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