Molecular physiology and stress responses of the Pacific white shrimp, Litopenaeus vannamei: impacts of hypoxia and hypercapnic hypoxia
Rathburn, Charles Kolo
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Many crustaceans inhabit estuarine ecosystems where they are frequently exposed to hypoxia (H) and elevated levels of CO2 (hypercapnia). These factors may impair the ability of crustaceans to maintain optimal metabolic processes. Marine crustaceans employ various tactics to cope with environmental H and hypercapnia, which can require changes in biochemistry such as altered activities of metabolic enzymes. Furthermore, by regulating their gene expression marine crustaceans may coordinate specific and general stress responses to H and hypercapnic H (HH) which may include metabolic depression. This study set out to determine the impacts of H and HH on the molecular physiology of the Pacific white shrimp, L. vannamei. First, we tested whether moderate H or HH would augment lactate dehydrogenase (LDH), a tissue enzyme associated with anaerobic metabolism. LDH activity did not change in abdominal muscle of L. vannamei exposed to moderate H for 24 h or HH for 4 h; LDH activity also did not change in L. vannamei hepatopancreas (HP) after 24 h HH exposure. These results suggest that moderate H and HH do not increase glycolysis in L. vannamei. Second, we tested the hypothesis that H and HH elicit down-regulation of genes associated with metabolic depression, specifically protein synthesis and transcription. Shrimp were held in H, HH, or normoxia (N) for 4 h or 24 h. RNA from HP tissue was hybridized to microarrays containing 21,864 unigenes expressed by L. vannamei. Transcriptional profiles of H and HH animals were compared to respective 4 and 24 h N controls. Genes involved in amino acid metabolism, RNA metabolism, and translation (including numerous tRNA synthetases) were down-regulated in 4 h H, 24 h H and 4 h HH shrimp. Additionally, unique patterns of gene expression were tied to specific treatments and times. Overall, these results suggest that crustacean molecular responses to environmental changes in O2 and CO2 pressure involve both general and stress-specific gene sets, with shifts to metabolic depression occurring rather than increased anaerobic metabolism. This work contributes insight to the effects human perturbations might have on estuarine organisms.