While it has been established for some time that coral and seaweed do not mix, what is less clear is the reason why. New research addresses this issue.
A study by Katie Barott and coauthors to be published in a forthcoming issue of the Proceedings of the Royal Society B examines the exact nature of the interactions between the reef-building boulder star coral (Montastraea annularis) and four types of benthic macroalgae (Dictyota bartayresiana, Halimeda opuntia, turf algae and crustose coralline algae, CCA) seeking to understand the reasons for their incompatibility.
The authors conducted their study on the island of Curacao and used microprobes to measure the dissolved oxygen levels at the zones where the coral and algae interacted with and without algae removal in order to study and compare the physiological changes across these four types of coral-algal interfaces.
They also identified the taxonomic composition of the coral-associated bacteria across the four types of coral-algal interactions by DNA sequencing in order to identify any algal-induced changes. Lastly, they carried out visual surveys to quantify the types and abundances of interactions between corals and algae.
The authors found that all the algae except for the CCA induced hypoxia on adjacent coral tissues. Furthermore, they found the bacterial communities in the interaction zones to be significantly altered by turf algae, in which more pathogens predominate.
In contrast to the other three types of algae, the authors found little antagonistic interactions between CCA and the coral. Interestingly, the visual surveys found CCA-coral interactions to be at their most abundant in areas with the least human disturbance, falling to zero in disturbed areas.
Based on their results, the authors propose that some fleshy algae adversely affect adjacent coral by stimulating bacterial respiration (causing dissolved oxygen levels to drop and inducing hypoxia in the coral) and promoting the invasion of pathogenic bacteria on corals.
These proposed mechanisms allow algae to outcompete corals, especially in regions where controls to algal abundance have been removed by overfishing and eutrophication.
For more information, see the paper: Barott, KL, B Rodriguez-Mueller, M Youle, KL Marhaver, MJA Vermeij, JE Smith and FL Rohwer (2011) Microbial to reef scale interactions between the reef-building coral Montastraea annularis and benthic algae. Proceedings of the Royal Society B: Biological Sciences doi:10.1098/rspb.2011.2155
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