Macroalgal terpenes function as allelopathic agents against reef corals - PubMed (original) (raw)

Macroalgal terpenes function as allelopathic agents against reef corals

Douglas B Rasher et al. Proc Natl Acad Sci U S A. 2011.

Abstract

During recent decades, many tropical reefs have transitioned from coral to macroalgal dominance. These community shifts increase the frequency of algal-coral interactions and may suppress coral recovery following both anthropogenic and natural disturbance. However, the extent to which macroalgae damage corals directly, the mechanisms involved, and the species specificity of algal-coral interactions remain uncertain. Here, we conducted field experiments demonstrating that numerous macroalgae directly damage corals by transfer of hydrophobic allelochemicals present on algal surfaces. These hydrophobic compounds caused bleaching, decreased photosynthesis, and occasionally death of corals in 79% of the 24 interactions assayed (three corals and eight algae). Coral damage generally was limited to sites of algal contact, but algae were unaffected by contact with corals. Artificial mimics for shading and abrasion produced no impact on corals, and effects of hydrophobic surface extracts from macroalgae paralleled effects of whole algae; both findings suggest that local effects are generated by allelochemical rather than physical mechanisms. Rankings of macroalgae from most to least allelopathic were similar across the three coral genera tested. However, corals varied markedly in susceptibility to allelopathic algae, with globally declining corals such as Acropora more strongly affected. Bioassay-guided fractionation of extracts from two allelopathic algae led to identification of two loliolide derivatives from the red alga Galaxaura filamentosa and two acetylated diterpenes from the green alga Chlorodesmis fastigiata as potent allelochemicals. Our results highlight a newly demonstrated but potentially widespread competitive mechanism to help explain the lack of coral recovery on many present-day reefs.

PubMed Disclaimer

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Effects of macroalgae and algal extracts on corals. (A_–_C) Coral bleaching (2D percentage of area; mean ± SE) and (D_–_I) effective quantum yield (Y; mean ± SE) of three species of corals when in contact with macroalgae for 20 d (A–F) or in contact with gel strips containing hydrophobic extracts from the same algae for 24 h (G_–_I), relative to controls (n = 10–11). Analyzed by Kruskal–Wallis ANOVA on ranks. Letters above bars indicate significant groupings by post hoc Student–Newman–Kuels tests. Numbers inset within bars indicate number of replicates experiencing 100% mortality.

Fig. 2.

Effects of surface-bound algal extracts on corals. Effective quantum yield (Y; mean ± SE) of three coral species when in direct contact for 24 h with gel strips containing hydrophobic extracts from the surfaces of macroalgae, relative to controls (n = 5–10). Analysis and symbols as in Fig. 1.

Fig. 3.

Effects of G. filamentosa and C. fastigiata extract fractions on corals. Effective quantum yield (Y; mean ± SE) of the coral Acropora millepora when in contact with extract fractions from G. filamentosa (A_–_D) or C. fastigiata (E_–_H) for 24 h (n = 10 per fraction), relative to controls. Methanol-soluble crude extracts of both algal species were fractionated by HP20ss reversed-phase chromatography before initial bioassay (A and E). Vertical lines indicate path and boxes indicate method of subsequent extract fractionation based on bioactivity. Asterisks indicate significant (P < 0.05) differences between fractions and controls by posthoc Student Newman–Kuels tests. Bioassays led to the isolation of allelopathic compounds 1 and 2 (Fig. 4) from G. filamentosa and compounds 3 and 4 (Fig. 4) from C. fastigiata.

Fig. 4.

Allelopathic compounds isolated from G. filamentosa (compounds 1 and 2) and C. fastigiata (compounds 3 and 4). Assigned carbon positions are noted for each molecule, and corresponding NMR spectroscopic data are available in

Tables S2

and

References

1. Hoegh-Guldberg O, et al. Coral reefs under rapid climate change and ocean acidification. Science. 2007;318:1737–1742. - PubMed
1. Baker AC, Glynn PW, Riegl B. Climate change and coral reef bleaching: An ecological assessment of long-term impacts, recovery trends and future outlook. Estuar Coast Shelf Sci. 2008;80:435–471.
1. De'ath G, Lough JM, Fabricius KE. Declining coral calcification on the Great Barrier Reef. Science. 2009;323:116–119. - PubMed
1. Bruno JF, et al. Thermal stress and coral cover as drivers of coral disease outbreaks. PLoS Biol. 2007;5:1220–1227. - PMC - PubMed
1. Jackson JBC, et al. Historical overfishing and the recent collapse of coastal ecosystems. Science. 2001;293:629–637. - PubMed

Macroalgal terpenes function as allelopathic agents against reef corals - PubMed (original) (raw)