High nutrient availability modulates photosynthetic performance and biochemical components of the economically important marine macroalga Kappaphycus alvarezii (Rhodophyta) in response to ocean acidification

Increased atmospheric CO₂ concentrations not only change the components of inorganic carbon system in seawater, resulting in ocean acidification, but also lead to decreased seawater pH, resulting in ocean acidification. Consequently, increased inorganic carbon concentrations in seawater provide a sufficient carbon source for macroalgal photosynthesis and growth. Increased domestic sewage and industrial wastewater discharge into coastal areas has led to nutrient accumulation in coastal seawaters. Combined with elevated pCO₂ (1200 ppmv), increased nutrient availability always stimulates the growth of non-calcifying macroalgae, such as red economical macroalga Gracilariopsis lemaneiformis. Here, we evaluated the interactive effects of nutrients with elevated pCO₂ on the economically important marine macroalga Kappaphycus alvarezii (Rhodophyta) in a factorial 21-day coupling experiment. The effects of increased nutrient availability on photosynthesis and photosynthetic pigments of K. alvarezii were greater than those of pCO₂ concentration. The highest F_v/F_m values (0.660 ± 0.019 and 0.666 ± 0.030, respectively) were obtained at 2 μmol L⁻¹ of NO₃–N at two pCO₂ levels. Under the elevated pCO₂ condition, the Chl-a content was lowest (0.007 ± 0.004 mg g⁻¹) at 2 μmol L⁻¹ of NO₃–N and highest (0.024 ± 0.002 mg g⁻¹) at 50 μmol L⁻¹ of NO₃–N. The phycocyanin content was highest (0.052 ± 0.012 mg g⁻¹) at 150 μmol L⁻¹ of NO₃–N under elevated pCO₂ condition. The malondialdehyde content declined from 32.025 ± 4.558 nmol g⁻¹ to 26.660 ± 3.124 nmol g⁻¹ with the increased nutrients at under low pCO₂. To modulate suitable adjustments, soluble biochemical components such as soluble carbohydrate, soluble protein, free amino acids, and proline were abundantly secreted and were likely to protect the integrity of cellular structures under elevated nutrient availability. Our findings can serve as a reference for cultivation and bioremediation methods under future environmental conditions.