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.