Effects of seawater acidification and alkalization on the farmed seaweed, Pyropia haitanensis (Bangiales, Rhodophyta), grown under different irradiance conditions

The thalli of Pyropia haitanensis were cultured under different pH levels (7.8, 8.2, and 9.0) and under decreased (60 μmol photons m−2 s−1) and ambient (300 μmol photons m−2 s−1) levels of light irradiance conditions, aiming to examine the influence of different pH and decreased light irradiance on this farmed seaweed species in Southern China. Either the decreased (7.8) or increased (9.0) pH values in seawater inhibited nitrogen uptake rates and nitrate reductase activity of P. haitanensis. The capacity of nitrogen uptake and maximum inorganic carbon (Ci)-saturated photosynthetic rate (Vmax) were reduced in P. haitanensis grown at decreased irradiance compared with the algae grown at ambient irradiance. Decreased pH had no significant effect on the algal growth and photosynthesis under ambient light conditions, but it significantly inhibited growth and photosynthesis under decreased light conditions. Increased seawater pH resulted in decreased relative growth rate (RGR), maximal quantum yield of photosystem II (Fv/Fm), and non-photochemical quenching (NPQ) of P. haitanensis when the algae were grown under ambient light conditions. However, a slight decrease was observed with decreasing growth irradiances. Collectively, our results indicated that either the changed pH (acidification and alkalization) or reduced irradiance displayed a disadvantageous influence on nitrogen metabolism of P. haitanensis. We suggested that, during P. haitanensis mariculture, the decreased light irradiance resulting from increasing algal mats density alleviates the negative effects of seawater alkalization, but it aggravates the adverse effects of seawater acidification on the growth and photosynthesis of the algae.