River outflow into the Gulf of Mexico and intensification of precipitation periods may contribute to increased variability of salinity regimes in the future Gulf coastal and marine oceans. Increased eutrophication as a result of agricultural runoff leading into oceans, along with increases in turbidity, are also expected to occur in coastal areas of the Gulf of Mexico. Co-occurring with changes in lighting environment due to shifts in depth associated with possible sea level rise, these environmental stressors are likely to influence the future physiological adaptations and carbon acquisition strategies of seagrasses and other photosynthetic organisms. We studied the photo-physiology of the seagrass Thalassia testudinum (Banks & Sol. ex K. D.) in response to water browning, elevated nutrients, and low salinity crossed with contrasting light regimes. Water environments were simulated outdoors within 30-L buckets filled with free-flowing natural seawater from the warm temperate North Florida Gulf coast. In addition to examining the respiration and photosynthetic stress responses of T. testudinum, we quantified the relative use of carbon concentrating mechanisms (CCMs) in response to these climatic stressors and light regimes. We also examined chlorophyll fluorescence of seaweed samples within the various treatments.
Results/Conclusions
We observed a marginally significant decrease in the respiration of samples within the high light regimes, and a significant decrease in photosynthetic oxygen production of seagrass in the low salinity environmental treatments. There was also a significant decrease in photosynthetic oxygen production within the low salinity environmental treatment following the addition of CCM inhibitors. There were no significant interactions between environmental treatments and the contrasting light regimes on the photosynthetic responses of the seagrasses. Possible deregulation of CCMs and increased reliance on passive CO2 diffusion as a source of carbon for photosynthesis use may have occurred due to the increased metabolic costs of maintaining CCM machinery in response to the lower salinity treatment. Seagrass ecosystems are important not only for the sequestration of carbon and nutrients in oceans but also serve as important nursery habitat for many juvenile marine species. Understanding the links between CCM function and photosynthetic metabolic responses will be important for interpreting seagrass habitat responses to fluctuating dissolved inorganic carbon (DIC) in future coastal and marine oceans.