Molecular and biochemical response of the freshwater bivalve Diplodon chilensis to experimental oxygen depletion.

MARÍA S. YUSSEPPONE; IARA ROCCHETTA; STEFANIE MEYER; SEBASTIAN E. SABATINI; CARLOS LUQUET; RIOS DE MOLINA MARÍA DEL CARMEN; CHRISTOPH HELD; DORIS ABELE

Congreso; SETAC Latin America 11th Biennial Meeting; 2015

Hypoxic conditions in freshwater and marine systems are spreading as a consequence of anthropogenic pollution and eutrophication. The bivalve Diplodon chilensis is a key species for Patagonian lakes and rivers as important filter feeder involved in water clearance. We tested its biochemical and molecular response to 10 days of exposure to anoxia (<0.6% air saturation), hypoxia (10% air saturation), and normoxia (control group).Both treatments had differential effects on gene expression in D. chilensis which further differed between gills and mantle. Some key genes related to the antioxidant defense system (glutathione synthase, glutathione peroxidase) and glycogen mobilization (glycogen phosphorylase) were down regulated under oxygen depletion, suggesting depression of metabolic rate to reduce consumption of energy. Up-regulation of Heat Shock Protein HSP70 and HSP90 expression indicates impaired protein folding in anoxia and hypoxia. Since HSP70 has anti-apoptotic function, its up-regulation, together with the observed down-regulation of apoptosis related laminin receptor, suggest another energy saving strategy by delaying apoptosis. Lower protein carbonyl content in gills indicates less oxidative protein damage under anoxic condition and is in agreement with lower oxidative stress, less tissue damage and lower apoptotic activity. Higher expression of the alternative oxidase under anoxia and hypoxia compared to the normoxic controls indicates a shortcut of mitochondrial electron transport system (ETS), circumventing at least the third complex. A clear induction of mitochondrial anaerobic pathway enzymes including phosphoenol-pyruvate-carboxy-kinase (PEPCK) was not detected within 10 days of exposure. However, accumulation of succinate and higher Malate Dehydrogenase (MDH) activity in mantle under hypoxic and anoxic exposure is linked with intensified anaerobic fermentation of malate in the mitochondria. Our results indicate this freshwater bivalve to responds both hypoxia and anoxia through decreasing metabolic energy turnover within the first 10 days of exposure. It is likely that the switch to mitochondrial anaerobic pathways was induced through reversible phosphorylation of pyruvate kinase and that a final shift to hypoxic gene transcription will occur upon longer exposure to hypoxia/anoxia. It is still not clear how long these bivalves can survive without oxygen, but based on the present results they seem to be of considerable anoxia tolerance.