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
The ability to directly measure neutralizing antibodies on live SARS-CoV-2 virus in individuals can play an important role in understanding the efficacy of therapeutic interventions or vaccines. In contrast to functional neutralization assays, serological assays only quantify the presence of antibodies as a proxy of immune protection.
The major histocompatibility complex (MHC) is critical to host-pathogen interactions. Class II MHC is a heterodimer, with α and β subunits encoded by different genes. The peptide binding groove is formed by the first domain of both subunits (α 1 and β 1 ), but studies of class II variation or natural selection focus primarily on the β subunit and II B genes. We explored MHC II A in Leach's storm-petrel, a seabird with two expressed, polymorphic II B genes. We found two II A genes, Ocle-DAA and Ocle-DBA, in contrast to the single II A gene in chicken and duck. In exon 2 which encodes the α 1 domain, the storm-petrel II A genes differed strongly from each other but showed little within-gene polymorphism in 30 individuals: just one Ocle-DAA allele, and three Ocle-DBA alleles differing from each other by single non-synonymous substitutions. In a comparable sample, the two II B genes had nine markedly diverged alleles each. Differences between the α 1 domains of Ocle-DAA and Ocle-DBA showed signatures of positive selection, but mainly at non-peptide binding site (PBS) positions. In contrast, positive selection within and between the II B genes corresponded to putative PBS codons. Phylogenetic analysis of the conserved α 2 domain did not reveal deep or well supported lineages of II A genes in birds, in contrast to the pronounced differentiation of DQA, DPA, and DRA isotypes in mammals. This uncertain homology complicates efforts to compare levels of functional variation and modes of evolution of II A genes across taxa.
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