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) and phosphate (PO) 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 and PO 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.

Dissolved organic matter (DOM) composition influences microbial community metabolism and benthic primary producers are a source of DOM in coral reefs. As reef benthic communities change, in part due to nutrient pollution, understanding impacts on reef microbial processes requires knowledge of DOM sources and composition. We conducted a multi-week mesocosm experiment dosing four coral reef benthic constituents with three levels of nitrate and phosphate to contrast exudate composition and quantify the effects of nutrient enrichment on exudate release. Moderate nutrient enrichment enhanced bulk dissolved organic carbon exudation by all producers. Corals exuded rapidly accumulating DOM with a markedly high concentration of aromatic amino acid-like fluorescent DOM components that clearly distinguishes them from algal exudates, *Correspondence: craig.nelson@hawaii.edu Author Contribution Statement: ZAQ and CAC conducted the laboratory measurements. ZAQ analyzed the data and conducted all spectral modeling. KR, NJS, MJD, and CEN designed the experiments. KR, MDF, TAO, HMP, and CEN ran the experiment. All authors contributed to data interpretation and edited the manuscript. ZAQ and CEN wrote the paper and are accountable for the integrity of the data, analysis and presentation of findings as a whole.Data Availability Statement: All data and metadata from this experiment has been made publicly available via the US National Science Foundation Biological and Chemical Oceanography Data Management Office Dataset 723868 (https://www.bco-dmo.org/dataset/723868).Additional Supporting Information may be found in the online version of this article.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance StatementOn coral reefs, four primary groups of benthic organisms dominate photosynthetic production: corals, macroalgae, microphytobenthos, and encrusting algae on rubble--all of which exude significant quantities of dissolved organic matter (DOM). However, little is known about whether and how DOM exudates differ among these four groups and whether nutrient enrichment alters exudate quantity or composition. Our mesocosm experiment showed that nutrients stimulated exudation in all groups, but there were key differences in composition among the groups. Corals exuded DOM with characteristics that clearly distinguish them from algal exudates; coral exudates also accumulated throughout the experiment whereas algal exudates did not. Our results clarify a mechanism whereby anthropogenic activities that alter benthic cover and nutrient pollution on reefs may alter microbial communities and metabolism in reefs.1

Plant-associated microbes are critical players in host health, fitness and productivity. Despite microbes’ importance in plants, seeds are mostly sterile, and most plant microbes are recruited from an environmental pool. Surprisingly little is known about the processes that govern how environmental microbes assemble on plants in nature. In this study we examine how bacteria are distributed across plant parts, and how these distributions interact with spatial gradients. We sequenced amplicons of bacteria from the surfaces of six plant parts and adjacent soil of Scaevola taccada, a common beach shrub, along a 60 km transect spanning O’ahu island’s windward coast, as well as within a single intensively-sampled site. Bacteria are more strongly partitioned by plant part as compared with location. Within S. taccada plants, microbial communities are highly nested: soil and rhizosphere communities contain much of the diversity found elsewhere, whereas reproductive parts fall at the bottom of the nestedness hierarchy. Nestedness patterns suggest either that microbes follow a source/sink gradient from the ground up, or else that assembly processes correlate with other traits, such as tissue persistence, that are vertically stratified. Our work shines light on the origins and determinants of plant-associated microbes across plant and landscape scales.

Plant-associated microbes are critical players in host health, fitness and productivity. Despite microbes’ importance in plants, seeds are mostly sterile, and most plant microbes are recruited from an environmental pool. Surprisingly little is known about the processes that govern how environmental microbes assemble on plants in nature. In this study we examine how bacteria are distributed across plant parts, and how these distributions interact with spatial gradients. We sequenced amplicons of bacteria from six plant parts and adjacent soil of Scaevola taccada, a common beach shrub, along a 60 km transect spanning Oʻahu island’s windward coast, as well as within a single intensively-sampled site. Bacteria are more strongly partitioned by plant part as compared with location. Within S. taccada plants, microbial communities are highly nested: soil and rhizosphere communities contain much of the diversity found elsewhere, whereas reproductive parts fall at the bottom of the nestedness hierarchy. Nestedness patterns suggest either that microbes follow a source/sink gradient from the ground up, or else that assembly processes correlate with other traits, such as tissue persistence, that are vertically stratified. Our work shines light on the origins and determinants of plant-associated microbes across plant and landscape scales.

The effects of nutrient pollution on coral reef ecosystems are multifaceted. Numerous experiments have sought to identify the physiological effects of nutrient enrichment on reef‐building corals, but the results have been variable and sensitive to choices of nutrient quantity, chemical composition and exposure duration. To test the effects of chronic, ecologically relevant nutrient enrichment on coral growth and photophysiology, we conducted a 5‐week continuous dosing experiment on two Hawaiian coral species, Porites compressa and Pocillopora acuta. We acclimated coral fragments to five nutrient concentrations (0.1–7 µM NO3‐ and 0.06–2.24 µM PO43‐) with constant stoichiometry 2.5:1 nitrate to phosphate) bracketing in situ observations from reefs throughout the Pacific. Nutrient enrichment linearly increased photophysiological performance of both species within 3 weeks. The effect of nutrients on P. acuta photochemical efficiency increased through time while a consistent response in P. compressa indicated acclimation to elevated nutrients within 5 weeks. Endosymbiont densities and total chlorophyll concentrations also increased proportionally with nutrient enrichment in P. acuta, but not in P. compressa, revealing contrasting patterns of host–symbiont acclimatization. The two species also exhibited contrasting effects of nutrient enrichment on skeletal growth. Calcification was enhanced at low nutrient enrichment (1 µM NO3‐) in P. acuta, but comparable to the control at higher concentrations, whereas calcification was reduced in P. compressa (30%–35%) above 3 µM NO3‐. Stable isotope analysis revealed species‐specific nitrogen uptake dynamics in the coral–algal symbiosis. The endosymbionts of P. acuta exhibited increased nitrogen uptake (decreased δ15N) and incorporation (19%–31% decrease in C:N ratios) across treatments. In contrast, P. compressa endosymbionts maintained constant δ15N values and low levels of nitrogen incorporation (9%–11% decrease in C:N ratios). The inability of P. acuta to regulate endosymbiont nutrient uptake may indicate an emerging destabilization in the coral–algal symbiosis under nutrient enrichment that could compromise resistance to additional environmental stressors. Our results highlight species‐specific differences in the coral–algal symbiosis, which influence responses to chronic nutrient enrichment. These findings showcase how symbioses can vary among closely related taxa and underscore the importance of considering how life‐history traits modify species response to environmental change. A free Plain Language Summary can be found within the Supporting Information of this article.