MEPS 671:111-128 (2021)  -  DOI: https://doi.org/10.3354/meps13777

ABSTRACT: Seagrasses account for approximately 10% of the total carbon stored in the ocean, although photosynthesis of seagrasses is carbon-limited at present oceanic pH levels. 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, but the acclimation of photoprotective mechanisms in this context has not been characterized. We 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 aquaria at different aqueous CO₂ concentrations ranging from ~50 to ~2100 μM from May 2013 to October 2014 and examined for differences in leaf optical properties. 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 acclimation responses suggest that a common photosynthetic sensory function, such as redox regulation, can be manipulated by CO₂ availability, as well as light, and may serve to optimize photosynthetic carbon gain by seagrasses into the Anthropocene.

KEY WORDS: Ocean acidification · Photoprotection · Photosynthesis · Pigments · Seagrass · Temperature · Light