Description: Highlights: • Transcriptional response to hypoxia-reoxygenation was studied in an OMZ bivalve. • Anaerobic glycolysis and protein quality control pathways were transcriptionally upregulated in hypoxia. • Hypoxia elevated mRNA levels of UCP2 but had no effect on thiol-dependent antioxidants. • No impact of hypoxia-reoxygenation was found on aerobic marker enzymes. • Responses of an OMZ bivalve show parallels to other hypoxia-tolerant bivalves. Abstract: Benthic animals inhabiting the edges of marine oxygen minimum zones (OMZ) are exposed to unpredictable large fluctuations of oxygen levels. Sessile organisms including bivalves must depend on physiological adaptations to withstand these conditions. However, as habitats are rather inaccessible, physiological adaptations of the OMZ margin inhabitants to oxygen fluctuations are not well understood. We therefore investigated the transcriptional responses of selected key genes involved in energy metabolism and stress protection in a dominant benthic species of the northern edge of the Namibian OMZ, the nuculanid clam Lembulus bicuspidatus,. We exposed clams to normoxia (~5.8 ml O2 l−1), severe hypoxia (36 h at ~0.01 ml O2 l−1) and post-hypoxic recovery (24 h of normoxia following 36 h of severe hypoxia). Using newly identified gene sequences, we determined the transcriptional responses to hypoxia and reoxygenation of the mitochondrial aerobic energy metabolism (pyruvate dehydrogenase E1 complex, cytochrome c oxidase, citrate synthase, and adenine nucleotide translocator), anaerobic glycolysis (hexokinase (HK), phosphoenolpyruvate carboxykinase (PEPCK), phosphofructokinase, and aldolase), mitochondrial antioxidants (glutaredoxin, peroxiredoxin, and uncoupling protein UCP2) and stress protection mechanisms (a molecular chaperone HSP70 and a mitochondrial quality control protein MIEAP) in the gills and the labial palps of L. bicuspidatus. Exposure to severe hypoxia transcriptionally stimulated anaerobic glycolysis (including HK and PEPCK), antioxidant protection (UCP2), and quality control mechanisms (HSP70 and MIEAP) in the gills of L. bicuspidatus. Unlike UCP2, mRNA levels of the thiol-dependent mitochondrial antioxidants were not affected by hypoxia-reoxygenation stress. Transcript levels of marker genes for aerobic energy metabolism were not responsive to oxygen fluctuations in L. bicuspidatus. Our findings highlight the probable importance of anaerobic succinate production (via PEPCK) and mitochondrial and proteome quality control mechanisms in responses to oxygen fluctuations of the OMZ bivalve L. bicuspidatus. The reaction of L. bicuspidatus to oxygen fluctuations implies parallels to that of other hypoxia-tolerant bivalves, such as intertidal species.

Description: Hypoxia is a major stressor in estuarine and coastal habitats leading to adverse effects in aquatic organisms. Estuarine bivalves such as the blue mussels Mytilus edulis and the Pacific oysters Crassostrea gigas can survive periodic oxygen deficiency but the molecular mechanisms that underlie cellular injury during hypoxia-reoxygenation are not well understood. We examined the molecular markers of autophagy, apoptosis and inflammation during the short-term (1 day) and long-term (6 days) hypoxia and post-hypoxic recovery (1 h) in the mussels and oysters by measuring the lysosomal membrane stability, activity of a key autophagic enzyme (cathepsin D) and mRNA expression of the genes involved in the cellular survival and inflammation, including caspases 2, 3 and 8, Bcl-2, BAX, TGF-β-activated kinase 1 (TAK1), nuclear factor kappa B-1 (NF-κB), and NF-κB activating kinases IKKα and TBK1. C gigas exhibited higher hypoxia tolerance as well as blunted or delayed inflammatory and apoptotic response to hypoxia and reoxygenation shown by the later onset and/or the lack of transcriptional activation of caspases, BAX and an inflammatory effector NF-κB compared with M.edulis. Long-term hypoxia resulted in upregulation of Bcl-2 in the oysters and the mussels implying activation of the anti-apoptotic mechanisms. Our findings indicate the potential importance of the cell survival pathways in hypoxia tolerance of marine bivalves and demonstrate the utility of the molecular markers of apoptosis and autophagy for assessment of the sublethal hypoxic stress in bivalve populations.

Description: Mitochondrial Complex IV in hypoxia-tolerant marine bivalves, namely, the blue mussel Mytilus edulis, the Arctic quahog Arctica islandica and the Pacific oyster Crassostrea gigas, exposing them for one or six days to extreme hypoxia (〈0.1% O2) followed by one hour of reoxygenation.

Description: Total phosphatases in hypoxia-tolerant marine bivalves, namely, the blue mussel Mytilus edulis, the Arctic quahog Arctica islandica and the Pacific oyster Crassostrea gigas, exposing them for one or six days to extreme hypoxia (〈0.1% O2) followed by one hour of reoxygenation.

Description: Mitochondrial Complex I in hypoxia-tolerant marine bivalves, namely, the blue mussel Mytilus edulis, the Arctic quahog Arctica islandica and the Pacific oyster Crassostrea gigas, exposing them for one or six days to extreme hypoxia (〈0.1% O2) followed by one hour of reoxygenation.

Description: Serine/threonine protein kinase A (PKA) and protein kinase C (PKC) in hypoxia-tolerant marine bivalves, namely, the blue mussel Mytilus edulis, the Arctic quahog Arctica islandica and the Pacific oyster Crassostrea gigas, exposing them for one or six days to extreme hypoxia (〈0.1% O2) followed by one hour of reoxygenation.