Potential Impacts of Climate Change on Photochemistry of Zostera Marina L.

Abstract

Seagrasses account for approximately 10% of the ocean’s total carbon storage, although photosynthesis of seagrasses is carbon limited at today’s oceanic pH. Therefore, increasing atmospheric CO2 concentration, which results in ocean acidification/carbonation, is predicted to have a positive impact on seagrass productivity. Previous studies have confirmed the positive influence of increasing CO2 on photosynthesis and survival of the temperate eelgrass Zostera marina L., but the acclimation of photoprotective mechanisms in this context has not been characterized. This study aimed to quantify the long-term impacts of ocean acidification on photochemical control mechanisms that promote photosynthesis while simultaneously protecting eelgrass from photodamage. Eelgrass were grown in controlled outdoor aquarium tanks at different aqueous CO2 concentrations ranging from ~50 to ~2100 μM from May 2013 to October 2014, and compared for differences in optical properties and photochemistry. Even with daily and seasonal variations of temperature and light, CO2 enrichment consistently increased plant size, leaf thickness and chlorophyll use efficiency, and decreased pigment content and the package effect while maintaining similar light harvesting efficiency. These CO2 responses resembled high light acclimation suggesting a common photosynthetic sensory function, such as redox regulation, controls long-term acclimation of leaf morphology. Laboratory incubations resolved this mutual regulation of redox state via carbon and light availability, by measuring O2 production, total CO2 uptake and fluorescence of the acclimated leaves. The morphological acclimations due to CO2 enrichment were facilitated by improved photosynthetic capacity. Increasing CO2 availability, relative to oxygen concentrations, maximized chlorophyll specific photosynthesis to its physiological limits at pH 6.2 by minimizing photorespiration, and increased the light requirement to saturate photosynthesis. The instantaneous increase of photosynthesis up to 8 fold reduced the role of alternative electron pathways and non-photochemical quenching for photoprotection, therefore increasing quantum yield of oxygen production. These findings explained how seagrasses resist photodamage in shallow high light environments, while maintaining long daily period of light-saturated photosynthesis to compensate carbon limitation and sustain growth. The quasi-mechanistic models generated by this study provide a pathway for including the photoprotection and photoacclimation processes in understanding the dynamic response of seagrasses to fluctuating coastal environments and climate change.