Infrared gas analysis as a method of measuring seagrass photosynthetic rate in the face of desiccation stress

Photosynthesis, a core autotrophic metabolic process for aquatic and terrestrial organisms, is the backbone of the global carbon biogeochemical cycle. Inorganic assimilation of carbon in photosynthesis is relative difficult to measure in an aqueous medium since carbon readily reacts with ions in water. Therefore, aquatic photosynthesis is often measured using secondary methods that introduce uncertainty into measurements (e.g., oxygen evolution). One technique, infrared gas analysis (IRGA), uses a closed gas loop to calculate an accurate carbon budget. Multiple studies have successfully used IRGA with intertidal seagrasses, but it remains unknown how applicable the technology is for underwater plants. Here, we evaluate the potential of IRGA to mea-sure carbon assimilation of subtidal seagrasses temporarily removed from seawater, and evaluate how carbon fixation rates and chlorophyll fluorescence characteristics of subtidal seagrasses change as they desiccate. We use IRGA for four common seagrass species from the Western Gulf of Mexico (Halophila engelmannii, Halodule wrightii, Syringodium filiforme, and Thalassia testudinum) paired with pulse amplitude modulated fluorometry to measure desiccation stress. Halophila had the highest maximum carbon assimilation rate (6.06 {micro}mol C m-2s-1), followed by Thalassia (5.58 {micro}mol C m-2 s-1), Halodule (4.75 {micro}mol C m-2 s-1), and Syringodium (3.63 {micro}mol C m-2 s-1). Thalassia was most resistant to desiccation stress as reflected by the plants ability to maintain high maximum leaf quantum efficiency (Fv/Fm) while the other species were not. Additionally, Thalassia had a slower desiccation rate (2.3% min-1 cm-2) than 4.79% Syringodium filiforme (4.79% min-1 cm-2) and Halodule wrightii (30.17% min-1 cm-2). Together, our provide reasonable measures of carbon assimilation and support previous studies of seagrass desiccation stress gradients along depth. Overall, we recognize IRGA as a promising direction for future studies of seagrass productivity and recommend further investigation.