Effects of increasing temperature on the photosynthetic activity and oxygen balance of sheath-covered seagrass Zostera marina seeds

ABSTRACT: The seed sheaths of eelgrass Zostera marina L. have recently been shown to possess photosynthetic capacity that can alleviate intra-seed hypoxic conditions and thereby enhance biosynthetic activity. However, nothing is known about how increasing seawater temperatures affect physiological responses in developing Z. marina seeds. We used an optical multi-analyte sensor system in combination with O₂-sensitive sensor spots to measure rates of photosynthesis and dark respiration within custom-made gas exchange chambers. Exponential saturation models were then applied to determine key photosynthetic parameters, such as maximum photosynthesis rate, light use efficiency, saturating photon irradiance (E_K), compensation photon irradiance, and net diel O₂ budget. Our results showed that both photosynthetic activity and dark respiration rates in sheath-covered seeds increased with increasing seawater temperature (from 10 to 25°C) but with a 2-fold stronger response in dark respiration than in gross photosynthesis over the measured temperature range. These temperature responses resulted in increasing light requirements (from 47 to 183 µmol photons m^-2 s^-1 in E_K) and decreasing net diel O₂ budgets (from -5.4 to -126 nmol O₂ mg wet weight [WW]^-1 h^-1) of the eelgrass seeds with increasing temperature. Eelgrass seed exposure to high temperature led thus to highly reduced net diel O₂ balances, which is expected to have detrimental effects on seed development and germination success owing to negative effects on synthesis rates of storage products in the endosperm.