Physiological response of a golden tide alga (<i>Sargassum muticum</i>) to the interaction of ocean acidification and phosphorus enrichment

The evolvement of golden tides would be influenced by global change factors, such as ocean acidification and eutrophication, but the related studies are very scarce. In this study, we cultured a golden tide alga, Sargasssum muticum, at two levels of pCO2 (400, 1000 µatm) and phosphate (0.5 µM, 40 µM) conditions to investigate the interactive effects of elevated pCO2 and phosphate on physiological properties of the thalli. The higher pCO2 level and phosphate (P) level alone increased the relative growth rate by 40.82 % and 47.78 %, net photosynthetic rate by 46.34 % and 55.16 %, soluble carbohydrates by 32.78 % and 61.83 % respectively whilst the combination of these two levels did not promote growth or soluble carbohydrates further. The higher levels of pCO2 and P alone also enhanced the nitrate uptake rate by 68.27 % and 35.89 %, nitrate reductase activity by 89.08 % and 39.31 %, and soluble protein by 19.05 % and 15.13 % respectively. The nitrate uptake rate and soluble protein was further enhanced although the nitrate reductase activity was reduced when the higher levels of pCO2 and P worked together. The higher pCO2 level and higher P level alone did not affect the dark respiration rate of thalli but they together increased it by 32.30 % compared to the condition of the lower pCO2 and lower P. The mute effect of the higher level of pCO2 and higher P on growth, soluble carbohydrates, combined with the promoting effect of it on soluble protein and dark respiration, suggests more energy was drawn from carbon assimilation to nitrogen assimilation at the condition of higher pCO2 and higher P, probably to act against the higher pCO2 caused acid-base perturbation via synthesizing H+ transport-related protein. Our results indicate ocean acidification and eutrophication may not boost the gold tides events synergistically although each of them alone has a promoting effect.