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

Seabird-derived nitrogen supply enhances photosynthetic activity in a reef-building coral

Nutrient subsidies from seabirds (SDN) significantly impact coral reef ecosystems, but access to remote seabird nesting islands limits our understanding of how corals respond to this nutrient source. This study investigates the in natura effects of SDN on coral metabolism at Surprise Island in the southwestern Pacific. Specifically, we examine how the photobiology of the ubiquitous reef-building coral Pocillopora damicornis responds to SDN availability. On Surprise Island, seabird-derived nitrogen follows a well-defined gradient across the land-ocean continuum and significantly contributes to scleractinian coral nitrogen uptake. At stations exposed to SDN discharge, seawater exhibited an elevated N:P ratio due to high nitrate concentrations as phosphate and ammonium concentrations remained similar to those at the reference station. Corals exposed to SDN-enriched coastal waters displayed a 50 % increase in photosynthetic efficiency and a 40 % increase in photosynthetic rate, coupled with a 115 % increase in both symbiont density and chlorophyll concentrations. These findings demonstrate that nitrogen inputs from seabirds stimulate the photosynthetic activity of coral symbionts, highlighting the sensitivity of coral photophysiology to SDN. This underscores the critical link between seabird ecology and coral reefs functioning and emphasizes the need for integrated conservation efforts on coral islands.

Nutrition-induced changes in the microbiota can cause dysbiosis and disease development

Eukaryotic organisms are associated with complex microbial communities. Changes within these communities have been implicated in disease development. Nonetheless, it remains unclear whether these changes are a cause or a consequence of disease. Here, we report a causal link between environment-induced shifts in the microbiota and disease development. Using the model organism Hydra, we observed changes in microbial composition when transferring laboratory-grown Hydra to natural lake environments. These shifts were caused not only by new colonizers, through the process of community coalescence (merging of previously separate microbial communities), but also by lake water nutrients. Moreover, selective manipulation of the nutrient environment induced compound-specific shifts in the microbiota followed by disease development. Finally, L-arginine supplementation alone caused a transition in Pseudomonas from symbiotic to pathogenic, leading to an upregulation of immune response genes, tissue degradation, and host death. These findings challenge the notion that the host-associated microbiota is exclusively controlled by the host, highlighting the dynamic interplay between host epithelial environment, microbial colonizer pool, and nutrient conditions of the surrounding water. Furthermore, our results show that overfeeding of the microbiota allows for uncontrolled microbial growth and versatile interactions with the host. Environmental conditions may thus render symbionts a potential hazard to their hosts, blurring the divide between pathogenic and non-pathogenic microbes.IMPORTANCEThis study highlights the critical need to understand the dynamic interplay between host-associated microbiota and environmental factors to obtain a holistic view on organismal health. Our results demonstrate that ecosystem-wide microbial trafficking (community coalescence) and environmental nutrient conditions reshape microbial communities with profound implications for host health. By exploring nutrient-driven changes in microbial composition, our research finds experimental support for the "overfeeding hypothesis," which states that overfeeding alters the functionality of the host microbiota such that an overabundance in nutrients can facilitate disease development, transforming non-pathogenic microbes into pathogens. These findings emphasize the critical role of metabolic interactions driving microbial pathogenicity. Furthermore, our research provides empirical evidence for the "pathogenic potential" concept, challenging traditional distinctions between pathogenic and non-pathogenic microbes and supporting the idea that any microbe can become pathogenic under certain conditions.

High-resolution temporal assessment of physicochemical variability and water quality in tropical semi-enclosed bays and coral reefs

Tropical coastlines featuring mangrove, seagrass, and coral habitats are of immense ecological and socio-economic importance, supporting biodiversity, carbon storage, coastal protection, fisheries, and tourism. However, climate change, coastal development, and low water quality increasingly threaten these interconnected coastal ecosystems, particularly in semi-enclosed bays where the impacts of these stressors are often amplified. Yet, physicochemical conditions are rarely assessed at sufficient temporal resolution (i.e., diel and seasonal variation) and time-integrated pollution monitoring is rarely performed. Here, we used a multi-disciplinary approach to assess >20 abiotic parameters characterizing two mangrove- and seagrass-dominated inland bays and two nearby coral reefs in Curaçao (southern Caribbean) during the cool, dry season and warm, wet season. This was combined with time-integrated pollution monitoring using bioindicators to assess nutrients and trace metal pollution (inland bays only), and passive samplers and bioassays to assess organic chemical pollution (all four sites) during the wet season. This approach revealed a previously undocumented extent of strong diel and seasonal environmental variability in Curaçao's inland bays, with temperature, pH, and dissolved oxygen frequently reaching values predicted under moderate-to-severe future climate scenarios as outlined by the IPCC (2021). In addition, the inland bays had greater nutrient concentrations (especially ammonium) and potential ecotoxicological risks than the nearby reefs during the wet season due to run-off and anthropogenic activities. These findings emphasize the importance of high-resolution monitoring to understand risks across appropriate temporal scales and establish an environmental baseline against which future monitoring can be benchmarked. Moreover, our study provides a robust water quality assessment framework that can be used by natural resource managers to monitor reef-associated habitats and conserve their high ecological and socio-economic value. Overall, our work highlights the urgent need to improve monitoring, water quality, and protection of these valuable reef-associated habitats.

Anthropogenic Impacts on Coral-Algal Interactions of the Subtropical Lagoonal Reef, Norfolk Island

Reef building corals are important in subtropical marine ecoregions, shaping ecosystems and providing habitats for fish and benthic species. Algal communities contribute substantially to the benthic population structure across subtropical coral reefs, however increasing algal cover on subtropical reefs is also linked to degraded ecosystems as has been shown on tropical systems. As such, the dynamics of coral-algal interactions on subtropical reefs are also likely to be an indicator of ecosystem health on subtropical ecosystems. The subtropical lagoonal coral reef of Norfolk Island within the Norfolk Marine Park has been impacted by a regime of disturbance since 2020 including flooding, sedimentation, and heat stress events. Assessing the type and extent of algal interactions with the dominant coral Pocillopora damiconis within the reef sites of Emily Bay, Slaughter Bay, and Cemetery Bay has the potential to provide insight into drivers of ecosystem decline within the reef. Similarly, photochemical efficiency, as measured by yield (Fv/Fm) using pulse amplitude modulated fluorometry, can be used to provide a measure of the health of corals on reefs during degradation events. Here we assess the extent of coral-algal interactions and health of colonies of P. damicornis prior to the onset of summertime conditions (April 2023) and during summertime conditions (December 2023). Seasonal and within site dynamics of coral-algal interactions were evident by a significant bloom of red cyanobacteria (P < 0.0001, April 2023) and Lyngbya {P < 0.01 [Slaughter Bay West (SBW)], P < 0.01 [Slaughter Bay East (SBE)], December 2023}. Within reef, variability of coral-algal interactions was most evident for Lyngbya, and on the Norfolk reef, interactions of Lyngbya with P. damincornis were found to be significantly higher at slaughter bay west (SBW 30.2% of interactions) and east (SBE 24.6% of interactions) in December 2023 than at neighboring Emily (11.6% of interactions) and Cemetery Bay (0.6% of interactions). Pulse Amplitude Modulated (PAM) fluorometry also highlighted the potential for algal interactions to influence the photochemical efficiency of Pocillopora damicornis. Benthic structure, as measured by coral-algal interactions, and coral health within the Norfolk lagoonal, both highlight the potential for anthropogenic drivers of reef decline to influence the health of the ecosystem. Further investigation is therefore necessary to elucidate the specific causes and consequences of algae linked to poor water quality, such as red cyanobacteria and Lyngbya, interacting with corals.

Impact of Nutrient Enrichment on Community Structure and Co-Occurrence Networks of Coral Symbiotic Microbiota in Duncanopsammia peltata: Zooxanthellae, Bacteria, and Archaea

Symbiotic microorganisms in reef-building corals, including algae, bacteria, archaea, fungi, and viruses, play critical roles in the adaptation of coral hosts to adverse environmental conditions. However, their adaptation and functional relationships in nutrient-rich environments have yet to be fully explored. This study investigated Duncanopsammia peltata and the surrounding seawater and sediments from protected and non-protected areas in the summer and winter in Dongshan Bay. High-throughput sequencing was used to characterize community changes, co-occurrence patterns, and factors influencing symbiotic coral microorganisms (zooxanthellae, bacteria, and archaea) in different environments. The results showed that nutrient enrichment in the protected and non-protected areas was the greatest in December, followed by the non-protected area in August. In contrast, the August protected area had the lowest nutrient enrichment. Significant differences were found in the composition of the bacterial and archaeal communities in seawater and sediments from different regions. Among the coral symbiotic microorganisms, the main dominant species of zooxanthellae is the C1 subspecies (42.22-56.35%). The dominant phyla of bacteria were Proteobacteria, Cyanobacteria, Firmicutes, and Bacteroidota. Only in the August protected area did a large number (41.98%) of SAR324_cladeMarine_group_B exist. The August protected and non-protected areas and December protected and non-protected areas contained beneficial bacteria as biomarkers. They were Nisaea, Spiroplasma, Endozoicomonas, and Bacillus. No pathogenic bacteria appeared in the protected area in August. The dominant phylum in Archaea was Crenarchaeota. These symbiotic coral microorganisms' relative abundances and compositions vary with environmental changes. The enrichment of dissolved inorganic nitrogen in environmental media is a key factor affecting the composition of coral microbial communities. Co-occurrence analysis showed that nutrient enrichment under anthropogenic disturbances enhanced the interactions between coral symbiotic microorganisms. These findings improve our understanding of the adaptations of coral holobionts to various nutritional environments.

Spatio-temporal distribution of sea surface chlorophyll-a in coral reefs of the South China Sea over the past decade based on Landsat-8 Operational Land Images

The concentration of chlorophyll-a (Chl-a) in seawater reflects phytoplankton growth and water eutrophication, which are usually assessed for evaluation of primary productivity and carbon source/sink of coral reefs. However, the precise delineation of Chl-a concentration in coral reefs remains a challenge when ocean satellites with low spatial resolution are utilized. In this study, a remote sensing inversion model for Chl-a was developed in fringing reefs (R2 = 0.76, RMSE =0.41 μg/L, MRE = 14 %) and atolls (R2 = 0.79, RMSE =0.02 μg/L, MRE = 8 %), utilizing reflectance data from the sensitive band of the Landsat-8 Operational Land Imagers (OLI) with a spatial resolution of 30 m. The aforementioned model was utilized to invert high-resolution distribution maps of Chl-a concentration in six major coral reef regions of the South China Sea from 2013 to 2022 and subsequently used to analyze the variations in Chl-a concentration and its influencing factors. The results indicate a Chl-a concentration gradient among coral reefs Daya Bay, Weizhou Island, Luhuitou, Xuwen, Huangyan Island, and Xisha Island in that order. The Chl-a concentration in coral reefs exhibited an overall increasing trend, with significant seasonal fluctuations, characterized by higher concentrations during winter and spring and lower concentrations during summer and autumn. The concentration of Chl-a in coral reefs was positively correlated with the average wind speed.

Coral Lipids

Reef-building corals, recognized as cornerstone species in marine ecosystems, captivate with their unique duality as both symbiotic partners and autotrophic entities. Beyond their ecological prominence, these corals produce a diverse array of secondary metabolites, many of which are poised to revolutionize the domains of pharmacology and medicine. This exhaustive review delves deeply into the multifaceted world of coral-derived lipids, highlighting both ubiquitous and rare forms. Within this spectrum, we navigate through a myriad of fatty acids and their acyl derivatives, encompassing waxes, sterol esters, triacylglycerols, mono-akyl-diacylglycerols, and an array of polar lipids such as betaine lipids, glycolipids, sphingolipids, phospholipids, and phosphonolipids. We offer a comprehensive exploration of the intricate biochemical variety of these lipids, related fatty acids, prostaglandins, and both cyclic and acyclic oxilipins. Additionally, the review provides insights into the chemotaxonomy of these compounds, illuminating the fatty acid synthesis routes inherent in corals. Of particular interest is the symbiotic bond many coral species nurture with dinoflagellates from the Symbiodinium group; their lipid and fatty acid profiles are also detailed in this discourse. This exploration accentuates the vast potential and intricacy of coral lipids and underscores their profound relevance in scientific endeavors.

Response of coral bacterial composition and function to water quality variations under anthropogenic influence

Microbial communities play key roles in the adaptation of corals living in adverse environments, as the microbiome flexibility can enhance environmental plasticity of coral holobiont. However, the ecological association of coral microbiome and related function to locally deteriorating water quality remains underexplored. In this work, we used 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) to investigate the seasonal changes of bacterial communities, particularly their functional genes related to carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) cycle, of the scleractinian coral Galaxea fascicularis from nearshore reefs exposed anthropogenic influence. We used nutrient concentrations as the indicator of anthropogenic activities in coastal reefs, and found a higher nutrient pressure in spring than summer. The bacterial diversity, community structure and dominant bacteria of coral shifted significantly due to seasonal variations dominated by nutrient concentrations. Additionally, the network structure and nutrient cycling gene profiles in summer under low nutrient stress was distinct from that under poor environmental conditions in spring, with lower network complexity and abundance of CNPS cycling genes in summer compared with spring. We further identified significant correlations between microbial community (taxonomic composition and co-occurrence network) and geochemical functions (abundance of multiple functional genes and functional community). Nutrient enrichment was proved to be the most important environmental fluctuation in controlling the diversity, community structure, interactional network and functional genes of the coral microbiome. These results highlight that seasonal shifts in coral-associated bacteria due to anthropogenic activities alter the functional potentials, and provide novel insight about the mechanisms of coral adaptation to locally deteriorating environments.

Corals at the edge of environmental limits: A new conceptual framework to re-define marginal and extreme coral communities

The worldwide decline of coral reefs has renewed interest in coral communities at the edge of environmental limits because they have the potential to serve as resilience hotspots and climate change refugia, and can provide insights into how coral reefs might function in future ocean conditions. These coral communities are often referred to as marginal or extreme but few definitions exist and usage of these terms has therefore been inconsistent. This creates significant challenges for categorising these often poorly studied communities and synthesising data across locations. Furthermore, this impedes our understanding of how coral communities can persist at the edge of their environmental limits and the lessons they provide for future coral reef survival. Here, we propose that marginal and extreme coral communities are related but distinct and provide a novel conceptual framework to redefine them. Specifically, we define coral reef extremeness solely based on environmental conditions (i.e., large deviations from optimal conditions in terms of mean and/or variance) and marginality solely based on ecological criteria (i.e., altered community composition and/or ecosystem functioning). This joint but independent assessment of environmental and ecological criteria is critical to avoid common pitfalls where coral communities existing outside the presumed optimal conditions for coral reef development are automatically considered inferior to coral reefs in more traditional settings. We further evaluate the differential potential of marginal and extreme coral communities to serve as natural laboratories, resilience hotspots and climate change refugia, and discuss strategies for their conservation and management as well as priorities for future research. Our new classification framework provides an important tool to improve our understanding of how corals can persist at the edge of their environmental limits and how we can leverage this knowledge to optimise strategies for coral reef conservation, restoration and management in a rapidly changing ocean.

Influence of upwelling on coral reef benthic communities: a systematic review and meta-analysis

Highly competitive coral reef benthic communities are acutely sensitive to changes in environmental parameters such as temperature and nutrient concentrations. Physical oceanographic processes that induce upwelling therefore act as drivers of community structure on tropical reefs. How upwelling impacts coral communities, however, is not fully understood; upwelling may provide a natural buffer against climate impacts and could potentially enhance the efficacy of spatial management and reef conservation efforts. This study employed a systematic review to assess existing literature linking upwelling with reef community structure, and a meta-analysis to quantify upwelling impact on the percentage cover of coral reef benthic groups. We show that upwelling has context-dependant effects on the cover of hard coral and fleshy macroalgae, with effect size and direction varying with depth, region and remoteness. Fleshy macroalgae were found to increase by 110% on inhabited reefs yet decrease by 56% around one well-studied remote island in response to upwelling. Hard coral cover was not significantly impacted by upwelling on inhabited reefs but increased by 150% when direct local human pressures were absent. By synthesizing existing evidence, this review facilitates adaptive and nuanced reef management which considers the influence of upwelling on reef assemblages.

Determining reclaimed water quality thresholds and farming practices to improve food crop yield: A meta-analysis combined with random forest model

Irrigated agricultural systems with reclaimed water (RW) play a crucial role in alleviating global water scarcity and increased food demand. However, appropriate reclaimed water quality thresholds and farming practices to improve food crop yield is virtually unclear. Therefore, for the first time, this study made a large compilation of previous studies using meta-analysis combined with a random forest (RF) model and analyzed the impact of RW versus freshwater (FW) on the yield of food crops (cereals, vegetables, and fruits). It was found that magnesium ion (Mg²⁺), calcium ion (Ca²⁺), electrical conductivity (EC), total nitrogen (TN), and potential of hydrogen (pH) were the most important factors for RW quality indicators. Based on the results, water managers should establish more conservative RW quality thresholds to promote food crop production, especially for salts and pollutants in RW. Compared to international water quality standards, it could be slightly relaxed the restrictions of TN in RW. The optimal farming practices obtained that irrigation amount of the mixed RW and FW (RW + FW) was from 1000 m³ ha^-1 to 5000 m³ ha^-1, and the cultivation period was no more than three years. Flood irrigation (FI) and drip irrigation (DI) for cereals were also recommended. Finally, a comparison of the determined results from this method with other scenarios published, finding a good agreement.