Quantifying Potential N Turnover Rates in Hypersaline Microbial Mats by Using ¹⁵ N Tracer Techniques

Abstract

Microbial mats, due to stratification of the redox zones, have a potential to include a complete N cycle, however an attempt to evaluate a complete N cycle in these ecosystems has not been yet made. In this study, occurrence and rates of major N cycle processes were evaluated in intact microbial mats from Elkhorn Slough, Monterey Bay, CA, USA, and Baja California Sur, Mexico under oxic and anoxic conditions using 15N-labeling techniques. All of the major N transformation pathways, with the exception of anammox, were detected in both microbial mats. Nitrification rates were found to be low at both sites for both seasons investigated. The highest rates of ammonium assimilation were measured in Elkhorn Slough mats in April and corresponded to high in situ ammonium concentration in the overlying water. Baja mats featured higher ammonification than ammonium assimilation rates and this, along with their higher affinity for nitrate compared to ammonium and low dissimilatory nitrate reduction to ammonium rates, characterized their differences from Elkhorn Slough mats. Nitrogen fixation rates in Elkhorn Slough microbial mats were found to be low implying that other processes such as recycling and assimilation from water are main sources of N for these mats at the times sampled. Denitrification in all of the mats was incomplete with nitrous oxide as end product and not dinitrogen. Our findings highlight N cycling features not previously quantified in microbial mats and indicate a need of further investigations in these microbial ecosystems. Importance: Nitrogen is essential for life. The nitrogen cycle on Earth is mediated by microbial activity and has had a profound impact on both the atmosphere and the biosphere throughout geologic time. Microbial mats, present in many modern environments, have been regarded as living records of the organisms, genes, and phylogenies of microbes, as they are one of the most ancient ecosystems on Earth. While rates of major nitrogen metabolic pathways have been evaluated in a number of ecosystems, it remains elusive in microbial mats. In particular it is unclear what factors affect nitrogen cycling in these ecosystems and how morphological differences between mats impact nitrogen transformations. In this study we investigate nitrogen cycling in two microbial mats having morphological differences. Our findings provide insight for further understanding of biogeochemistry and microbial ecology of microbial mats.

Millimeter-scale vertical partitioning of nitrogen cycling in hypersaline mats reveals prominence of genes encoding multi-heme and prismane proteins

“…Similar results were reported in a study based on metagenome assembled genomes from ammonium-rich geothermal groundwater microbial mat [53]. In a recent study based on stable isotope tracer measurements in the same microbial mat studied here, nitrification metabolism was detected only at very low rates [22]. Previous discussions of reasons for low rates of nitrification in microbial mat communities have noted sensitivities of nitrifying organisms to light and salinity [47,54].…”

“…However, while qPCR assays detect both the presence and expression of Planctomycetes-16S rRNA gene marker in the mat, with a sharp peak located in Layer 4, no sequences of the enzyme hydrazine oxidoreductase (hzo) and hydrazine synthase (hzs), normally present in ammonium oxidizing organisms, were detected; 277 genes related to microorganisms described as anammox bacteria such as Candidatus Scalindia sp, Candidatus Scalindua rubra, Candidatus Jettenia ecosi, Candidatus Jettenia caeni, Candidatus Kuenenia stuttgartiensis, Candidatus Brocadiae bacterium, Candidatus Brocadia sp, were identified in the taxonomy from the metagenome (Table S7). This, coupled with a lack of anammox activity detected in 15 N-labeling experiments [22], is consistent with the possibility that there may be Planctomycetes in these Guerrero Negro mats that are not performing anammox. No hzs genes were detected in another hypersaline mat from Australia in a study employing a metagenome-assembled genomes approach [34].…”

“…High light, low dissolved ammonium concentrations, and a greater metabolic requirement for the production of new biomass are all characteristics of the surface layers of microbial mats. Higher ammonium concentrations in deeper layers of the mat may be explained by ammonification, which provides an additional source of fixed nitrogen to microorganisms removed from the surface layers and/or lacking the capacity for diazotrophy, as has been previously speculated [39], and more recently experimentally verified [22].…”

“…They are characterized by high rates of metabolic processes carried out by cyanobacteria, phototrophic and chemotrophic sulfur-oxidizing bacteria, sulfate-reducing bacteria, and methanogens, among others [19][20][21]. Transformations of the nitrogen cycle including N 2fixation, nitrification, denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction were recently quantified in Guerrero Negro microbial mats [22]. Nitrogen necessary for growth of the mat is primarily understood to be obtained through nitrogen fixation, uptake of inorganic and organic sources of nitrogen from the overlying water, and through recycling of previously produced biomass [22,23].…”

Community homeostasis of coastal microbial mats from the Camargue during winter (cold) and summer (hot) seasons

“…Denitrification (e.g., K00374, K02567, K02568, K00368, K02305, K00376) is a bacterial process in which nitrate or nitrite undergoes a stepwise reduction through a series of nitrogen intermediates to dinitrogen. Denitrification mostly takes place under anaerobic conditions in which nitrate/nitrite is the electron acceptor (anaerobic respiration), and organic carbon is the primary electron donor for heterotrophic denitrifying bacteria (Coban et al, 2021). Denitrification was a major metabolic activity in layer 3 (Figure 4) and was related to Bacteroidetes (Flacobacteriales), Gammaproteobacteria (Marinicellales), and Alphaproteobacteria (Rhodobacterales).…”