In Situ Oxygen Dynamics in Coral-Algal Interactions

Daniel Wangpraseurt,Dirk De Beer,Hans Røy,Lubos Polerecky,Suharsono Suharsono,Miriam Weber,Maggy M Nugues,Sharyn Jane Goldstien

doi:10.1371/journal.pone.0031192

Abstract

BackgroundCoral reefs degrade globally at an alarming rate, with benthic algae often replacing corals. However, the extent to which benthic algae contribute to coral mortality, and the potential mechanisms involved, remain disputed. Recent laboratory studies suggested that algae kill corals by inducing hypoxia on the coral surface, through stimulated microbial respiration.Methods/FindingsWe examined the main premise of this hypothesis by measuring in situ oxygen microenvironments at the contact interface between the massive coral Porites spp. and turf algae, and between Porites spp. and crustose coralline algae (CCA). Oxygen levels at the interface were similar to healthy coral tissue and ranged between 300–400 µM during the day. At night, the interface was hypoxic (∼70 µM) in coral-turf interactions and close to anoxic (∼2 µM) in coral-CCA interactions, but these values were not significantly different from healthy tissue. The diffusive boundary layer (DBL) was about three times thicker at the interface than above healthy tissue, due to a depression in the local topography. A numerical model, developed to analyze the oxygen profiles above the irregular interface, revealed strongly reduced net photosynthesis and dark respiration rates at the coral-algal interface compared to unaffected tissue during the day and at night, respectively.Conclusions/SignificanceOur results showed that hypoxia was not a consistent feature in the microenvironment of the coral-algal interface under in situ conditions. Therefore, hypoxia alone is unlikely to be the cause of coral mortality. Due to the modified topography, the interaction zone is distinguished by a thicker diffusive boundary layer, which limits the local metabolic activity and likely promotes accumulation of potentially harmful metabolic products (e.g., allelochemicals and protons). Our study highlights the importance of mass transfer phenomena and the need for direct in situ measurements of microenvironmental conditions in studies on coral stress.

Highlights

In the last three decades, coral reefs have been exposed to an increasing intensity and frequency of human stressors
We focused on communities of turf algae [26] and crustose coralline algae (CCA) that interacted with the Indo-Pacific reef building massive coral Porites
Abundance of Coral-algal Interactions We found that massive Porites spp. colonies had a mean interaction border of 39 cm (60.96 SE, n = 39 colonies)

Summary

Introduction

In the last three decades, coral reefs have been exposed to an increasing intensity and frequency of human stressors. Degradation of coral reefs typically involves a shift in community structure from a coral-dominated reef to an algaldominated reef, a process known as ‘coral-algal phase shift’ [5,6,7]. Despite being a well-documented phenomenon, the underlying dynamics and mechanistic processes leading to algal dominance are still unclear. It remains disputed whether algae acquire space by colonizing open substrates after a coral has died, or by actively overgrowing and out-competing neighboring corals [8]. Coral reefs degrade globally at an alarming rate, with benthic algae often replacing corals. The extent to which benthic algae contribute to coral mortality, and the potential mechanisms involved, remain disputed. Recent laboratory studies suggested that algae kill corals by inducing hypoxia on the coral surface, through stimulated microbial respiration

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