DOI: 10.5061/dryad.1ns1rn8ww
title: Nocturnal dissolved organic matter release by turf algae and its role in the microbialization of reefs

description: The increased release of dissolved organic matter (DOM) by algae has been
 associated with the fast but inefficient growth of opportunistic microbial
 pathogens and the ongoing degradation of coral reefs. Turf algae
 (consortia of micro- and macroalgae commonly including cyanobacteria)
 dominate benthic communities on many reefs worldwide. Opposite to other
 reef algae that predominantly release DOM during the day, turf algae
 containing cyanobacteria may additionally release large amounts of DOM at
 night. However, this night-DOM release and its potential contribution to
 the microbialization of reefs remains to be investigated. We first tested
 the occurrence of hypoxic conditions at the turf algae-water interface, as
 a lack of oxygen will facilitate the production and release of
 fermentation intermediates as night-time DOM. Secondly, the dissolved
 organic carbon (DOC) release by turf algae was quantified during day- and
 nighttime, and the quality of day and night exudates as food for
 bacterioplankton was tested. Finally, DOC release rates of turf algae were
 combined with estimates of DOC release based on benthic community
 composition in 1973 and 2013 to explore how changes in benthic community
 composition affected the contribution of night-DOC to the reef-wide DOC
 production. A rapid shift from supersaturated to hypoxic conditions at the
 turf algae-water interface occurred immediately after the onset of
 darkness, resulting in night-DOC release rates similar to those during
 daytime. Bioassays revealed major differences in the quality between day
 and night exudates: Night-DOC was utilized by bacterioplankton two times
 faster than day-DOC, but yielded a four-times lower growth efficiency.
 Changes in benthic community composition were estimated to have resulted
 in a doubling of DOC release since 1973, due to an increasing abundance of
 benthic cyanobacterial mats (BCMs), with night-DOC release by BCMs and
 turf algae accounting for &gt;50 % of the total release over a diurnal
 cycle.  Night-DOC released by BCMs and turf algae is likely an
 important driver in the microbialization of reefs by stimulating microbial
 respiration at the expense of energy and nutrient transfer to higher
 trophic levels via the microbial loop and thereby threatens the
 productivity and biodiversity of these unique ecosystems.
description: <strong>Dissolved organic and inorganic nutrients for daytime and nighttime incubations with turf algae.</strong> Dissolved organic carbon (DOC), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and surface area recovered with turf algae.   <strong>Dissolved organic and inorganic nutrients for bioassays with daytime and nighttime exudates of turf algae.</strong> Dissolved organic carbon (DOC), total dissolved nitrogen (TDN), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and bacterial concentrations.   <strong>Dissolved oxygen concentrations recorded during daytime incubations with turf algae.</strong> μmol O2 L<sup>-1</sup>   <strong>Light intensities recorded outside of flow chambers during daytime incubations with turf algae.</strong> Lux