Predicting carbon isotope discrimination in Eelgrass (Zostera marina L.) from the environmental parameters—light, flow, and [DIC

Abstract

AbstractIsotopic discrimination against 13C during photosynthesis is determined by a combination of environmental conditions and physiological mechanisms that control delivery of CO2 to RUBISCO. This study investigated the effects of light, flow, dissolved inorganic carbon (DIC) concentration, and its speciation, on photosynthetic carbon assimilation of Zostera marina L. (eelgrass) using a combination of laboratory experiments and theoretical calculations leading to a mechanistic understanding of environmental conditions that influence leaf carbon uptake and determine leaf stable carbon isotope signatures (δ13C). Photosynthesis was saturated with respect to flow at low velocity (∼ 3 cm s−1), but was strongly influenced by [DIC], and particularly aqueous CO2 (CO2(aq)) under all flow conditions. The non‐linear responses of light‐ and flow‐saturated photosynthesis to [DIC] were used to quantify the maximum physiological capacity for photosynthesis, and to determine the degree of photosynthetic carbon limitation for light‐saturated photosynthesis, which provided a mechanistic pathway for modeling regulation of carbon uptake and 13C discrimination. Model predictions of δ13C spanned the typical range of values reported for a variety of seagrass taxa, and were most sensitive to [DIC] (predominantly [CO2(aq)]) and flow, but less sensitive to DIC source [CO2(aq) vs. ]. These results provide a predictive understanding of the role of key environmental parameters (light, flow, and DIC availability) can have in driving δ13C of seagrasses, which will become increasingly important for predicting the response of these ecosystem engineers to local processes that affect light availability and flow, as well as global impacts of climate warming and ocean acidification in the Anthropocene.