The roles of carbonate, borate, and bicarbonate ions in affecting zooplankton hatching success under ocean acidification

Two ocean acidification studies about egg hatching success (HS) in geographically important marine copepods, Calanus finmarchicus and C. helgolandicus, were reanalyzed with improved statistical procedures. The new results at low and moderate levels of seawater acidification showed no HS inhibition at normal habitat temperatures but statistically significant inhibition at warmer and colder temperatures. These HS results were compared with seawater carbonate system and borate concentrations from precise seawater measurements. The temperature dependent differences in HS could not be directly explained by changes in the seawater concentrations of either H⁺, bicarbonate (HCO₃⁻), or CO₂* (CO₂* being the sum of unhydrated CO₂ and H₂CO₃). In contrast, HS differences did match trends in seawater carbonate (CO₃²⁻) concentrations. A numerical model was developed which evaluates the concentrations of O₂ or CO₂*, HCO₃⁻, and CO₃²⁻ at the cellular level across an egg and embryo by considering both gas diffusion with the seawater and respiration by the embryo. Again, temperature-dependent trends in HS could not be explained changes in intracellular CO₂* or HCO₃⁻ concentrations, but HS did trend with the changes in intracellular CO₃²⁻ concentrations. Carbonate ions form strong coordination complexes with metals, so acidification-driven decreases in external seawater carbonate concentrations, which are amplified at warmer temperatures, could release injurious metals, thus driving the HS inhibition at warmer temperatures. Increases in cytoplasmic carbonate concentrations at warmer temperatures caused by seawater acidification could complex with biochemically-needed nutrient-type metals within the cells, also causing the increased HS inhibition at warmer temperatures. Furthermore, boron is essential in chemically signaling within and between cells. Seawater borate concentrations were closely correlated with HS inhibition via Michaelis-Menton equations, suggesting that acidification-driven decreases in seawater borate concentrations may also inhibit HS. Finally, the acidification-driven increases in CO₂ diffusion into cells dramatically increased intracellular bicarbonate concentrations. At mild levels of seawater acidification, an organism might compensate by exporting bicarbonate from the cells to the haemolymph and then to the seawater. Although the energetic cost, as percentage of ATP production, might be high, increased respiration rates at warmer temperatures might better allow the organism to survive. However, as temperature is lowered, the cellular respiration rate declines more rapidly with respect to temperature than does the gas diffusion coefficient. Consequently, bicarbonate accumulation driven by inward CO₂ diffusion might overwhelm the egg's bicarbonate export capacity at colder temperatures, explaining the colder temperature HS inhibition.