Impact of low pH/high pCO2 on the physiological response and exopolysaccharide content in cyanobacteria Trichodesmium erythraeum

The acidification of the ocean caused by the diffusion of anthropogenic carbon dioxide (CO₂) into seawater has been believed to threaten the stability of the marine ecosystem. As one of the major contributors to the primary production in oligotrophic oceans, the response of Trichodesmium to the acidification of the ocean has attracted a lot of attention. Therefore, in this study, we applied physiological and biochemical methods to identify the influences of high pCO₂ and low pH conditions on the growth of T. erythraeum. Our results showed that the low pH during the acidification of the ocean was the main factor inhibiting the growth of T. erythraeum. In addition, low pH caused oxidative stress to T. erythraeum, as evidenced by the increase of the reactive oxygen species and antioxidant enzyme’s activities. The activity of carbonic anhydrase (CA) enzyme is dually regulated by pCO₂ and pH, and T. erythraeum can adapt to different levels of pCO₂ and pH in seawater by flexibly adjusting CA enzyme activity. We also discovered that the stimulatory effect of high pCO₂ on the exopolysaccharide (EPS) content of T. erythraeum outweighed the inhibitory effect of low pH during the process of ocean acidification. In conclusion, this study systematically revealed the effects of high pCO₂ and low pH caused by the acidification of the ocean on the growth and EPS of T. erythraeum. These results provide new insights into the response mechanisms of T. erythraeum in the acidified ocean under future climate conditions.