Effects of ocean acidification on the potency of macroalgal allelopathy to a common coral

Abstract

Many coral reefs have phase shifted from coral to macroalgal dominance. Ocean acidification (OA) due to elevated CO₂ is hypothesised to advantage macroalgae over corals, contributing to these shifts, but the mechanisms affecting coral-macroalgal interactions under OA are unknown. Here, we show that (i) three common macroalgae are more damaging to a common coral when they compete under CO₂ concentrations predicted to occur in 2050 and 2100 than under present-day conditions, (ii) that two macroalgae damage corals via allelopathy, and (iii) that one macroalga is allelopathic under conditions of elevated CO₂, but not at ambient levels. Lipid-soluble, surface extracts from the macroalga Canistrocarpus (=Dictyota) cervicornis were significantly more damaging to the coral Acropora intermedia growing in the field if these extracts were from thalli grown under elevated vs ambient concentrations of CO₂. Extracts from the macroalgae Chlorodesmis fastigiata and Amansia glomerata were not more potent when grown under elevated CO₂. Our results demonstrate increasing OA advantages seaweeds over corals, that algal allelopathy can mediate coral-algal interactions, and that OA may enhance the allelopathy of some macroalgae. Other mechanisms also affect coral-macroalgal interactions under OA, and OA further suppresses the resilience of coral reefs suffering blooms of macroalgae.

Figure 1. Mean rate of tissue loss (percent per day) in Acropora intermedia corals exposed to varying levels of ocean acidification and coral-macroalgal competition treatments.

The rate was calculated by estimating the amount of coral tissue loss per day and values were then averaged by the number of days until complete loss of coral tissue. N = 12 coral branches (±SEM). Corals were exposed to contact with the macroalgae Canistrocarpus (=Dictyota) cervicornis, Chlorodesmis fastigiata, Amansia glomerata or a plastic mimic under three levels of CO₂: ambient (380 ppm); medium (540 ppm) or high (936 ppm). Controls were corals without macroalgal contact. Data were analysed with a two-way ANOVA. Lowercase (a,b) and uppercase (A,B,C) letters indicate significant differences among CO₂ and competition treatments, respectively, via Tukey tests. Complete ANOVA and Tukey test results are in Table 1.

Figure 2. Percentage of coral tissue loss through time during a coral-algal competition and ocean acidification experiment.

Acropora intermedia corals were exposed to contact with Canistrocarpus (=Dictyota) cervicornis, Chlorodesmis fastigiata, Amansia glomerata or a plastic mimic under three levels of CO₂: ambient (380 ppm); medium (540 ppm) or high (936 ppm). Controls were corals without macroalgal contact. Coral tissue loss was estimated daily by counting the number of coral branches that exhibited any tissue loss and data are presented as frequency of dead colonies over the experimental period (25 days).

Figure 3. Mean effective quantum yield (±SEM, n = 15) of symbiotic dinoflagellates in in-situ corals exposed to contact with lipid-soluble surface extracts from the macroalgae Canistrocarpus (=Dictyota) cervicornis, Chlorodesmis fastigiata and Amansia glomerata grown under elevated CO₂ (936 CO₂ ppm) and ambient conditions (380 ppm).

Control corals were in contact with gel pads treated with carrier solvent but without inclusion of algal extract (i.e. Phytagel controls). Extract from C. cervicornis grown under enriched CO₂ was significantly more suppressive of photosynthetic efficiency (EQY) compared to extracts from C. cervicornis grown under ambient CO₂. Elevated CO₂ did not affect the potency of extracts from the other two algal species. Data were analysed with a two-way ANOVA: Asterisks and uppercase (A,B,C) letters indicate significant differences among CO₂ and macroalgal treatments, respectively, via posthoc Tukey tests (*p < 0.05, ns: not significant). Complete ANOVA and Tukey test results are in Table 2. Lowercase letters (a,b) indicate significant differences in EQY of the coral compared to Phytagel controls via Dunnett test. Note y-axis break.