Reactive oxygen intermediate production by oyster hemocytes exposed to hypoxia

Oysters are frequently exposed to severely hypoxic conditions, especially during summer months. During the summer, there are also large numbers of disease-related oyster mortalities. This research was conducted to determine whether exposure to environmental hypoxia reduces the ability of oyster hemocytes to produce reactive oxygen intermediates (ROIs), an important part of their defense system. Oysters of the species Crassostrea virginica were held in normoxic (P(O)(2)=20.0-20.7 kPa, pH 7.8-8.0) and hypoxic conditions (P(O)(2)=4.0-6.7 kPa, pH 7.1-7.4). In vivo hemolymph variables (P(O)(2), P(CO)(2) and pH) were measured after both 1 hour and 2 days in each treatment to determine the appropriate environment for subsequent hemocyte experiments. Production of reactive oxygen intermediates by hemocytes was measured using luminol-enhanced chemiluminescence (CL). During CL tests, hemocytes were held under the following conditions: air (P(O)(2)=20.7, P(CO)(2)<0.07, pH 7.6), in vivo hemolymph conditions of normoxic oysters (P(O)(2)=5.2, P(CO)(2)=0.27, pH 7.6), and in vivo hemolymph conditions of hypoxic oysters (P(O)(2)=1.47, P(CO)(2)=0.53, pH 7.1). Production of ROIs under hypoxic conditions was 33 % of that under normoxia. This decrease was the result of specific and independent effects of lower oxygen levels and decreased pH. It was not due to any direct effect of CO(2).