Nitrogen cycling in a tropical coral reef ecosystem under severe anthropogenic disturbance in summer: Insights from isotopic compositions

Nitrate enrichment exacerbates microbiome and metabolism disturbances of the coral holobiont under heat stress

Coral reef ecosystems are facing severe deterioration due to escalating global temperatures and human-induced activities. Combined nitrate and heat stress can exacerbate coral bleaching, however, the underlying mechanism is still unclear. In the present study, we assessed the bleaching status of Acropora hyacinthus, a reef-building coral species, under high temperature and nitrate stress conditions using chemostat cultivation. We observed nitrate enrichment (9 μM) induced a significant reduction in photosystem efficiency (Fv/Fm) of Symbiodiniaceae and an increased thermal bleaching of corals under high temperature (30 °C). Nitrate exposure promoted the proliferation of Enterobacteriaceae and Vibrionaceae, which are bacterial families, potentially augmenting the coral's susceptibility to disease while exerting negligible effects on the fungal community. Alterations were observed in the metabolic pathways of both the coral hosts and Symbiodiniaceae, including down-regulated folate biosynthesis and inflammatory mediator regulation of TRP channels. Our findings indicate that nitrate enrichment under heat stress disrupts the metabolism of coral holobionts through altering bacterial communities, ultimately leading to increased coral bleaching.

Understanding nitrate dynamics in urbanized and forested river ecosystems: A study integrating natural-abundance and paired isotopes

Nitrate pollution in river ecosystems poses significant global environmental and public health challenges. However, the underlying drivers behind the elevated nitrate concentrations within river systems remain unclear. This study employed natural abundance isotopes combined with 15N-pairing techniques to elucidate the origins and transformation pathways of nitrate in two contrasting river ecosystems: a heavily urbanized river and a minimally disturbed forested river. Results revealed that domestic sewage was the dominant nitrate source in both rivers, contributing 56.9 % in the urban river and 37.8 % in the forested river. Denitrification was identified as the primary nitrate removal process, although its efficiency was constrained in the urban river due to excessive nitrate inputs and limited in-stream removal capacity. Furthermore, soil denitrification was largely regulated by moisture. The elevated nitrate loadings within urban river were attributed to substantial domestic sewage inputs coupled with limited nitrate removal capabilities. These findings highlight the impact of urbanization on nitrate dynamics and underscore the urgent need for integrated management strategies. Effective measures, such as upgrading wastewater treatment infrastructure, preserving riparian vegetation, and implementing nutrient reduction programs, are critical for mitigating nitrate pollution and enhancing water quality in both urban and natural river systems. This study provides insights applicable to riverine nitrogen management globally, contributing to the sustainable development of aquatic ecosystems.

White lies for coral reefs: Dynamics of two-patch coral reefs model with asymmetric dispersal

Coral reef ecosystem is a crucial component of marine ecosystems and is undergoing severe degradation due to the combined dural impact of environmental changes and human activities. Soundscape technology is an innovative coral reef restoration approach that attracts fish to degraded reefs. Inspired by such technique, a five-dimensional mathematical dynamical model incorporating the asymmetric dispersal of parrotfish is formulated to characterize the dynamic interaction among macroalgae, coral, algal turf, and parrotfish in coral reef ecosystem. Theoretical analyses are conducted and the impact of dispersal on the stability of coral reef ecosystems is systematically studied. The global sensitivity analysis is presented by using PRCC method and the impact of dispersal coefficients on the dynamic behavior of the model is explored through numerical simulations, which provide deeper insights into the influence of key parameters on the stability of the model. The main findings indicate that, adopting soundscape technology facilitates the recovery of coral reefs, make it easier to maintain a coral-dominated state, and reduce the possibility of phase shifts, thereby enhance the stability, biodiversity, and recovery of coral reef ecosystem.

Nitrate budget of a terrestrial-to-marine continuum in South China: Insights from isotopes and a Markov chain Monte Carlo model

Anthropogenic nitrate (NO3-) production has been increasing and is exported to the ocean via river networks, causing eutrophication and ecological damage. While studies have focused on river NO3- pollution, what has been lacking is the quantification of the sources of NO3- in coastal rivers. This study applied the dual isotopes (δ15N/δ18O-NO3-) to quantify the sources and their fluxes of NO3- in two inflow rivers of the Qinzhou Bay. By adding our results to the NO3- source apportionment in Qinzhou Bay, we, for the first time, established the NO3- budgets of the terrestrial-to-marine continuum in both high- and low-flow seasons. We quantitatively showed the direct and indirect roles (e.g., the stimulation of nitrification by sewage ammonium-NH4+) of terrestrial sources in driving the high NO3- loading in the estuary. The results highlighted the necessity to consider coastal rivers and estuary as a whole, which could shed light on the effective reduction of NO3- pollution in coastal environments.

Current trends and challenges in the analysis of marine environmental contaminants by isotope ratio mass spectrometry

An increasing number of organic and inorganic pollutants are being detected in the marine environment, posing a severe threat to the ecosystem and human health, even in trace concentrations. Isotope ratio mass spectrometry (IRMS) is one of the critical methods for determining the origin and fate of environmental pollutants and characterising their transformation processes. It has been used for a relatively long time for ecological monitoring of some well-studied industrial hydrocarbons at contaminated sites. However, the method still faces many analytical challenges. This review provides a comprehensive overview of recent technical advances concerning IRMS analysis of various contaminants and discusses typical pitfalls encountered in marine environment analysis. Particular attention is given to the study of sampling techniques and sample preparation for examination, often the keys to successful research given the complexity of marine matrices and the diverse and numerous nature of contaminants. Prospects for developing IRMS to monitor pollution sources and pollutant transformation in the marine environment are outlined.

Synergistic/antagonistic effects of nitrate/ammonium enrichment on fatty acid biosynthesis and translocation in coral under heat stress

Superimposed on ocean warming, nitrogen enrichment caused by human activity puts corals under even greater pressure. Biosynthesis of fatty acids (FA) is crucial for coral holobiont survival. However, the responses of FA biosynthesis pathways to nitrogen enrichment under heat stress in coral hosts and Symbiodiniaceae remain unknown, as do FA translocation mechanisms in corals. Herein, we used the thermosensitive coral species Acropora hyacinthus to investigate changes in FA biosynthesis pathways and polyunsaturated FA translocation of coral hosts and Symbiodiniaceae with respect to nitrate and ammonium enrichment under heat stress. Heat stress promoted pro-inflammatory FA biosynthesis in coral hosts and inhibited FA biosynthesis in Symbiodiniaceae. Nitrate enrichment inhibited anti-inflammatory FA biosynthesis in Symbiodiniaceae, and promoted pro-inflammatory FA biosynthesis in coral hosts and translocation to Symbiodiniaceae, leading to bleaching after 14 days of culture. Intriguingly, ammonium enrichment promoted anti-inflammatory FA biosynthesis in Symbiodiniaceae and translocation to hosts, allowing corals to better endure heat stress. We constructed schematic diagrams of the shift in FA biosynthesis and translocation in and between A. hyacinthus and its Symbiodiniaceae under heat stress, heat and nitrate co-stress, and heat and ammonium co-stress. The findings provide insight into the mechanisms of coral bleaching under environmental stress from a fatty acid perspective.

From Soil to River: Revealing the Mechanisms Underlying the High Riverine Nitrate Levels in a Forest Dominated Catchment

Elevated riverine nitrate (NO₃^-) levels have led to increased eutrophication and other ecological implications. While high riverine NO₃^- levels were generally ascribed to anthropogenic activities, high NO₃^- levels in some pristine or minimally disturbed rivers were reported. The drivers of these unexpectedly high NO₃^- levels remain unclear. This study combined natural abundance isotopes, ¹⁵N-labeling techniques, and molecular techniques to reveal the processes driving the high NO₃^- levels in a sparsely populated forest river. The natural abundance isotopes revealed that the NO₃^- was mainly from soil sources and that NO₃^- removal processes were insignificant. The ¹⁵N-labeling experiments also quantitatively showed that the biological NO₃^- removal processes, i.e., denitrification, dissimilatory NO₃^- reduction to ammonium (DNRA), and anaerobic ammonia oxidation (anammox), in the soils and sediments were weak relative to nitrification in summer. While nitrification was minor in winter, the NO₃^- removal was insignificant relative to the large NO₃^- stock in the catchment. Stepwise multiple regression analyses and structural equation models revealed that in summer, nitrification in the soils was regulated by the amoA-AOB gene abundances and NH₄⁺-N contents. Low temperature constrained nitrification in winter. Denitrification was largely controlled by moisture content in both seasons, and anammox and DNRA could be explained by the competition with nitrification and denitrification on their substrate (nitrite-NO₂^-). We also revealed the strong hydrological control on the transport of soil NO₃^- to the river. This study effectively revealed the mechanisms underlying the high NO₃^- levels in a nearly pristine river, which has implications for the understanding of riverine NO₃^- levels worldwide.

Nitrate with enriched heavy oxygen isotope linked to changes in nitrogen source and transformation as groundwater table rises

Nitrate is a significant constituent of the total nitrogen pool in shallow aquifers and poses an escalating threat to groundwater resources, making it crucial to comprehend the source, conversion, and elimination of nitrogen using appropriate techniques. Although dual-isotope dynamics in nitrate have been widely used, uncertainties remain regarding the asynchronously temporal changes in δ¹⁸O-NO₃^- and δ¹⁵N-NO₃^- observed in hypoxic aquifers. This study aimed to investigate changes in nitrogen sources and transformations using temporal changes in field-based NO₃^- isotopic composition, hydro-chemical variables, and environmental DNA profiling, as the groundwater table varied. The results showed that the larger enrichment in δ¹⁸O-NO₃^- (+13‰) compared with δ¹⁵N-NO₃^- (-2‰) on average during groundwater table rise was due to a combination of factors, including high ¹⁸O-based atmospheric N deposition, canopies nitrification, and soil nitrification transported vertically by rainfalls, and ¹⁸O-enriched O₂ produced through microbial and root respiration within denitrification. The strong association between functional gene abundance and nitrogen-related indicators suggests that anammox was actively processed with nitrification but in small bacterial population during groundwater table rise. Furthermore, bacterial species associated with nitrogen-associated gradients provided insight into subsurface nitrogen transformation, with Burkholderiaceae species and Pseudorhodobacter potentially serving as bioindicators of denitrification, while Candidatus Nitrotogn represents soil nitrification. Fluctuating groundwater tables can cause shifts in hydro-chemical and isotopic composition, which in turn can indicate changes in nitrogen sources and transformations. These changes can be used to improve input sources for mixture models and aid in microbial remediation of nitrate.

Anthropogenic impacts on the nitrate pollution in an urban river: Insights from a combination of natural-abundance and paired isotopes

Urban rivers are often characterized by high nitrate (NO₃^-) loadings. High NO₃^- loadings cause water quality and ecological damages, which undermines the sustainable development of cities. To date, the drivers of these high NO₃^- loadings remain unclear. This study, for the first time, integrated natural-abundance isotopes (δ¹⁵ N/δ¹⁸O-NO₃^- and δD/δ¹⁸O-H₂O) and ¹⁵N-pairing techniques to comprehensively reveal the anthropogenic impacts on the NO₃^- pollution in an urban river. Natural-abundance isotopes suggested that in both the wet and dry seasons, the NO₃^- was predominantly from the conservative mixing of different sources, and biological NO₃^- removal was minor. The ¹⁵N-pairing experiments supported the natural-abundance isotope data, quantitatively showing that in-soil nitrification was prevailing, while NO₃^- removal processes (denitrification, anammox, and dissimilatory NO₃^- reduction to ammonium) were weak. A Bayesian isotope-mixing model showed that soil sources (soil organic nitrogen and chemical fertilizer) dominated the NO₃^- in the upper reaches, while in the lower reaches, the impermeable riparian zone short-circuited the access of soils to the river. Here, the wastewater treatment plants became a significant source of NO₃^-. This study quantitatively revealed the drivers of high NO₃^- loadings in an urban river, and generated important clues for effective NO₃^- pollution control and remediation in urban rivers.

High vulnerability and a big conservation gap: Mapping the vulnerability of coastal scleractinian corals in South China

Scleractinian corals build the most complex and diverse ecosystems in the ocean with various ecosystem services, yet continue to be degraded by natural and anthropogenic stressors. Despite the rapid decline in scleractinian coral habitats in South China, they are among the least concerning in global coral vulnerability maps. This study developed a rapid assessment approach that combines vulnerability components and species distribution models to map coral vulnerability within a large region based on limited data. The approach contained three aspects including, exposure, habitat suitability, and coral-conservation-based adaptive capacity. The exposure assessment was based on seven indicators, and the habitat suitability was mapped using Maximum Entropy and Random Forest models. Vulnerability of scleractinian corals in South China was spatially evaluated using the approach developed here. The results showed that the average exposure of the study region was 0.62, indicating relatively high pressure. The highest exposure occurred from the east coast of the Leizhou Peninsula to the Pearl River Estuary. Aquaculture and shipping were the most common causes of exposure. Highly suitable habitats for scleractinian corals are concentrated between 18°N-22°N. Only 21.6 % of the potential coral habitats are included in marine protected areas, indicating that there may still be large conservation gaps for scleractinian corals in China. In total, 37.7 % of the potential coral habitats were highly vulnerable, with the highest vulnerability appearing in the Guangdong Province. This study presents the first attempt to map the vulnerability of scleractinian corals along the coast of South China. The proposed approach and findings provide an essential tool and information supporting the sustainable management and conservation of coral reef ecosystems, addressing an important gap on the world's coral reef vulnerability map.

Effects of watershed land use on coastal marine environments: A multiscale exploratory analysis with multiple biogeochemical indicators in fringing coral reefs of Okinawa Island

The analytical spatial scale and selection of biogeochemical indicators affect interpretations of land-use impacts on coastal marine environments. In this study, nine biogeochemical indicators were sampled from 36 locations of coral reefs fringing a subtropical island, and their relationships with watershed land use were assessed by spatial autoregressive models with spatial weight matrixes based on distance thresholds of a few to 30 km. POM-relevant indicators were associated with agricultural and urban lands of watersheds within relatively small ranges (6-14 km), while the concentrations of inorganic nutrients were associated with watersheds within 20 km or more. The macroalgal δ¹⁵N showed a strong relationship with agricultural lands of watersheds within 7 km and urban/forest lands of watersheds within 24 km. These results demonstrate significant effects of land use on the coral reef ecosystems of the island, and the importance of appropriate combinations of analytical scales and biogeochemical indicators.