Ocean Acidification

Gracilariopsis lemaneiformis (Gracilariales, Rhodophyta) is an important economic alga in Southern China. In the present study, G. lemaneiformis thalli collected from Nan’ao Island, China, were cultured in six different treatments with three carbon supply levels (20, 400 and 1000 μatm) and two nitrogen supply levels (15 and 300 μmol L⁻¹). The thalli were used in the examination of the effects of altered carbon supply and high nitrogen content in seawater on the growth, photosynthetic characteristics and amino acid (AA) content of this farmed algal species. Regardless of nitrogen levels, the relative growth rate of G. lemaneiformis increased with CO₂ supply. Decrease and increase in the carbon supply of the culture both decreased the maximum quantum yield of photosystem PS II (F_v/F_m), initial slope of the rapid light curves (ɑ), non-photochemical quenching, maximum relative electron transport rate and AA content of G. lemaneiformis thalli. Moreover, under low and high CO₂ supply conditions, the growth rates, F_v/F_m, antioxidant activities (SOD, CAT and POD) and AAs of G. lemaneiformis increases with higher nitrogen application (300 μmol L⁻¹). Our results indicated that the inhibition of growth, photosynthesis, and AA accumulation of G. lemaneiformis can be alleviated by appropriately increasing the concentration of nitrogen in seawater. We suggest that during G. lemaneiformis mariculture, the appropriate application of nitrogen fertiliser may be an effective way to increase algal yield with improved nutrient quality, and seawater nitrogen enrichment may alleviate the physiological stress caused by high CO₂ on G. lemaneiformis in the future.