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Link to original content: http://www.ncbi.nlm.nih.gov/pubmed/20457927
Chemically rich seaweeds poison corals when not controlled by herbivores - PubMed Skip to main page content
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. 2010 May 25;107(21):9683-8.
doi: 10.1073/pnas.0912095107. Epub 2010 May 10.

Chemically rich seaweeds poison corals when not controlled by herbivores

Douglas B Rasher  1 , Mark E Hay
Affiliations

Chemically rich seaweeds poison corals when not controlled by herbivores

Douglas B Rasher et al. Proc Natl Acad Sci U S A. .

Abstract

Coral reefs are in dramatic global decline, with seaweeds commonly replacing corals. It is unclear, however, whether seaweeds harm corals directly or colonize opportunistically following their decline and then suppress coral recruitment. In the Caribbean and tropical Pacific, we show that, when protected from herbivores, approximately 40 to 70% of common seaweeds cause bleaching and death of coral tissue when in direct contact. For seaweeds that harmed coral tissues, their lipid-soluble extracts also produced rapid bleaching. Coral bleaching and mortality was limited to areas of direct contact with seaweeds or their extracts. These patterns suggest that allelopathic seaweed-coral interactions can be important on reefs lacking herbivore control of seaweeds, and that these interactions involve lipid-soluble metabolites transferred via direct contact. Seaweeds were rapidly consumed when placed on a Pacific reef protected from fishing but were left intact or consumed at slower rates on an adjacent fished reef, indicating that herbivory will suppress seaweeds and lower frequency of allelopathic damage to corals if reefs retain intact food webs. With continued removal of herbivores from coral reefs, seaweeds are becoming more common. This occurrence will lead to increasing frequency of seaweed-coral contacts, increasing allelopathic suppression of remaining corals, and continuing decline of reef corals.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Experimental design. (A) A rack holding experimental corals in cones. (B) A coral replicate showing a seaweed transplanted against a coral. (C) A coral replicate wrapped with a gel containing the lipid-soluble extract of a seaweed.
Fig. 2.
Fig. 2.
Effects of intact seaweeds and extracts on coral health. (A and B) Visual coral tissue bleaching (percent 2D area; mean ± SEM) and (C–F) photosynthetic efficiency (Y; mean ± SEM) of the corals Porites porites in Panama and Porites cylindrica in Fiji when in contact with intact seaweeds for 20 d (A–D: n = 9–11), or in contact with gel strips containing lipid-soluble extracts from the same seaweeds for 24 h (E and F: n = 10). Analyzed by Kruskal-Wallis ANOVA on Ranks. Letters indicate homogeneous subgroups by posthoc Student-Newman-Kuels tests.
Fig. 3.
Fig. 3.
Linear correlation between coral bleaching and photosynthetic efficiency for both corals. Values determined for corals in direct contact with seaweeds for 20 d (mean ± SEM; n = 9–11 per seaweed-coral treatment). Analyzed by Pearson's correlation coefficients.
Fig. 4.
Fig. 4.
Effects of seaweed surface extracts on coral health. Photosynthetic efficiency (Y; mean ± SEM) of Porites cylindrica in direct contact for 24 h with gel strips containing lipid-soluble extracts from the surfaces of seaweeds (n = 10). Analyzed as in Fig. 2.
Fig. 5.
Fig. 5.
Consumption of seaweeds in a marine protected area (MPA) and adjacent fished reef. Seaweeds consumed (percent; mean ± SEM) by herbivores during a 24-h feeding assay on a protected (n = 20) and fished (n = 20) reef (~300 m apart) in 2008 (A) and 2009 (B). Stars indicate differences in the consumption of a seaweed between reefs, within a year, by Mann-Whitney U tests.
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