Dissolved organic carbon (DOC)

Biological oxygen demand optode analysis of coral reef-associated microbial communities exposed to algal exudates

Algae-derived dissolved organic matter has been hypothesized to induce mortality of reef building corals. One proposed killing mechanism is a zone of hypoxia created by rapidly growing microbes. To investigate this hypothesis, biological oxygen demand (BOD) optodes were used to quantify the change in oxygen concentrations of microbial communities following exposure to exudates generated by turf algae and crustose coralline algae (CCA). BOD optodes were embedded with microbial communities cultured from Montastraea annularis and Mussismilia hispida, and respiration was measured during exposure to turf and CCA exudates. The oxygen concentrations along the optodes were visualized with a low-cost Submersible Oxygen Optode Recorder (SOOpR) system. With this system we observed that exposure to exudates derived from turf algae stimulated higher oxygen drawdown by the coral-associated bacteria than CCA exudates or seawater controls. Furthermore, in both turf and CCA exudate treatments, all microbial communities (coral-, algae-associated and pelagic) contributed significantly to the observed oxygen drawdown. This suggests that the driving factor for elevated oxygen consumption rates is the source of exudates rather than the initially introduced microbial community. Our results demonstrate that exudates from turf algae may contribute to hypoxia-induced coral stress in two different coral genera as a result of increased biological oxygen demand of the local microbial community. Additionally, the SOOpR system developed here can be applied to measure the BOD of any culturable microbe or microbial community.

 

Date
2013
Data type
Scientific article
Theme
Research and monitoring
Journal
Geographic location
Curacao

High dissolved organic carbon release by benthic cyanobacterial mats in a Caribbean reef ecosystem.

Abstract

Benthic cyanobacterial mats (BCMs) are increasing in abundance on coral reefs worldwide. However, their impacts on biogeochemical cycling in the surrounding water and sediment are virtually unknown. By measuring chemical fluxes in benthic chambers placed over sediment covered by BCMs and sediment with BCMs removed on coral reefs in Curaçao, Southern Caribbean, we found that sediment covered by BCMs released 1.4 and 3.5 mmol C m(-2) h(-1) of dissolved organic carbon (DOC) during day and night, respectively. Conversely, sediment with BCMs removed took up DOC, with day and night uptake rates of 0.9 and 0.6 mmol C m(-2) h(-1). DOC release by BCMs was higher than reported rates for benthic algae (turf and macroalgae) and was estimated to represent 79% of the total DOC released over a 24 h diel cycle at our study site. The high nocturnal release of DOC by BCMs is most likely the result of anaerobic metabolism and degradation processes, as shown by high respiration rates at the mat surface during nighttime. We conclude that BCMs are significant sources of DOC. Their increased abundance on coral reefs will lead to increased DOC release into the water column, which is likely to have negative implications for reef health.

 

Date
2015
Data type
Scientific article
Theme
Research and monitoring
Geographic location
Curacao

Effect of light availability on dissolved organic carbon release by Caribbean reef algae and corals

Dissolved organic carbon (DOC) release of three algal and two coral species was determined at three light intensities (0, 30–80, and 200–400 μmol photons m−2 s−1) in ex situ incubations to quantify the effect of light availability on DOC release by reef primary producers. DOC release of three additional algal species was quantified at the highest light intensity only to infer inter-specific differences in DOC release. For species tested at different light intensities, highest net release of DOC occurred under full light (200–400 μmol photons m−2 s−1). DOC released by benthic algae under full light differed (up to 16-fold) among species, whereas DOC release by scleractinian corals was minimal (Orbicella annularis Ellis and Solander, 1786) or net uptake occurred (Madracis mirabilis Duchassaing and Michelotti, 1860) independent of light availability. DOC concentrations and light intensities were also measured in situ near seven benthic primary producers, sediment, and in the water column at nine sites evenly distributed along the leeward coast of Curaçao. In situ DOC concentrations increased with light availability, although the magnitude of this positive effect differed among species and bottom types tested. In situ DOC concentrations were on average lower in November–December [87 (SD 45) μmol L−1] compared to May–June [186 (SD 136) μmol L−1], which can, at least partly, be explained by the lower light availability in the latter period. Our results suggest that DOC release by Caribbean benthic primary producers varies considerably among species and depends on light availability in reef algae. 

Date
2014
Data type
Scientific article
Theme
Research and monitoring
Geographic location
Curacao