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General Commentary ARTICLE

Front. Environ. Sci., 29 August 2017 | https://doi.org/10.3389/fenvs.2017.00055

Commentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variations

  • 1Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, Burwood, VIC, Australia
  • 2Centre for Marine Ecosystems Research, School of Sciences, Edith Cowan University, Joondalup, WA, Australia
  • 3Red Sea Research Center, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
  • 4School of Biological Sciences, Monash University, Clayton, VIC, Australia
  • 5Southern Cross Geoscience, Southern Cross University, Lismore, NSW, Australia

A commentary on
Evaluating the Role of Seagrass in Cenozoic CO2 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 CO2 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 (CaCO3; 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 CO2 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 pCO2 (by depleting CO32- and therefore reducing alkalinity), which facilitates the return of CO2 to the atmosphere (Frankignoulle et al., 1994). Specifically, for every mole of CaCO3 precipitated as carbonate, the process also consumes 2 moles of HCO3- and releases 1 mole of CO2:

Ca2++2HCO3CaCO3 dissolutionCalcificationCaCO3+H2O+CO2

However, the stoichiometry of this reaction is complicated by buffering effects in seawater (Frankignoulle et al., 1995), leading to less than 1 mole of CO2 being released to the atmosphere. This ratio is ~0.63 under current atmospheric CO2 concentrations (known as the “rule of the 0.6”; Ware et al., 1992), but is predicted to increase at higher atmospheric CO2 concentrations (Suzuki, 1998), with unaddressed implications for the CO2 atmospheric fluxes since the Cenozoic reported by Brandano et al. (2016) (their Figure 4). Hence, carbonate production results in CO2 release, but the authors do not account for this. Instead, the authors treat carbonate production as resulting in net CO2 sequestration, which they add to the CO2 sink capacity of seagrasses, when in reality it needs to be treated as a CO2 source.

Previous studies have pointed out that although Posidonia meadows (as well as other seagrass species; Mazarrasa et al., 2015) host significant CaCO3 stocks and accumulation rates, it has been shown that calcification represents a global CO2 source to the atmosphere (Smith and Gattuso, 2009), and therefore seagrass meadows (all species) could represent a significant net CO2 source (Mateo and Serrano, 2012; Serrano et al., 2012). A global estimate for the entire Mediterranean indicates that calcification in P. oceanica meadows could be responsible for the emission of 0.7 to 4.2 Tg C year−1 to the atmosphere (Mateo and Serrano, 2012). Indeed, they conclude that the net carbon balance between the organic carbon (CO2 sink) and the inorganic carbon (CO2 source) pools in Tyrrhenian P. oceanica meadows could range from −7.4 to +3.9 Tg C year−1. Coral reefs are also not considered natural atmospheric CO2 sinks because production and respiration are balanced (i.e., there is no net accumulation of organic carbon) and as a result of high calcification rates in many reef systems, they are considered a net source of CO2 to the atmosphere (e.g., Ware et al., 1992; Gattuso et al., 1995).

We acknowledge the complexity of estimating the CO2 sequestration capacity of ecosystems over geological time scales, and the multiple assumptions on which such type of estimates are based. However, we feel that their analysis is weakened by not taking proper account of the chemistry of carbonate formation and that accordingly the values they report must represent an overestimate of the true rates of draw-down of CO2.

Author Contributions

PM led the paper. All authors contributed to the writing.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

PM was supported by an Australian Research Council DECRA Fellowship (DE130101084) and a Linkage Project (LP160100242). OS was supported by the CSIRO Flagship Marine and Coastal Carbon Biogeochemical Cluster and an ARC DECRA Fellowship DE170101524. DM is supported by an ARC DECRA Fellowship (DE150100581). CD is supported by baseline funding from KAUST.

References

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Keywords: seagrass, CO2, biosequestration, ecosystem, coast, carbonate, blue carbon, calcification

Citation: Macreadie PI, Serrano O, Duarte CM, Beardall J and Maher D (2017) Commentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variations. Front. Environ. Sci. 5:55. doi: 10.3389/fenvs.2017.00055

Received: 07 December 2016; Accepted: 18 August 2017;
Published: 29 August 2017.

Edited by:

Cody Sheik, University of Minnesota Duluth, United States

Reviewed by:

Anna R. Armitage, Texas A&M University at Galveston, United States

Copyright © 2017 Macreadie, Serrano, Duarte, Beardall and Maher. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Peter I. Macreadie, p.macreadie@deakin.edu.au