Commentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variations

Abstract

A commentary onEvaluating the Role of Seagrass in Cenozoic CO 2 Variations by Brandano, M., Cuffaro, M., Gaglianone, G., Pettricca, P., Stagno, V., and Mateu-Vicens, G. (2016). Front. Environ. Sci. 4:72. doi: 10.3389/fenvs.2016.00072 Brandano et al. (2016 sought to quantify the role of seagrasses in removing atmospheric CO 2 during the past 65 million years. To date, this estimate has been missing from the literature. Moreover, as the authors point out, there has so far been little attention paid to the role of calcium carbonate formation (CaCO 3 ; inorganic carbon precipitated by calcifying organisms) in seagrass carbon budgets; much of the literature has focused on organic carbon only. The authors conclude that seagrasses have had globally-significant impacts on atmospheric CO 2 fluxes throughout the Cenozoic era. While we appreciate the ambitious nature and difficulty of the study, we argue that the authors have made fundamental misconceptions about the contribution of carbonate production (calcification) and sequestration to ocean carbon budgets.The authors have not accounted for the fact that calcification increases pCO 2 (by depleting CO 2− 3 and therefore reducing alkalinity), which facilitates the return of CO 2 to the atmosphere (Frankignoulle et al., 1994). Specifically, for every mole of CaCO 3 precipitated as carbonate, the process also consumes 2 moles of HCO − 3 and releases 1 mole of CO 2 :

Response: Commentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variations

“…So, the balance of the carbonate precipitation is a net CO 2 sink. This is well illustrated in the different calcification processes (Figure 1) Macreadie et al (2017) opinion that calcification represents a global CO 2 source to the atmosphere, and therefore seagrass meadows could represent a significant net CO 2 source is just the vision of one half of the processes, and cannot be sustained when looking at the entire carbon cycle.…”

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We would like to acknowledge Macreadie et al (2017) for their interest in clarifying some key points on the processes involved in the balance of CO 2 exerted by the seagrass ecosystems during the Cenozoic. However, their criticisms seem to be based on some misunderstandings regarding the biocalcification processes.

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“…So we take this opportunity to make clear some concepts on this topic. Macreadie et al (2017) criticize Brandano et al (2016) when considering the epiphytic carbonate production to result in net CO 2 sequestration, in addition to the CO 2 sequestration by the growth of the seagrasses. In fact, they assert "the authors have not accounted for the fact that calcification increases pCO 2 (by depleting CO −2 3 and therefore reducing alkalinity), which facilitates the return of CO 2 to the atmosphere.…”

“…The only process releasing the CO 2 trapped in the carbonate rocks is the recycling of the lithosphere in the subduction zones and the associated volcanism. When Macreadie et al (2017) use the reaction (5) to illustrate calcification as a source of CO 2 , they ignore the stoichiometry of the complete system (6):…”

“…So why are people promoting to restore and protect P. oceanica meadows, in order to fulfill their key role in carbon cycle and storage by adopting efficient regulation of human pressures such as fishing? Some of the coauthors of the Macreadie et al (2017) commentary (e.g., Fourqurean et al, 2012) state that the rapid loss of seagrasses resulted in a substantial decrease in C sequestration by seagrass ecosystems of 6 to 24 Tg C yr −1 . So the authors of the Macreadie et al (2017) paper have to take a decision: are seagrasses a sink or a source of CO 2 ?…”