Graduation Year

2013

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

Joseph J. Torres

Keywords

coral, depth, light, metabolism, respiration, temperature

Abstract

Coral habitats span the range from tropical to polar, extremely shallow to thousands of meters deep. The differences in light and temperature experienced in these varied habitats likely affect the metabolic rates of the corals residing there. The metabolism of three coral species from different habitats have been examined to elucidate the effects of these environmental parameters on metabolism, an under-studied aspect of coral biology. For all three species, measurements of oxygen uptake, ammonium excretion, and activity of the enzymes lactate dehydrogenase (LDH), malate dehydrogenase (MDH), and citrate synthase (CS) were used to characterize their metabolism. Off Florida's Gulf coast, Cladocora arbuscula is known to be one of the species least damaged by bleaching events and is one of the quickest to recover, making it an ideal candidate for studying the effects of symbionts. The first set of experiments was designed to reveal the effect of disrupting the coral-algal symbiosis between this subtropical shallow-water coral and its dinoflagellate symbiont, Symbiodinium. The metabolic effects were described for "normal" C. arbuscula and those "bleached" by being held in total darkness for 4 months. Normal C. arbuscula had a relatively low rate of oxygen consumption at 21°C, averaging 2.43±0.65 µmol O2 gwm-1 h-1 (±S.E.), using tissue wet mass, while the bleached colonies had an average rate of 2.46±0.49 µmol O2 gwm-1 h-1. Ammonium excretion averaged 0.07±0.02 and 0.10±0.03 µmol NH4+ gwm-1 h-1 (±S.E.) for normal and bleached C. arbuscula, respectively. The activity values of the metabolic enzymes citrate synthase (CS) fell within the normal range expected for a cnidarian, averaging around 0.09±0.02 activity units (U) gwm-1 for both treatments, indicating normal aerobic ability. MDH was extremely high for the normal corals, compared to other cnidarians, averaging 2.5±0.4 U gwm-1, and a bit lower for the bleached corals, averaging 1.2±0.3 U gwm-1, indicating high MDH activity during both normoxia and hypoxia. LDH activity, also high, averaged 1.3±0.2 U gwm-1 for both treatments, indicating anaerobic competence. These experiments show that C. arbuscula is adept at maintaining almost completely normal metabolic function when bleached, although the corals quickly become re-inoculated with symbionts upon return to normal light conditions in a tank with normal corals.

The second set of experiments served to characterize the metabolism of Lophelia pertusa, an azooxanthellate cold-water coral that thrives in water depths between 36 and 3383 m. L. pertusa is rather stenothermal, commonly found between 6-8°C, but in the Gulf of Mexico can be subjected to warm water incursions. This makes it an ideal candidate for the examination of the effects of temperature. L. pertusa exhibited a respiration rate of 1.14 µmol O2 gwm-1 h-1 at the control temperature of 8°C. Calculating the Q10 for bringing L. pertusa up to the environmental temperature of C. arbuscula results in a value of 1.8. The 11°C treatment group exhibited an 11% increase in respiration, while at 13°C, the corals showed a 23% rise from normal. The 5°C group showed a 32% decrease in respiration. The activity values of the metabolic enzyme citrate synthase (CS) fell into the normal range expected for a cnidarian, averaging 0.15, 0.20, 0.10, and 0.18 activity units (U) gwm-1 for the 8°C, 11°C, 13°C, and 5°C treatments, respectively. Malate dehydrogenase (MDH) values were unexpectedly high, averaging 2.05, 1.48, 1.48, and 1.82U gwm-1 for the 8°C, 11°C, 13°C, and 5°C treatments, respectively. Lactate dehydrogenase (LDH) was undetectable in this species, suggesting it has a different terminal glycolytic enzyme. Nonetheless, the other two enzymes indicate metabolic competence in both normoxic and hypoxic conditions. L. pertusa is adaptable to temperatures within its range, although its respiration rate is lower than that of tropical corals.

The third set of experiments characterized the metabolism of the endemic Antarctic coral Flabellum impensum, one of the world's largest solitary corals. It resides at roughly the same depths as L. pertusa, but the water temperature in its habitat never strays far from 0°C. F. impensum had a low rate of oxygen consumption at 0°C, averaging 0.31 µmol O2 g-1 h-1, calculated using tissue wet mass. Calculating a Q10 for this species at C. arbuscula's habitat temperature results in a value of 2.7. Ammonium excretion averaged 4.21 nmol NH4+ gwm-1 h-1. The activity values of the metabolic enzymes citrate synthase (CS), malate dehydrogenase (MDH), and lactate dehydrogenase (LDH) fell within the normal range expected for a cnidarian, averaging 0.13, 1.01, and 0.42 activity units (U) gwm-1, respectively. A count of the skeletal growth bands on the calyx suggests that this species has a linear extension rate of approximately 1 mm per year. F. impensum is a long-lived, slow-growing coral, with a low metabolic rate.

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