Effects of Hydrostatic Pressure on the Metabolic Enzymes of Ctenophores from Different Habitat Depths.

Deep-sea animals have evolved various biochemical strategies to live at great depths. Our knowledge of evolutionary changes in enzymes is limited to very few enzymes and species, mostly fishes. Functionality of enzymatic machinery may decrease with increasing depth due to increasing pressure, which affects not only physiological performance, but also influences evolutionary and phylogenetic patterns of deep-sea organisms. To better understand biochemical adaptations to high hydrostatic pressure, we chose to study the phylum Ctenophora, since both closely related and phylogenetically distant species inhabit the deep sea. Enzymatic activities of native enzymes were recorded at pressures corresponding to depths of 1, 2000, 4000 , and 6000 meters. The glycolytic enzyme pyruvate kinase (PK), used both in anaerobic and aerobic metabolism, has shown adaptive pressure resistance in deep-sea fishes, and was targeted first for comparison. Pressure effects on malate dehydrogenase (MDH) and creatine (CK) kinase were also investigated. Shallow and deep-sea ctenophore species were examined to explore flexibility/rigidity of depth ranges. Initial results indicate that enzymes from species inhabiting similar vertical ranges can display unique pressure tolerance characteristics. The PK of both shallow and deep-water Beroe species exhibited a high pressure tolerance. PK from the shallow species Lampea sp. displayed a low capacity to function under high pressures. When MDH was subjected to high pressures, it exhibited low activation volume across all taxa. Creatine kinase displayed high sensitivity to pressure treatments. Phylogenetically, results indicate that is not necessarily convergent at the scale of a single enzyme. Further assessing functional diversity of ctenophore metabolism will indicate parallel or convergent protein adaptation in the deep sea. The effects of pressure reported herein are novel for invertebrates, and they offer a good comparison to fish biochemical adaptations.