Response to pH stress in the reef-building coral Pocillopora capitata (Anthozoa: Scleractinia)

To evaluate the metabolic response of the symbiotic coral Pocillopora capitata to reduced seawater pH in an in vitro system, 112 branches of P. capitata with no apparent damage from bleaching were collected from the La Boquita (LB) coral community. Two pH treatments were evaluated: a) pH 7.85/7.95 (Treatment C₁), b) 7.60/7.70 (Treatment C₂), and 8.00/8.40 pH control treatment (CC). The coral branches were randomly assigned to experimental units (n= 38 per treatment). Samples were taken at 2 separate times for biochemical analysis and qualitative assessment of microstructural aspects with scanning electron microscopy: initial time, sample taken at 12 h (T₁); end time, samples from the final day of exposure (T_f, day 7 of the experiment). Unlike the symbiont tissue, the biochemical analysis of the host tissue revealed that P. capitata displayed an immediate response as reflected in the RNA/DNA and protein/DNA ratios as well as in the concentrations of RNA and protein, particularly in the initial hours of the experiment in treatment C₂. Qualitative microstructural analysis primarily identified effects in treatment C₂ which was influenced by the presence of micro surface detachments in the terminal regions of the skeletal fibers.