Nutrient pollution disrupts key ecosystem functions on coral reefs

Benzoyl Chloride Derivatization Advances the Quantification of Dissolved Polar Metabolites on Coral Reefs

Extracellular chemical cues constitute much of the language of life among marine organisms, from microbes to mammals. Changes in this chemical pool serve as invisible signals of overall ecosystem health and disruption to this finely tuned equilibrium. In coral reefs, the scope and magnitude of the chemicals involved in maintaining reef equilibria are largely unknown. Processes involving small, polar molecules, which form the majority components of labile dissolved organic carbon, are often poorly captured using traditional techniques. We employed chemical derivatization with mass spectrometry-based targeted exometabolomics to quantify polar dissolved phase metabolites on five coral reefs in the U.S. Virgin Islands. We quantified 45 polar exometabolites, demonstrated their spatial variability, and contextualized these findings in terms of geographic and benthic cover differences. By comparing our results to previously published coral reef exometabolomes, we show the novel quantification of 23 metabolites, including central carbon metabolism compounds (e.g., glutamate) and novel metabolites such as homoserine betaine. We highlight the immense potential of chemical derivatization-based exometabolomics for quantifying labile chemical cues on coral reefs and measuring molecular level responses to environmental stressors. Overall, improving our understanding of the composition and dynamics of reef exometabolites is vital for effective ecosystem monitoring and management strategies.

Impacts of humic substances, elevated temperature, and UVB radiation on bacterial communities of the marine sponge Chondrilla sp

Sponges are abundant components of coral reefs known for their filtration capabilities and intricate interactions with microbes. They play a crucial role in maintaining the ecological balance of coral reefs. Humic substances (HS) affect bacterial communities across terrestrial, freshwater, and marine ecosystems. However, the specific effects of HS on sponge-associated microbial symbionts have largely been neglected. Here, we used a randomized-controlled microcosm setup to investigate the independent and interactive effects of HS, elevated temperature, and UVB radiation on bacterial communities associated with the sponge Chondrilla sp. Our results indicated the presence of a core bacterial community consisting of relatively abundant members, apparently resilient to the tested environmental perturbations, alongside a variable bacterial community. Elevated temperature positively affected the relative abundances of ASVs related to Planctomycetales and members of the families Pseudohongiellaceae and Hyphomonadaceae. HS increased the relative abundances of several ASVs potentially involved in recalcitrant organic matter degradation (e.g., the BD2-11 terrestrial group, Saccharimonadales, and SAR202 clade). There was no significant independent effect of UVB and there were no significant interactive effects of HS, heat, and UVB on bacterial diversity and composition. The significant, independent impact of HS on the composition of sponge bacterial communities suggests that alterations to HS inputs may have cascading effects on adjacent marine ecosystems.

Riverine pollution influences the intraspecific variation in the gut microbiome of an invasive fish, Cyprinus carpio (Linn., 1758)

Humans are significantly impacting riverine systems worldwide, prompting us to investigate the effects of water pollution on the gut microbiome of Cyprinus carpio (common carp). Using 16S rRNA gene sequencing, we compared the gut microbiomes of common carp from two sites along river Yamuna with different pollution levels. Water pollution significantly altered the fish gut microbiome structure and microbial composition. Proteobacteria dominated in both sampling sites, while Bacteroidota prevailed in polluted water samples, indicating sewage and fecal contamination. Less polluted samples exhibited Verrucomicrobiae and Planctomycetes, negatively correlated with pollution levels. The polluted site had higher prevalence of potentially pathogenic and heavy metal-resistant bacteria, as well as microbial communities associated with wastewater treatment systems. Functional prediction highlighted the significant role of the gut microbiome in digestion and metabolism, with active enzymes for breaking down various organic substances. Biosynthetic pathways for leucine, valine, and isoleucine were present in both sites, known to be involved fish immunity. The host maintained a stable and diverse bacterial consortium, while microbial diversity became more specialized due to human activities, adapting to anthropogenic stress and selection pressures.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03747-0.

Microbial Interactions with Dissolved Organic Matter Are Central to Coral Reef Ecosystem Function and Resilience

To thrive in nutrient-poor waters, coral reefs must retain and recycle materials efficiently. This review centers microbial processes in facilitating the persistence and stability of coral reefs, specifically the role of these processes in transforming and recycling the dissolved organic matter (DOM) that acts as an invisible currency in reef production, nutrient exchange, and organismal interactions. The defining characteristics of coral reefs, including high productivity, balanced metabolism, high biodiversity, nutrient retention, and structural complexity, are inextricably linked to microbial processing of DOM. The composition of microbes and DOM in reefs is summarized, and the spatial and temporal dynamics of biogeochemical processes carried out by microorganisms in diverse reef habitats are explored in a variety of key reef processes, including decomposition, accretion, trophictransfer, and macronutrient recycling. Finally, we examine how widespread habitat degradation of reefs is altering these important microbe-DOM interactions, creating feedbacks that reduce reef resilience to global change.

A systematic review and meta-analysis of the direct effects of nutrients on corals

Chronic exposure of coral reefs to elevated nutrient conditions can modify the performance of the coral holobiont and shift the competitive interactions of reef organisms. Many studies have now quantified the links between nutrients and coral performance, but few have translated these studies to directly address coastal water quality standards. To address this management need, we conducted a systematic review of peer-reviewed studies, public reports, and gray literature that examined the impacts of dissolved inorganic nitrogen (DIN: nitrate, nitrite, and ammonium) and dissolved inorganic phosphorus (DIP: phosphate) on scleractinian corals. The systematic review resulted in 47 studies with comparable data on coral holobiont responses to nutrients: symbiont density, chlorophyll a (chl-a) concentration, photosynthesis, photosynthetic efficiency, growth, calcification, adult survival, juvenile survival, and fertilization. Mixed-effects meta-regression meta-analyses were used to determine the magnitude of the positive or negative effects of DIN and DIP on coral responses. Zooxanthellae density (DIN & DIP), chl-a concentration (DIN), photosynthetic rate (DIN), and growth (DIP) all exhibited positive responses to nutrient addition; maximum quantum yield (DIP), growth (DIN), larval survival (DIN), and fertilization (DIN) exhibited negative responses. In lieu of developing specific thresholds for the management of nutrients as a stressor on coral reefs, we highlight important inflection points in the magnitude and direction of the effects of inorganic nutrients and identify trends among coral responses. The responses of corals to nutrients are complex, warranting conservative guidelines for elevated nutrient concentrations on coral reefs.

Uptake of inorganic and organic phosphorus compounds by two marine sponges and their associated bacterial communities in aquaria

Marine sponges are abundant filter-feeders in benthic ecosystems and many host copious microorganisms. Sponges and their symbionts have emerged as major players within marine biogeochemical cycles, facilitating uptake and release of carbon, nitrogen, and sulfur. Sponge holobionts' role in transforming dissolved carbon and nitrogen is well established; however, the same depth of understanding has not yet been extended to phosphorus. In this aquaria-based study, ³² P-labelled orthophosphate and ATP were used to determine that two sponges, Lendenfeldia chondrodes and Hymeniacidon heliophila, both take up ambient dissolved inorganic phosphate (DIP) and dissolved organic phosphorus (DOP). Subsequent genetic analyses and chemical extraction showed that sponge symbionts have the potential to synthesise polyphosphate (poly-P) and that this energy-rich form of stored phosphorus is present in both sponges. L. chondrodes, an oligotrophic sponge with a microbiome dominated by cyanobacteria, stores more phosphorus as poly-P (6%-8% of total phosphorus) than H. heliophila (0.55%), a eutrophic sponge with low cyanobacterial abundance. DIP/DOP uptake, as well as poly-P storage, may be driven by two factors: cyanobacterial abundance and nutrient availability. Considering their prevalence in phosphorus-limited ecosystems and their ability to pump large amounts of seawater, sponge holobionts are likely to be key players within benthic phosphorus cycles.

A half-century record of coral skeletal P/Ca reveals late 20th century nutrient pollution in Port Dickson, Malaysia

Anthropogenic nutrient pollution has been identified as one of the key stressors of coastal ecosystems. However, the paucity of long-term nutrient records limits our understanding of both the extent of nutrient pollution as well as of the ecological impacts. Here, using coral skeletal phosphorus (P/Ca), we reconstructed a half-a-century record of seawater phosphate at Port Dickson, Malaysia. The P/Ca in the coral revealed an up to 8-fold increase in coral P/Ca from the late 1970s to 2000s, likely linked to increases in fertilizer use (R² = 0.47) and variabilities in rainfall (R² = 0.17). The rise in coral P/Ca in coincided with a contemporaneous 18 % decrease in coral skeletal density, suggesting phosphate enrichment may impact the growth and structural integrity of reef-building corals. Given the importance of both agriculture and heavy reliance on coral reefs by populations in Southeast Asia, our study highlights continue the need to develop environmental management upstream of coastal zones.

Nutrients dynamics in water and sediment of the Bonny Estuary, Niger Delta, Nigeria

Surface water and sediment from Bonny Estuary, Nigeria, were assessed to determine the nutrient dynamics. Nutrients (nitrates, phosphates, and total organic carbon (TOC)) and some physicochemical parameters of water samples are collected from three sampling stations along the Bonny Estuary during wet seasons (April 2017, 2018, and 2019) and dry seasons (November 2017, 2018, and 2019). Physicochemical parameters analyses were conducted in situ using Horiba water checker (Model U-10). Surface water nitrate and phosphate were analysed by APHA 4500-NO3-B Colorimetric method, while sediment nitrates and phosphate by Chemical Analysis for Ecological Matter CAEM/APHA 4500-NO3-E Colorimetric method and TOC by CAEM-Wet-Oxidation Titration method. There was no significant seasonal variation in pH, temperature, dissolved oxygen, biological oxygen demand, conductivity, and turbidity nor across the sampling stations. TDS values showed significant seasonal variation with higher values in the wet seasons. Surface water nitrate and phosphate had higher concentrations in wet seasons though within recommended limits. Conversely, nitrate, phosphate, and TOC in the sediments were quite high, with higher values in wet seasons, especially the downstream of the estuary. The high levels of nutrients in the sediments as opposed to surface water could be a result of rapid deposition of nutrients which could lead to possible nutrient enrichment of the sediment. Nutrient levels in the sediment have increased fourfold over the study period. This suggests an influx of nutrients into the estuary, due to human activities. Nutrient enrichment could result in deterioration in aquatic water quality and pose a threat to the ecology of the estuary.

Reviewing the effects of contamination on the biota of Brazilian coastal ecosystems: Scientific challenges for a developing country in a changing world

Pollution is a major worldwide problem that is increasing with urban growth, mainly along coastal areas. Pollution is often worse, governance is poorer and managerial strategies to improve environmental quality are less advanced in developing than developed countries. Here, we present an overview of the current scientific knowledge of the impacts of contamination on the biota of coastal ecosystems of Brazil and evaluate the scientific challenges to provide baseline information for local managerial purposes. We compiled data from 323 peer-reviewed published papers from the extensive Brazilian coast. We critically evaluated the produced knowledge (target contaminants, sources, ecosystems, taxa, response variables) and the science behind it (rigour and setting) within its socioenvironmental context (land occupation, use of the coast, sanitation status, contamination history). Research was driven largely by environmental outcomes of industrial development with a focus on the single effects of metals on the biota. The current knowledge derives mainly from laboratory manipulative experiments or from correlative field studies of changes in the biota with varying levels of contamination. Of these, 70% had problems in their experimental design. Environmental impacts have mainly been assessed using standard indicators of populations, mostly in ecotoxicological studies. Benthic assemblages have mostly been studied using structural indicators in field studies. Future assessments of impacts should expand research to more taxonomic groups and ecosystem compartments, adding combined functional and structural responses. Furthermore, further investigations need to consider the interactive effects of contaminants and other environmental stressors. By doing so, researchers would deliver more robust and effective results to solve problems of pollution.

Coral reef biofilm bacterial diversity and successional trajectories are structured by reef benthic organisms and shift under chronic nutrient enrichment

Work on marine biofilms has primarily focused on host-associated habitats for their roles in larval recruitment and disease dynamics; little is known about the factors regulating the composition of reef environmental biofilms. To contrast the roles of succession, benthic communities and nutrients in structuring marine biofilms, we surveyed bacteria communities in biofilms through a six-week succession in aquaria containing macroalgae, coral, or reef sand factorially crossed with three levels of continuous nutrient enrichment. Our findings demonstrate how biofilm successional trajectories diverge from temporal dynamics of the bacterioplankton and how biofilms are structured by the surrounding benthic organisms and nutrient enrichment. We identify a suite of biofilm-associated bacteria linked with the orthogonal influences of corals, algae and nutrients and distinct from the overlying water. Our results provide a comprehensive characterization of marine biofilm successional dynamics and contextualize the impact of widespread changes in reef community composition and nutrient pollution on biofilm community structure.

Eutrophication on Coral Reefs: What Is the Evidence for Phase Shifts, Nutrient Limitation and Coral Bleaching

Coral reefs continue to experience extreme environmental pressure from climate change stressors, but many coral reefs are also exposed to eutrophication. It has been proposed that changes in the stoichiometry of ambient nutrients increase the mortality of corals, whereas eutrophication may facilitate phase shifts to macroalgae-dominated coral reefs when herbivory is low or absent. But are corals ever nutrient limited, and can eutrophication destabilize the coral symbiosis making it more sensitive to environmental stress because of climate change? The effects of eutrophication are confounded not just by the effects of climate change but by the presence of chemical pollutants in industrial, urban, and agricultural wastes. Because of these confounding effects, the increases in nutrients or changes in their stoichiometry in coastal environments, although they are important at the organismal and community level, cannot currently be disentangled from each other or from the more significant effects of climate change stressors on coral reefs.

Asymmetric physiological response of a reef-building coral to pulsed versus continuous addition of inorganic nutrients

Coral reefs, especially those located near-shore, are increasingly exposed to anthropogenic, eutrophic conditions that are often chronic. Yet, corals under unperturbed conditions may frequently receive natural and usually temporary nutrient supplementation through biological sources such as fishes. We compared physiological parameters indicative of long- and short-term coral health (day and night calcification, fragment surface area, productivity, energy reserves, and tissue stoichiometry) under continuous and temporary nutrient enrichment. The symbiotic coral Acropora intermedia was grown for 7 weeks under continuously elevated (press) levels of ammonium (14 µmol L^-1) and phosphate (10 µmol L^-1) as separate and combined treatments, to discern the individual and interactive nutrient effects. Another treatment exposed A. intermedia twice-daily to an ammonium and phosphate pulse of the same concentrations as the press treatments to simulate natural biotic supplementation. Press exposure to elevated ammonium or phosphate produced mixed effects on physiological responses, with little interaction between the nutrients in the combined treatment. Overall, corals under press exposure transitioned resources away from calcification. However, exposure to nutrient pulses often enhanced physiological responses. Our findings indicate that while continuous nutrient enrichment may pose a threat to coral health, episodic nutrient pulses that resemble natural nutrient supplementation may significantly benefit coral health and physiology.

Experimental Techniques to Assess Coral Physiology in situ Under Global and Local Stressors: Current Approaches and Novel Insights

Coral reefs are declining worldwide due to global changes in the marine environment. The increasing frequency of massive bleaching events in the tropics is highlighting the need to better understand the stages of coral physiological responses to extreme conditions. Moreover, like many other coastal regions, coral reef ecosystems are facing additional localized anthropogenic stressors such as nutrient loading, increased turbidity, and coastal development. Different strategies have been developed to measure the health status of a damaged reef, ranging from the resolution of individual polyps to the entire coral community, but techniques for measuring coral physiology in situ are not yet widely implemented. For instance, while there are many studies of the coral holobiont response in single or limited-number multiple stressor experiments, they provide only partial insights into metabolic performance under more complex and temporally and spatially variable natural conditions. Here, we discuss the current status of coral reefs and their global and local stressors in the context of experimental techniques that measure core processes in coral metabolism (respiration, photosynthesis, and biocalcification) in situ, and their role in indicating the health status of colonies and communities. We highlight the need to improve the capability of in situ studies in order to better understand the resilience and stress response of corals under multiple global and local scale stressors.

Potential Pitfalls in Wastewater Phosphorus Analysis and How to Avoid Them

Due to the increasing adoption of nutrient discharge regulations, many research groups are stepping into new territory with phosphorus (P) measurements. Accurate reporting of P concentrations in effluent from novel wastewater treatment technologies is critical for protecting both environmental and human health. Analysis of P in wastewater is prone to pitfalls because of the (1) variety of chemical forms of P in wastewater (orthophosphate, condensed P, and organic P), (2) availability of different chemical assays for measuring different P forms, and (3) different conventions in the units for reporting P. Here, we present a case study highlighting how these pitfalls affect analysis and interpretation of P measurements. We show that, when used appropriately, commercially-available kits are indeed accurate tools for evaluating reactive P and total P concentrations. For both standard solutions and real wastewater, we systematically remove steps from the total P protocol to show how protocol deviations affect the results. While standard solutions are important for validating analytical methods, commercially-available wastewater standard solutions only contain P as orthophosphate (reactive P). We therefore demonstrate options for making a mixed-P standard solution containing acid-hydrolyzable and/or organic P compounds that can be used to validate both reactive P and total P assays.

Genetic structure of a remnant Acropora cervicornis population

Amongst the global decline of coral reefs, hope spots such as Cordelia Bank in Honduras, have been identified. This site contains dense, remnant thickets of the endangered species Acropora cervicornis, which local managers and conservation organizations view as a potential source population for coral restoration projects. The aim of this study was to determine the genetic diversity of colonies across three banks within the protected area. We identified low genetic diversity (F_ST = 0.02) across the three banks, and genetic similarity of colonies ranged from 91.3 to 95.8% between the banks. Clonality rates were approximately 30% across the three banks, however, each genotype identified was unique to each bank. Despite the low genetic diversity, subtle genetic differences within and among banks were demonstrated, and these dense thickets were shown not to be comprised of a single or a few genotypes. The presence of multiple genotypes suggests A. cervicornis colonies from these banks could be used to maintain and enhance genetic diversity in restoration projects. Management of hope spots, such as Cordelia Bank, and the incorporation of genetic information into restoration projects to ensure genetic diversity within out-planted populations, will be critical in the ongoing challenge of conserving and preserving coral reefs.

Neurotoxicity and physiological stress in brain of zebrafish chronically exposed to tributyltin

Tributyltin (TBT), an organotin compound, is hazardous in aquatic ecosystems. However, the mechanisms underlying TBT-induced central nervous system (CNS) toxicity remain to be determined especially in freshwater aquatic vertebrates. The aim of present study was to investigate the effects of chronic exposure to TBT on brain functions in a freshwater teleost the adult wild-type zebrafish (Danio rerio). Fish were exposed to sublethal concentrations of TBT (10, 100 or 300 ng/L) for 6 weeks. The influence of long-term TBT exposure was assessed in the brain of zebrafish with antioxidant related indices including malondialdehyde (MDA) levels and total antioxidant capacity, neurological parameters such as activities of acetylcholinesterase, and monoamine oxidase as well as levels of nitric oxide, dopamine, 5-hydroxytryptamine. In addition indices related to sensitivity of toxic insult such as cytochrome P450 1 regulation and heat shock protein 70 were determined. The regulation of related genes involved in endoplasmic reticulum stress (ERS), apoptosis and Nrf2 pathway were measured. Adverse physiological and biochemical responses were significantly enhanced in a concentration-dependent manner reflecting neurotoxicity attributed to TBT exposure. Our findings provide further insight into TBT-induced toxicity in wild-type zebrafish. and enhance our understanding of the molecular mechanisms underlying TBT-initiated CNS effects.

Submarine groundwater discharge alters coral reef ecosystem metabolism

Submarine groundwater discharge (SGD) influences near-shore coral reef ecosystems worldwide. SGD biogeochemistry is distinct, typically with higher nutrients, lower pH, cooler temperature and lower salinity than receiving waters. SGD can also be a conduit for anthropogenic nutrients and other pollutants. Using Bayesian structural equation modelling, we investigate pathways and feedbacks by which SGD influences coral reef ecosystem metabolism at two Hawai'i sites with distinct aquifer chemistry. The thermal and biogeochemical environment created by SGD changed net ecosystem production (NEP) and net ecosystem calcification (NEC). NEP showed a nonlinear relationship with SGD-enhanced nutrients: high fluxes of moderately enriched SGD (Wailupe low tide) and low fluxes of highly enriched SGD (Kūpikipiki'ō high tide) increased NEP, but high fluxes of highly enriched SGD (Kūpikipiki'ō low tide) decreased NEP, indicating a shift toward microbial respiration. pH fluctuated with NEP, driving changes in the net growth of calcifiers (NEC). SGD enhances biological feedbacks: changes in SGD from land use and climate change will have consequences for calcification of coral reef communities, and thereby shoreline protection.

Nutrient and sediment loading affect multiple facets of functionality in a tropical branching coral

Coral reefs, one of the most diverse ecosystems in the world, face increasing pressures from global and local anthropogenic stressors. Therefore, a better understanding of the ecological ramifications of warming and land-based inputs (e.g. sedimentation and nutrient loading) on coral reef ecosystems is necessary. In this study, we measured how a natural nutrient and sedimentation gradient affected multiple facets of coral functionality, including endosymbiont and coral host response variables, holobiont metabolic responses and percent cover of Pocillopora acuta colonies in Mo'orea, French Polynesia. We used thermal performance curves to quantify the relationship between metabolic rates and temperature along the environmental gradient. We found that algal endosymbiont percent nitrogen content, endosymbiont densities and total chlorophyll a content increased with nutrient input, while endosymbiont nitrogen content per cell decreased, likely representing competition among the algal endosymbionts. Nutrient and sediment loading decreased coral metabolic responses to thermal stress in terms of their thermal performance and metabolic rate processes. The acute thermal optimum for dark respiration decreased, along with the maximal performance for gross photosynthetic and calcification rates. Gross photosynthetic and calcification rates normalized to a reference temperature (26.8°C) decreased along the gradient. Lastly, percent cover of P. acuta colonies decreased by nearly two orders of magnitude along the nutrient gradient. These findings illustrate that nutrient and sediment loading affect multiple levels of coral functionality. Understanding how local-scale anthropogenic stressors influence the responses of corals to temperature can inform coral reef management, particularly in relation to the mediation of land-based inputs into coastal coral reef ecosystems.

Nitrogen eutrophication particularly promotes turf algae in coral reefs of the central Red Sea

While various sources increasingly release nutrients to the Red Sea, knowledge about their effects on benthic coral reef communities is scarce. Here, we provide the first comparative assessment of the response of all major benthic groups (hard and soft corals, turf algae and reef sands—together accounting for 80% of the benthic reef community) to in-situ eutrophication in a central Red Sea coral reef. For 8 weeks, dissolved inorganic nitrogen (DIN) concentrations were experimentally increased 3-fold above environmental background concentrations around natural benthic reef communities using a slow release fertilizer with 15% total nitrogen (N) content. We investigated which major functional groups took up the available N, and how this changed organic carbon (C_org) and N contents using elemental and stable isotope measurements. Findings revealed that hard corals (in their tissue), soft corals and turf algae incorporated fertilizer N as indicated by significant increases in δ¹⁵N by 8%, 27% and 28%, respectively. Among the investigated groups, C_org content significantly increased in sediments (+24%) and in turf algae (+33%). Altogether, this suggests that among the benthic organisms only turf algae were limited by N availability and thus benefited most from N addition. Thereby, based on higher C_org content, turf algae potentially gained competitive advantage over, for example, hard corals. Local management should, thus, particularly address DIN eutrophication by coastal development and consider the role of turf algae as potential bioindicator for eutrophication.

Diel, daily, and spatial variation of coral reef seawater microbial communities

Reef organisms influence microorganisms within the surrounding seawater, yet the spatial and temporal dynamics of seawater microbial communities located in proximity to corals are rarely investigated. To better understand reef seawater microbial community dynamics over time and space, we collected small-volume seawater samples during the day and night over a 72 hour period from three locations that differed in spatial distance from 5 Porites astreoides coral colonies on a shallow reef in St. John, U.S. Virgin Islands: near-coral (sampled 5 cm horizontally from each colony), reef-depth (sampled 2 m above each colony) and surface seawater (sampled 1 m from the seawater surface). At all time points and locations, we quantified abundances of microbial cells, sequenced small subunit rRNA genes of bacterial and archaeal communities, and measured inorganic nutrient concentrations. Prochlorococcus and Synechococcus cells were consistently elevated at night compared to day and these abundances changed over time, corresponding with temperature, nitrite, and silicate concentrations. During the day, bacterial and archaeal alpha diversity was significantly higher in reef-depth and near-coral seawater compared to the surface seawater, signifying that the reef influences the diversity of the seawater microorganisms. At night, alpha diversity decreased across all samples, suggesting that photosynthesis may favor a more taxonomically diverse community. While Prochlorococcus exhibited consistent temporal rhythmicity, additional taxa were enriched in reef seawater at night compared to day or in reef-depth compared to surface seawater based on their normalized sequence counts. There were some significant differences in nutrient concentrations and cell abundances between reef-depth and near-coral seawater but no clear trends. This study demonstrates that temporal variation supersedes small-scale spatial variation in proximity to corals in reef seawater microbial communities. As coral reefs continue to change in benthic composition worldwide, monitoring microbial composition in response to temporal changes and environmental fluctuations will help discern normal variability from longer lasting changes attributed to anthropogenic stressors and global climate change.

Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape

Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that heterogeneity in nitrogen pollution can shape spatial patterns of coral bleaching across a seascape. Using island-wide surveys of coral bleaching and nitrogen availability within a Bayesian hierarchical modeling framework, we tested the hypothesis that excess nitrogen interacts with temperature anomalies to alter coral bleaching for the two dominant genera of branching corals in Moorea, French Polynesia. For both coral genera, Pocillopora and Acropora, heat stress primarily drove bleaching prevalence (i.e., the proportion of colonies on a reef that bleached). In contrast, the severity of bleaching (i.e., the proportion of an individual colony that bleached) was positively associated with both heat stress and nitrogen availability for both genera. Importantly, nitrogen interacted with heat stress to increase bleaching severity up to twofold when nitrogen was high and heat stress was relatively low. Our finding that excess nitrogen can trigger severe bleaching even under relatively low heat stress implies that mitigating nutrient pollution may enhance the resilience of coral communities in the face of mounting stresses from global climate change.

High heritability of coral calcification rates and evolutionary potential under ocean acidification

Estimates of heritability inform evolutionary potential and the likely outcome of many management actions, but such estimates remain scarce for marine organisms. Here, we report high heritability of calcification rate among the eight most dominant Hawaiian coral species under reduced pH simulating future ocean conditions. Coral colonies were sampled from up to six locations across a natural mosaic in seawater chemistry throughout Hawai'i and fragmented into clonal replicates maintained under both ambient and high pCO2 conditions. Broad sense heritability of calcification rates was high among all eight species, ranging from a low of 0.32 in Porites evermanni to a high of 0.61 in Porites compressa. The overall results were inconsistent with short-term acclimatization to the local environment or adaptation to the mean or ideal conditions. Similarly, in 'local vs. foreign' and 'home vs. away' tests there was no clear signature of local adaptation. Instead, the data are most consistent with a protected polymorphism as the mechanism which maintains differential pH tolerance within the populations. Substantial individual variation, coupled with high heritability and large population sizes, imply considerable scope for natural selection and adaptive capacity, which has major implications for evolutionary potential and management of corals in response to climate change.

Extreme spatial heterogeneity in carbonate accretion potential on a Caribbean fringing reef linked to local human disturbance gradients

The capacity of coral reefs to maintain their structurally complex frameworks and to retain the potential for vertical accretion is vitally important to the persistence of their ecological functioning and the ecosystem services they sustain. However, datasets to support detailed along-coast assessments of framework production rates and accretion potential do not presently exist. Here, we estimate, based on gross bioaccretion and bioerosion measures, the carbonate budgets and resultant estimated accretion rates (EAR) of the shallow reef zone of leeward Bonaire - between 5 and 12 m depth - at unique fine spatial resolution along this coast (115 sites). Whilst the fringing reef of Bonaire is often reported to be in a better ecological condition than most sites throughout the wider Caribbean region, our data show that the carbonate budgets of the reefs and derived EAR varied considerably across this ~58 km long fringing reef complex. Some areas, in particular the marine reserves, were indeed still dominated by structurally complex coral communities with high net carbonate production (>10 kg CaCO₃  m^-2  year^-1 ), high live coral cover and complex structural topography. The majority of the studied sites, however, were defined by relatively low budget states (<2 kg CaCO₃  m^-2  year^-1 ) or were in a state of net erosion. These data highlight the marked spatial heterogeneity that can occur in budget states, and thus in reef accretion potential, even between quite closely spaced areas of individual reef complexes. This heterogeneity is linked strongly to the degree of localized land-based impacts along the coast, and resultant differences in the abundance of reef framework building coral species. The major impact of this variability is that those sections of reef defined by low-accretion rates will have limited capacity to maintain their structural integrity and to keep pace with current projections of climate change induced sea-level rise (SLR), thus posing a threat to reef functioning and biodiversity, potentially leading to trophic cascades. Since many Caribbean reefs are more severely degraded than those found around Bonaire, it is to be expected that the findings presented here are rather the rule than the exception, but the study also highlights the need for similar high spatial resolution (along-coast) assessments of budget states and accretion rates to meaningfully explore increasing coastal risk at the country level. The findings also more generally underline the significance of reducing local anthropogenic disturbance and restoring framework building coral assemblages. Appropriately focussed local preservation efforts may aid in averting future large-scale above reef water depth increases on Caribbean coral reefs and will limit the social and economic implications associated with the loss of reef goods and services.

Functional responses of filter feeders increase with elevated metal contamination: Are these good or bad signs of environmental health?

Fast urbanization in coastal areas has increased the load of contaminants entering estuaries worldwide, threatening the diversity and provision of services by these important systems. Contamination causes structural changes in ecosystems, but the consequences for their functioning are still overlooked. Here we investigated filtration and biodeposition rates of the mussel Mytilaster solisianus across different concentrations of metals, nutrients and suspended material, and levels of urbanization. As expected, filtration rates increased with the number of particles in the water column. However, in areas with low particle concentration, filtering increased in mussels with higher metal concentrations (Cu/Zn/Ni), which were, in turn, related to high urbanization. Similarly, biodeposition rates were positively related to metal concentration in mussels. The increased functional responses observed here is likely a symptom of stress, caused by potential compensatory mechanisms to the energetic costs of cell maintenance and body detoxification of mussels, rather than an indication of healthy systems/organisms. CAPSULE: Increased functional responses of mussels can be a sign of environmental stress.

Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery

Ocean warming and the increased prevalence of coral bleaching events threaten coral reefs. However, the biology of corals during and following bleaching events under field conditions is poorly understood. We examined bleaching and postbleaching recovery in Montipora capitata and Porites compressa corals that either bleached or did not bleach during a 2014 bleaching event at three reef locations in Kāne'ohe Bay, O'ahu, Hawai'i. We measured changes in chlorophylls, tissue biomass, and nutritional plasticity using stable isotopes (δ ¹³C, δ ¹⁵N). Coral traits showed significant variation among periods, sites, bleaching conditions, and their interactions. Bleached colonies of both species had lower chlorophyll and total biomass, and while M. capitata chlorophyll and biomass recovered 3 months later, P. compressa chlorophyll recovery was location dependent and total biomass of previously bleached colonies remained low. Biomass energy reserves were not affected by bleaching, instead M. capitata proteins and P. compressa biomass energy and lipids declined over time and P. compressa lipids were site specific during bleaching recovery. Stable isotope analyses did not indicate increased heterotrophic nutrition in bleached colonies of either species, during or after thermal stress. Instead, mass balance calculations revealed that variations in δ ¹³C values reflect biomass compositional change (i.e., protein : lipid : carbohydrate ratios). Observed δ ¹⁵N values reflected spatiotemporal variability in nitrogen sources in both species and bleaching effects on symbiont nitrogen demand in P. compressa. These results highlight the dynamic responses of corals to natural bleaching and recovery and identify the need to consider the influence of biomass composition in the interpretation of isotopic values in corals.

Adaptable mesocosm facility to study oil spill impacts on corals

Although numerous studies have been carried out on the impacts of oil spills on coral physiology, most have relied on laboratory assays. This scarcity is partly explained by the difficulty of reproducing realistic conditions in a laboratory setting or of performing experiments with toxic compounds in the field. Mesocosm systems provide the opportunity to carry out such studies with safe handling of contaminants while reproducing natural conditions required by living organisms. The mesocosm design is crucial and can lead to the development of innovative technologies to mitigate environmental impacts. Therefore, this study aimed to develop a mesocosm system for studies simulating oil spills with several key advantages, including true replication and the use of gravity to control flow-through that reduces reliance on pumps that can clog thereby decreasing errors and costs. This adaptable system can be configured to (a) have continuous flow-through; (b) operate as an open or closed system; (c) be fed by gravity; (d) have separate mesocosm sections that can be used for individual and simultaneous experiments; and (e) simulate the migration of oil from ocean oil spills to the nearby reefs. The mesocosm performance was assessed with two experiments using the hydrocoral Millepora alcicornis and different configurations to simulate two magnitudes of oil spills. With few exceptions, physical and chemical parameters remained stable within replicates and within treatments throughout the experiments. Physical and chemical parameters that expressed change during the experiment were still within the range of natural conditions observed in Brazilian marine environments. The photosynthetic potential (Fv/Fm ) of the algae associated with M. alcicornis decreased in response to an 1% crude-oil contamination, suggesting a successful delivery of the toxic contaminant to the targeted replicates. This mesocosm is customizable and adjustable for several types of experiments and proved to be effective for studies of oil spills.

Nutrient pollution disrupts key ecosystem functions on coral reefs

There is a long history of examining the impacts of nutrient pollution and pH on coral reefs. However, little is known about how these two stressors interact and influence coral reef ecosystem functioning. Using a six-week nutrient addition experiment, we measured the impact of elevated nitrate (NO^-₃) and phosphate (PO^3-₄) on net community calcification (NCC) and net community production (NCP) rates of individual taxa and combined reef communities. Our study had four major outcomes: (i) NCC rates declined in response to nutrient addition in all substrate types, (ii) the mixed community switched from net calcification to net dissolution under medium and high nutrient conditions, (iii) nutrients augmented pH variability through modified photosynthesis and respiration rates, and (iv) nutrients disrupted the relationship between NCC and aragonite saturation state documented in ambient conditions. These results indicate that the negative effect of NO^-₃ and PO^3-₄ addition on reef calcification is likely both a direct physiological response to nutrients and also an indirect response to a shifting pH environment from altered NCP rates. Here, we show that nutrient pollution could make reefs more vulnerable to global changes associated with ocean acidification and accelerate the predicted shift from net accretion to net erosion.