Coral physiology in response to thermal stress, nutritional status and seawater electrolysis

Abstract

To test the effect of food on the thermal tolerance of the symbiotic association, two feeding experiments were carried out at the Hasanuddin University Marine Field Station in southwest Sulawesi, Indonesia between June and October 2005. In both experiments corals were either starved, i.e. deprived of organic particles >0.5 μm or fed daily with freshly hatched Artemia salina nauplii. In Experiment 1 the effect of zooplankton feeding versus starvation on the bleaching susceptibility, photosynthetic activity of photosystem II (PSII) and non-photochemical quenching (NPQ) of zooxanthellae in the temperature sensitive species Stylophora pistillata under elevated temperature was assessed over a period of 10 days employing pulse-amplitude-modulated (PAM) chlorophyll fluorometry. Experiment 2 was run for 15 days in order to investigate the potential interactions between zooplankton feeding, basal metabolic functions (photosynthesis and respiration), energy status (lipid concentrations), total protein concentrations, and the bleaching susceptibility of S. pistillata and the more temperature tolerant species Galaxea fascicularis.The effects of mineral accretion on the physiology and rates of mortality of coral transplants were investigated in North Sulawesi, Indonesia between April and July 2006, using the congeneric species Acropora yongei and A. pulchra. In order to discriminate between the effects of electrochemical processes in the vicinity of a cathode and those of an electric field on coral performance, coral transplants were grown under 3 treatment conditions: 1) on a cathode in the presence of mineral accretion, 2) on bamboo inside an electric field, or 3) on bamboo outside the experimental matrix (control), for 4 months. At the end of the treatment phase coral survival, growth, zooxanthellae population characteristics and chlorophyll fluorescence responses of the two species were determined.The overall results of the two feeding experiments show that the thermal stress tolerance in terms of photosynthetic activity and pigmentation of both species was coupled to food availability. Zooxanthellae in fed corals in experiment 1 maintained high photosynthetic activity while starved corals displayed strong signs of chronic photoinhibition which was reflected by a progressive decline in the daily maximum quantum yield (Fv/Fm) of PSII and nocturnal recovery rates of PSII, which was manifest by a significant decrease in Fv/Fm, effective quantum yield (∆F/Fmà ¯Ã ¿Ã ½) and relative electron transport rate (rETR). This was paralleled by the progressive inability to dissipate excess excitation energy via non-photochemical quenching (NPQ). In addition, both gross (Pg) and net (Pn) primary production of starved S. pistillata and G. fascicularis in experiment 2 decreased significantly over the course of 15 days. Fed corals by comparison maintained higher levels of NPQ, which coincided with significantly higher rates of nocturnal PSII recovery. Sustained photosynthesis of fed corals was accompanied by a reduced loss in pigmentation compared to starved corals. After 10 and 15 days of exposure to elevated temperature the tissue of starved corals appeared pale relative to fed corals. The three most relevant findings of the investigation evaluating the proposed mechanisms of enhanced survival and skeletal growth rates of coral transplants in the presence of mineral accretion show that 1) the electric field and not the cathode resulted in the highest growth rates, 2) corals grown under the presence of mineral accretion can exhibit low growth and reduced health , and 3) the suitability of the mineral accretion technology for coral transplantation can vary greatly between closely related species. Contrary to previous observations, the presence of mineral accretion had either no or even a negative effect on coral survival over a period of 4 months. Mortality rates of A. yongei after 4 months were significantly higher for corals grown on the cathode than for those inside the electric field and on the control. By contrast there were no significant differences in mortality between treatments for A. pulchra.