Interaction between benthic algae (Lyngbya bouillonii, Dictyota dichotoma) and scleractinian coral Porites lutea in direct contact

Competition between the massive scleractinian coral Porites lutea and two benthic algal species, thin-filamentous blue-green Lyngbya bouillonii (Cyanophyta) commonly observed growing as mats and fleshy brown Dictyota dichotoma (Phaeophyta), was investigated. Experiments were designed to expose coral fragments to different treatments to test the role of abrasion, shading and allelopathy by Lyngbya and Dictyota on coral growth and physiology in direct physical contact. The growth rates of coral fragments were significantly lower in both the algal/coral and the net control (only plastic net touched the corals) treatments than in the manipulation control (contact with algae and plastic net was prevented), demonstrating the importance of abrasion in Porites-Lyngbya and Porites-Dictyota interactions. Furthermore, coral fragments exposed to Lyngbya grew significantly slowly than net controls, but this effect was not statistically significant for P. lutea maintained in contact with Dictyota. Light levels were reduced equally in the algal/coral and shading mimic (plastic net touched the corals shaded with neutral-density filters) treatments. However, there were no significant differences in the growth rates between the shading mimic and the net control treatments, suggesting that shading had no measurable effect on coral growth. Thus, the growth of P. lutea in contact with Dictyota was reduced by abrasion whereas in direct contact with L. bouillonii, abrasion was supplemented by additional factors unique to Lyngbya in mediating coral-algal competition. Physical contact with L. bouillonii induced a significant reduction in photochemical efficiency (F_v / F_m) of PSII and chlorophyll concentration of in hospite zooxanthellae of P. lutea fragments, as well as a decrease of the symbiotic dinoflagellate density. Analysis of the growth rate and F_v / F_m of the investigated algae revealed a significant reduction in both parameters for D. dichotoma but not for L. bouillonii when in direct contact with P. lutea fragments. Thus, the competitive inhibition by the coral P. lutea and the brown alga D. dichotoma was mutual, while L. bouillonii acted as a one-sided inhibitor for scleractinian corals inducing bleaching and severe damage of live coral tissue. The fact that mats-forming blue-green alga L. bouillonii acts as a poison for scleractinian corals and is able to kill live coral tissue is reported for the first time. Allelochemical mechanism of the effect on live coral tissue by this alga is suggested. Possible mechanisms of competitive interactions for substrate between the coral polyps of scleractinians and algal species investigated are discussed.