Light and Hydrogen Sulfide Cause Multilevel Disruption of Carbon Metabolism in the Seagrass Halophila ovalis

Seagrasses are critical global carbon sinks declining at a rapid pace. Phytotoxic hydrogen sulfides (H₂S) and light deprivation are known drivers of seagrass loss worldwide; however, the underlying physiological mechanisms are not well understood. To address this knowledge gap, we explored the fate of inorganic carbon (C_i) in Halophila ovalis which were exposed to either low light (88% shade), (ii) sediment H₂S stress, or (iii) both stressors combined in a mesocosm setting. Using a novel multidisciplinary approach in combination with a ¹³C tracer (NaH¹³CO₃), we investigated differences in C_i acquisition, metabolite incorporation, and carbon translocation. C_i acquisition into seagrass leaves was impacted by both H₂S and low light stress, synergistically reducing carbon acquisition rates by 10.9-fold. The incorporation of ¹³C into leaf sugar pools was also affected by both stressors. In addition, low light impacted critical intermediates of both glycolysis and the tricarboxylic acid cycle. Below-ground data suggest that H₂S interferes with carbon translocation from the leaf into the rhizome and caused an 85% reduction in rhizome growth, irrespective of light. Overall, this study suggests that it is likely a multilevel (acquisition, metabolism, translocation) disruption of the carbon budget that threatens seagrass health and survival under both H₂S and low light stress.