Elevated CO2 levels affect the activity of nitrate reductase and carbonic anhydrase in the calcifying rhodophyte Corallina officinalis

The concentration of CO₂ in global surface ocean waters is increasing due to rising atmospheric CO₂ emissions, resulting in lower pH and a lower saturation state of carbonate ions. Such changes in seawater chemistry are expected to impact calcification in calcifying marine organisms. However, other physiological processes related to calcification might also be affected, including enzyme activity. In a mesocosm experiment, macroalgal communities were exposed to three CO₂ concentrations (380, 665, and 1486 µatm) to determine how the activity of two enzymes related to inorganic carbon uptake and nutrient assimilation in Corallina officinalis, an abundant calcifying rhodophyte, will be affected by elevated CO₂ concentrations. The activity of external carbonic anhydrase, an important enzyme functioning in macroalgal carbon-concentrating mechanisms, was inversely related to CO₂ concentration after long-term exposure (12 weeks). Nitrate reductase, the enzyme responsible for reduction of nitrate to nitrite, was stimulated by CO₂ and was highest in algae grown at 665 µatm CO₂. Nitrate and phosphate uptake rates were inversely related to CO₂, while ammonium uptake was unaffected, and the percentage of inorganic carbon in the algal skeleton decreased with increasing CO₂. The results indicate that the processes of inorganic carbon and nutrient uptake and assimilation are affected by elevated CO₂ due to changes in enzyme activity, which change the energy balance and physiological status of C. officinalis, therefore affecting its competitive interactions with other macroalgae. The ecological implications of the physiological changes in C. officinalis in response to elevated CO₂ are discussed.

Seasonal coupling and de-coupling of net calcification rates from coral reef metabolism and carbonate chemistry at Ningaloo Reef, Western Australia

Rates of net production, net calcification, and nutrient uptake were measured in a coral-dominated reef flat community on Ningaloo Reef in northwestern Australia under seasonally minimum and maximum light levels. Daily integrated light decreased twofold while water temperatures remained relatively constant increasing by only 1°C on average from summer to winter. Rates of daily community gross primary production (GPP) were only 33% ± 9% higher in summer than in winter (1400 ± 70 versus 1050 ± 60 mmol C m⁻² d⁻¹), far less than the twofold seasonal changes reported for most shallow reef communities. Rates of daily community net calcification (G_net) were not significantly different between seasons (190 ± 40 mmol CaCO₃ m⁻² d⁻¹ in summer versus 200 ± 10 mmol CaCO₃ m⁻² d⁻¹ in winter). The average rate of total nitrogen uptake (dissolved + particulate) was also not significantly different between summer and winter (8.3 ± 3.8 versus 6.6 ± 3.4 mmol N m⁻² d⁻¹, respectively), despite evidence of sporadically high nitrate uptake in both seasons. In summer, rates of hourly net calcification (g_net) were linearly correlated with diurnal changes in net production, pH, and aragonite saturation state (Ω_ar); and were mostly correlated with light except at mid-day under heavy cloud cover. However, in winter, g_net was independent of diurnal changes in light, net production, pH, and Ω_ar indicating that the reef flat community had possibly reached a threshold above which rates of net calcification were insensitive to diurnal changes in their environment.