The critical roles of mitochondrial alternative chains in juvenile ark shells (Anadara broughtonii) exposed to acute hypoxia with or without sulfide

Reduced oxygen levels and increased sulfide concentrations have become a concern for marine animals. This study examines respiratory and energetic adaption to acute (0-96 h) hypoxia (0.5 mg/L dissolved oxygen) with or without sulfide (0.2 mM, 1 mM, 3 mM) in the hypoxia-resistant and sulfide-tolerant ark shell, Anadara broughtonii. The different states of aerobic respiration, energy-balance, and activity of the mitochondrial sulfide oxidation chain (MSOC) under these conditions were evaluated. The results indicated that the anaerobic pathway was activated by hypoxia at 24 h without sulfide, but was activated in the presence of sulfide at only 2 h. Exposure to sulfide resulted in significant accumulation of ATP, probably due to the activated MSOC and lowered metabolism via suppression of Na+-K+ ATPase activity and protein synthesis. During hypoxia, both enzyme activity and mRNA levels of alternative oxidase (AOX) increased while the key enzymes in MSOC, sulfide: quinone oxidoreductase (SQR) and sulfur dioxygenase (SDO), were not altered. With additional sulfide, the enzyme activity and mRNA levels of AOX, SQR, SDO significantly increased. Classical aerobic respiration was significantly inhibited, and induction of alternative respiration was detected. The corresponding alternative electron transport chain (AETC) accepted the electrons originating from both the tricarboxylic acid cycle and MSOC during the challenge, indicating that the capacity of aerobic respiration and sulfide-oxidation under a reduction state might greatly depend on AETC. The synergistically induced alternative chains (AETC and MSOC) and anaerobic pathway suggested energy-balance between respiration and sulfide-oxidation, which might contribute to the endurance of ark shells to acute sulfide exposure.