Efficient recycling of nutrients in modern and past hypersaline environments

Nutrient dynamics and budgeting in a semi-enclosed coastal hypersaline lagoon

Biogeochemical and ecological responses to limited external nutrient loading are poorly understood in tropical semi-enclosed coastal lagoons which are highly influenced by hydrological and salinity regimes. With objectives towards ecosystem sustainability via better management of the nutrient inputs, investigations were carried out to estimate the water, salt, and nutrient budget of "Pulicat" hypersaline coastal lagoon for the year 2018-2019. The budget revealed that the annual rate of precipitation and evaporation are the major driving factors regulating the annual residual flow in the lagoon. Limited exchange of water and material had resulted in a hypersaline condition with high spatial and temporal variation in salinity ranging from 20 to 103. In the absence of external loading, nutrient enrichment by internal compensation had resulted in DIN enrichment. DIN constituents are mainly contributed by ammonia, indicative of remineralisation through benthic regeneration. The extended water residence time and enhanced primary production has converted the inner lagoon into a limited phosphate system. The TRIX index also indicates a decline in the trophic status transforming the lagoon from a mesotrophic to a eutrophic system.

Salt to conserve: a review on the ecology and preservation of hypersaline ecosystems

When it comes to the investigation of key ecosystems in the world, we often omit salt from the ecological recipe. In fact, despite occupying almost half of the volume of inland waters and providing crucial services to humanity and nature, inland saline ecosystems are often overlooked in discussions regarding the preservation of global aquatic resources of our planet. As a result, our knowledge of the biological and geochemical dynamics shaping these environments remains incomplete and we are hesitant in framing effective protective strategies against the increasing natural and anthropogenic threats faced by such habitats. Hypersaline lakes, water bodies where the concentration of salt exceeds 35 g/l, occur mainly in arid and semiarid areas resulting from hydrological imbalances triggering the accumulation of salts over time. Often considered the 'exotic siblings' within the family of inland waters, these ecosystems host some of the most extremophile communities worldwide and provide essential habitats for waterbirds and many other organisms in already water-stressed regions. These systems are often highlighted as natural laboratories, ideal for addressing central ecological questions due to their relatively low complexity and simple food web structures. However, recent studies on the biogeochemical mechanisms framing hypersaline communities have challenged this archetype, arguing that newly discovered highly diverse communities are characterised by specific trophic interactions shaped by high levels of specialisation. The main goal of this review is to explore our current understanding of the ecological dynamics of hypersaline ecosystems by addressing four main research questions: (i) why are hypersaline lakes unique from a biological and geochemical perspective; (ii) which biota inhabit these ecosystems and how have they adapted to the high salt conditions; (iii) how do we protect biodiversity from increasing natural and anthropogenic threats; and (iv) which scientific tools will help us preserve hypersaline ecosystems in the future? First, we focus on the ecological characterisation of hypersaline ecosystems, illustrate hydrogeochemical dynamics regulating such environments, and outline key ecoregions supporting hypersaline systems across the globe. Second, we depict the diversity and functional aspects of key taxa found in hypersaline lakes, from microorganisms to plants, invertebrates, waterbirds and upper trophic levels. Next, we describe ecosystem services and discuss possible conservation guidelines. Finally, we outline how cutting-edge technologies can provide new insights into the study of hypersaline ecology. Overall, this review sheds further light onto these understudied ecosystems, largely unrecognised as important sources of unique biological and functional diversity. We provide perspectives for key future research avenues, and advocate that the conservation of hypersaline lakes should not be taken with 'a grain of salt'.

Quantifying potential N turnover rates in hypersaline microbial mats by 15N tracer techniques

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.

In silico determination of nitrogen metabolism in microbes from extreme conditions using metagenomics

The acid ponds of the Danakil Depression in northern Ethiopia are polyextreme environments that exceed the normal physicochemical limits of pH, salinity, ion content, and temperature. We tested for the occurrence of DNA-based life in this environment using Metagenomic Shotgun DNA sequencing approaches. The obtained sequences were examined by the bioinformatic tools MetaSpades, DIAMOND and MEGAN 6-CE, and we were able to bin more than 90% of the metagenomics contigs of Dallol and Black Water to the Bacteria domain, and to the Proteobacteria phylum. Predictions of gene function based on SEED disclosed the presence of different nutrient cycles in the acid ponds. For this study, we focused on partial or completely sequenced genes involved in nitrogen metabolism. The KEGG nitrogen metabolism pathway mapping results for both acid ponds showed that all the predicted genes are involved directly or indirectly in the assimilation of ammonia and no dissimilation or nitrification process was identified. Furthermore, the deduced nitrogen fixation in the two acid ponds based on SEED classification indicated the presence of different sets of nitrogen fixing (nif) genes for biosynthesis and maturation of nitrogenase. Based on the in silico analysis, the predicted proteins involved in nitrogen fixation, especially the cysteine desulfurase and [4Fe-4S] ferredoxin, from both acid ponds are unique with less than 80% sequence similarity to the next closest protein sequence. Considering the extremity of the environmental conditions of the two acid ponds in the Danakil depression, this metagenomics dataset can add to the study of unique gene functions in nitrogen metabolism that enable thriving biocommunities in hypersaline and highly acidic conditions.

Depthwise microbiome and isotopic profiling of a moderately saline microbial mat in a solar saltern

The solar salterns in Tuticorin, India, are man-made, saline to hypersaline systems hosting some uniquely adapted populations of microorganisms and eukaryotic algae that have not been fully characterized. Two visually different microbial mats (termed ‘white’ and ‘green’) developing on the reservoir ponds (53 PSU) were isolated from the salterns. Firstly, archaeal and bacterial diversity in different vertical layers of the mats were analyzed. Culture-independent 16S rRNA gene analysis revealed that both bacteria and archaea were rich in their diversity. The top layers had a higher representation of halophilic archaea Halobacteriaceae, phylum Chloroflexi, and classes Anaerolineae, Delta- and Gamma- Proteobacteria than the deeper sections, indicating that a salinity gradient exists within the mats. Limited presence of Cyanobacteria and detection of algae-associated bacteria, such as Phycisphaerae, Phaeodactylibacter and Oceanicaulis likely implied that eukaryotic algae and other phototrophs could be the primary producers within the mat ecosystem. Secondly, predictive metabolic pathway analysis using the 16S rRNA gene data revealed that in addition to the regulatory microbial functions, methane and nitrogen metabolisms were prevalent. Finally, stable carbon and nitrogen isotopic compositions determined from both mat samples showed that the δ¹³C_org and δ¹⁵N_org values increased slightly with depth, ranging from − 16.42 to − 14.73‰, and 11.17 to 13.55‰, respectively. The isotopic signature along the microbial mat profile followed a pattern that is distinctive to the community composition and net metabolic activities, and comparable to saline mats in other salterns. The results and discussions presented here by merging culture-independent studies, predictive metabolic analyses and isotopic characterization, provide a collective strategy to understand the compositional and functional characteristics of microbial mats in saline environments.

Volatile organic compounds in the salt-lake sediments of the Tibet Plateau influence prokaryotic diversity and community assembly

Volatile organic compounds (VOCs) are important environmental factors because they supply nutrients for microbial cells and mediate intercellular interactions. However, few studies have focused on the effects of VOCs on prokaryotic diversity and community composition. In this study, we examined the relationship between prokaryotic diversity and community composition and the content of VOCs in salt-lake sediments from the Tibet Plateau using amplicon sequencing of the 16S rRNA gene. Results showed that the alpha-diversity indices (Chao1, Shannon, and Simpson) were generally negatively correlated with the content of 36 VOCs (P < 0.05). The prokaryotic communities were significantly driven by multiple VOCs at the lineage-dependent pattern (P < 0.05). Further analysis indicated that VOCs, including 3-methylpyruvate, biuret, isocitric acid, and stearic acid, jointly explained 37.3% of the variations in prokaryotic communities. Supplemental VOCs-pyruvate, biuret, alanine, and aspartic acid-notably decreased the Chao1 and Shannon indices and significantly assembled co-occurrence networks for the bacterial communities in the saline sediments. Together, these results demonstrated that VOCs play a critical role in the regulation of the diversity, compositions, and network structures of prokaryotic communities in saline sediments.