Dataset: Galaxaura thalli allelopathy
Deployment: Fiji_2011

To confirm that induction of allelochemicals by Galaxaura increases coral damage under field conditions, we conducted an 8-day Galaxaura-Porites manipulation as before, but then placed the thalli (instead of extracts) of Galaxaura that had been in contact with Porites (‘treatment’) into competition with new Porites fragments (6-8 cm branches, planted as above; n = 15) and assessed the impacts of these seaweeds on corals relative to thalli that had been in contact with Porites skeletons (‘controls’; n = 15). We evaluated the effects of treatment and control seaweeds on the photophysiology of these new corals after 2, 4 and 12 days. We also deployed Porites without seaweed contact as environmental controls (‘environmental control’; n = 15). At each sampling interval, we took a single PAM fluorometry measurement (as above) on each coral at the most damaged location experiencing seaweed contact along the mid-point of the branch (i.e. excluding extremities). Environmental controls were sampled at the most damaged location along the midpoint using identical protocol. We caged the rack to prevent grazing by large herbivores.

We used PAM fluorometry (Diving-PAM, Walz, Germany) to assess changes in the photosystem II (PSII) quantum yield of zooxanthellae living within Porites, following contact with seaweed thalli or extracts. PAM fluorometry is commonly used to assess PSII function within the coral holobiont in response to biotic or abiotic stressors, and to investigate the processes leading to coral bleaching. Measurements of light-adapted corals (i.e. effective quantum yield (Phi_sub.PSII)) theoretically range from 0.0 to approximately 0.83. Empirical studies suggest that measurements of approximately 0.50-0.75 are indicative of a healthy coral and measurements of approximately 0.00-0.25 are indicative of coral bleaching and mortality. Effective quantum yield (Phi_sub.PSII) values are highly correlated with visual assessments of coral bleaching for Porites and other corals at our study site. We wanted to assess coral responses in nature with minimal disturbance of the test corals, so we sampled them in the field between 09.00 and 13.00 h. We interspersed readings for treatments and controls through time to prevent confounding seaweed effects with in situ temporal changes in non-photochemical quenching (i.e. temperature and UV). Coral fragments were (i) collected from colonies adjacent to our experimental rack (i.e. the same depth and local condition), (ii) allowed to acclimate on the rack for 1-24 months prior to experiments and (iii) haphazardly interspersed among treatments and controls to homogenize initial variance in zooxanthellae density and diversity among replicates. We avoided self-shading while sampling.

Relevant References:

* Rasher DB and ME Hay. "Competition induces allelopathy but suppresses growth and anti-herbivore defense in a chemically rich seaweed".  Proceedings of the Royal Society: B-Biological Sciences. vol. 281 no. 1777 20132615, 2014 (http://dx.doi.org/10.1098/rspb.2013.2615).

Rasher DB, Stout EP, Engel S, Kubanek J, and ME Hay. "Macroalgal terpenes function as allelopathic agents against reef corals", Proceedings of the National Academy of Sciences, v. 108, 2011, p. 17726.

Beattie AJ, ME Hay, B Magnusson, R de Nys, J Smeathers, JFV Vincent. "Ecology and bioprospecting," Austral Ecology, v.36, 2011, p. 341.

Rasher DB and ME Hay. "Seaweed allelopathy degrades the resilience and function of coral reefs," Communicative and Integrative Biology, v.3, 2010.

Hay ME, Rasher DB. "Corals in crisis," The Scientist, v.24, 2010, p. 42.

Hay ME and DB Rasher. "Coral reefs in crisis: reversing the biotic death spiral," Faculty 1000 Biology Reports 2010, v.2, 2010.

Rasher DB and ME Hay. "Chemically rich seaweeds poison corals when not controlled by herbivores", Proceedings of the National Academy of Sciences, v.107, 2010, p. 9683.