BURNETT, Louis E; MILARDO, Christopher S: Effects of Air Exposure in Oysters on the Growth and Metabolism of the Protozoan Oyster Parasite Perkinsus marinus

We examined the effects of pH, oxygen, carbon dioxide, and temperature on the metabolism of the Apicomplexan oyster parasite Perkinsus marinus simulating conditions that occur within the tissues of the Eastern oyster Crassostrea virginica during air exposure in the summer. P. marinus was cultured in a modified JL-ODRP medium in ambient air at high humidity. The culture medium was gassed with appropriate CO₂, N₂ and O₂ mixtures, and pH adjusted in order to simulate the conditions present in oyster hemolymph. Oxygen uptake of the parasite decreased proportionately with decreasing ambient oxygen, and decreasing pH increased P. marinus respiration under low CO₂ conditions (p=0.0006); under 15 torr CO₂, there was no relationship between pH and oxygen uptake. Increased CO₂ levels (simulating conditions during oyster air exposure) significantly increased parasite oxygen uptake (p<0.0001), as did increased temperature (p<0.0001). The effect of temperature cycling on the in vitro growth of P. marinus was also examined at pH 6.3, 7.1 and 7.6. Cycling in 6-hour intervals from 25^oC-35^oC did not affect culture growth; however, when cycled from 25^oC-40^oC, cells showed a significant decrease in growth (p<0.0001). Similarly, cells grown at 35^oC and 15 torr ambient CO₂ showed decreased growth. These results suggest that summer environmental conditions in the intertidal zone and especially in the tissues of oysters play a role in keeping P. marinus infections of C. virginica at sub-lethal levels.