Effects of future climate on coral-coral competition

and monitored using a pH stat computer (Aqua Medic) that bubbled 100% CO2 into each tank (25 mL/min) as necessary to maintain treatment levels. Temperature was monitored and con- trolled using independent dual-stage digital controllers attached to water-jacketed heat exchangers. pH, temperature, and salinity were also externally monitored and verified daily using a ThermoFisher Orion Star pH meter (relative accuracy ±0.01 units) and a YSI tempera- ture/salinity meter. Water changes (25%) occurred twice weekly with small additions of deion- ized (DI) water daily as needed to maintain salinity near 36ppt in both present and future treatments (Table 1). Total alkalinity was measured weekly via open-cell potentiometric titra- tion. Carbonate parameters within each tank were calculated in the CO2SYS program using measured parameters of pH, TA, temperature, and salinity, with the carbonate dissociation constants of Mehrbach et al. [44] as refit by Dickson & Millero [45]. Coral fragments were randomly assigned to treatments and tanks. Two pairs of fragments (one from each species) were positioned as follows: i) one pair was positioned ~10 cm apart from each other to prevent any interactions via mesenterial filaments or sweeper tentacles [36] and ii) a second pair was placed similarly but was then moved into contact after the acclima- tion period discussed below. This design resulted in the use of 24 fragments per species. Given the limited number of coral individuals (3) per species that were available for this experiment,

this design could have confounded tank and individual level effects, but the random allocation of coral fragments to tanks should have minimized this. Coral fragments acclimated to tank conditions for 7 days without competition, but after the 7-day acclimation contact was initi- ated for one of the pairs in each tank. One fragment of M. cavernosa was placed in direct con- tact with one fragment of P. astreoides, and this treatment was maintained for 7 days. The area of direct contact was 6–7 cm in the area of contact, and living surfaces were placed in contact with polyps oriented toward the surface of the other coral. The remaining fragments of each coral (no-contact controls) in each tank were positioned 10 cm away from the contact corals and from each other. Endosymbiont photosynthetic efficiency was monitored using PAM fluorometry, one method for assessing coral health [46] and for quantifying coral bleaching [47], to determine maximum quantum yield (Fv/Fm−a measure of photosynthetic efficiency) every other day over the course of the experiment. PAM fluorometry has been used previously in algal-coral compe- tition studies and has shown that reduced Fv/Fm commonly leads to coral bleaching and often death [48, 49]. Additionally, reduced Fv/Fm is also associated with rapid changes in coral immune responses, protein degradation, and changes in catalytic and metabolic activity, which can all lead to apoptosis and necrosis of coral tissues [50, 51]. Corals were dark-adapted for one hour prior to measuring Fv/Fm, and readings were taken between 1100 and 1300 hours

each day. For each coral in direct contact, a single Fv/Fm measurement was taken at the area of contact with the other species and two measurements were taken approximately 3 cm away from the area of contact, but on this same fragment. The average of these two measurements was designated as the average Fv/Fm for the “no-contact” portions of the coral, and the differ- ence between the direct contact and the average “no-contact” Fv/Fm on that same fragment was compared. For the “no-contact control” corals, three Fv/Fm measurements were taken haphazardly over the coral to get an average for that individual. This average value was com- pared to the aforementioned “no-contact” location Fv/Fm for the coral in contact with the other species. When coral lesions formed (this happened only for corals in contact), Fv/Fm thereafter increased to values outside of the average readings for no-contact corals in this experiment. This appeared to be due to rapid colonization of the exposed coral skeleton by dia- toms, cyanobacteria, or other epilithic algae (as also noted by McCook et al. 2001 [24]). Areas with a visible lesion (confirmed with photos and a zero or near zero Fv/Fm prior to epilithic algal growth) were recorded as a “zero” Fv/Fm−even following colonization by epilithic algae. PLOS ONE Climate change alters coral-coral competition PLOS ONE | https://doi.org/10.1371/journal.pone.0235465 August 13, 2020 4 / 16