (CACs). Subjects were healthy, nonsmoking men and women aged 18-35 yr and categorized by chronic physical activity habits. Blood was drawn from each subject for isolation and culture of CD34(+) and CD34(-)/CD31(+) CACs. No differences in redox status were found in any group across either cell type. Conditioned media (CM) was generated from the cultured CACs and used in an in vitro human umbilical vein endothelial cell-based tube assay. CM from CD34(+) cells from inactive individuals resulted in tube structures that were 29% shorter in length (P < 0.05) and 45% less complex (P < 0.05) than the endurance-trained group. CD34(-)/CD31(+) CM from inactive subjects resulted in tube structures that were 26% shorter in length (P < 0.05) and 42% less complex (P < 0.05) than endurance-trained individuals. Proteomics analyses identified S100A8 and S100A9 in the CM. S100A9 levels were 103% higher (P < 0.05) and S100A8 was 97% higher in the CD34(-)/CD31(+) CM of inactive subjects compared with their endurance-trained counterparts with no significant differences in either protein in the CM of CD34(+) CACs as a function of training status. Recombinant S100A8/A9 treatment at concentrations detected in inactive subjects' CD34(-)/CD31(+) CAC CM also reduced tube formation (P < 0.05). These findings are the first, to our knowledge, to demonstrate a differential paracrine role in CD34(+) and CD34(-)/CD31(+) CACs on tube formation as a function of chronic physical activity habits and identifies a differential secretion of S100A9 by CD34(-)/CD31(+) CACs due to habitual exercise.
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