Giovanni Gaglianone

Guillem Mateu-Vicens

Luis Pomar

Marco Brandano

Marco Cuffaro

Patrizio Petricca

Vincenzo Stagno

English

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

Brandano, Marco

Pomar, Luis

MATEU VICENS, GUILLEM

Cuffaro, Marco

Gaglianone, Giovanni

Petricca, Patrizio

STAGNO, VINCENZO

Archivio della ricerca- Università di Roma La Sapienza

GENERAL COMMENTARYpublished: 07 November 2017doi: 10.3389/fenvs.2017.00074Frontiers in Environmental Science | www.frontiersin.org 1 November 2017 | Volume 5 | Article 74Edited by:Cody Sheik,University of Minnesota Duluth,United StatesReviewed by:Hildegard Westphal,Leibniz Centre for Tropical MarineResearch (LG), Germany*Correspondence:Marco Brandanomarco.brandano@uniroma1.itSpecialty section:This article was submitted toMicrobiological Chemistry andGeomicrobiology,a section of the journalFrontiers in Environmental ScienceReceived: 01 September 2017Accepted: 19 October 2017Published: 07 November 2017Citation:Brandano M, Pomar L,Mateu-Vicens G, Cuffaro M,Gaglianone G, Petricca P andStagno V (2017) Response:Commentary: Evaluating the Role ofSeagrass in Cenozoic CO2 Variations.Front. Environ. Sci. 5:74.doi: 10.3389/fenvs.2017.00074Response: Commentary: Evaluatingthe Role of Seagrass in CenozoicCO2 VariationsMarco Brandano 1, 2*, Luis Pomar 3, Guillem Mateu-Vicens 3, 4, Marco Cuffaro 2,Giovanni Gaglianone 1, Patrizio Petricca 1 and Vincenzo Stagno 11Dipartimento Scienze della Terra, Università Roma La Sapienza, Rome, Italy, 2 Istituto di Geologia Ambientale eGeoingegneria (CNR), Sez. Sapienza, Dipartimento Scienze della Terra, Università Roma La Sapienza, Rome, Italy, 3CàtedraGuillem Colom Casasnovas, Universitat de les Illes Balears, Palma de Mallorca, Spain, 4 Laboratorio de Zoología,Departament de Biologia, Universitat de les Illes Balears, Palma de Mallorca, SpainKeywords: biocalcification, seagrass, CO2, carbonate, biosequestrationA commentary onCommentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variationsby Macreadie, P. I., Serrano, O., Duarte, C. M., Beardall, J., and Maher, D. (2017). Front. Environ.Sci. 5:55. doi: 10.3389/fenvs.2017.00055We would like to acknowledge Macreadie et al. (2017) for their interest in clarifying some keypoints on the processes involved in the balance of CO2 exerted by the seagrass ecosystems duringthe Cenozoic. However, their criticisms seem to be based on somemisunderstandings regarding thebiocalcification processes. 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 carbonateproduction to result in net CO2 sequestration, in addition to the CO2 sequestration by the growthof the seagrasses. In fact, they assert “the authors have not accounted for the fact that calcificationincreases pCO2 (by depleting CO−23 and therefore reducing alkalinity), which facilitates the return ofCO2 to the atmosphere. Specifically, for every mole of CaCO3 precipitated as carbonate, the processalso consumes 2 moles of HCO−3 and releases 1 mole of CO2”.This criticism is clearly questionable and contradicts themselves (Macreadie et al., 2014) whenconsidered the C to be stored by the seagrass system not only in the organic tissues (leaves, rootsand rhizomes) but also in the inorganic skeletons (carbonates). In fact they wrote “. . . carbonates –even though they are inorganic, and carbonate formation releases net CO2 – should be included aspart of the C stock because they consist of bound C that is locked away, and might otherwise end upas CO2 in the atmosphere.”The seagrass ecosystem uses the CO2 in twomain processes: in the photosynthetic production ofthe organic tissues –leafs, roots and steams, as well as the organic parts of all associated organismsthat live in this ecosystem, and in the carbonate precipitation forming skeletons of the epiphyticbiotas. In both processes, the DIC (CO2 andHCO−3 ) is used. The organicmatter is only a temporarysink of the CO2 because after death and decay the CO2 returns to the atmosphere.It is well-known that photosynthetic prokaryotes since the Archean, prokaryotes and eukaryotesduring the Proterozoic, and more evolved forms of photosynthetic life also during the Phanerozoichave been using the CO2 derived from the Earth Mantle outgassing. Nevertheless, following theGreat Oxidation Event, around 2000Ma ago, the oxygen started to accumulate in the atmosphereand dissolved in the oceans. This reverted the reaction and the organic matter that was notburied (isolated from the oxidizing environments) became oxidized and the carbon returned toBrandano et al. Reply: Commentary.Seagrass CO2the atmosphere in the form of CO2 (1). But, if organic matteris buried, it is isolated from oxidizing processes and becomes areal sink of C, as represented by are the hydrocarbon and coaldeposits.H2O+ CO2 <−> CH2O+O2 (1)In alkaline waters, bicarbonate is the most abundant DIC species,but it is inaccessible without a source of protons (2). Diffusionfrom ambient waters can supply protons that are consumedduring photosynthesis. But as diffusion is slow, the CO2 becomesdepleted, photosynthesis is inhibited and the photosyntheticorganisms are bathed in an alkaline microenvironment. ByFIGURE 1 | “Cis” and “trans” calcification mechanisms. Photosynthetic alcalinization of the water causes cis calcification, while enzyme Ca++ ATPase drives theCa++/2H+ exchange in the trans calcification (redrawn from McConnaughey and Whelan, 1997). Cis calcification occurs on the same side of the photosyntheticcarbon uptake, whereas trans calcification occurs away of the site of carbon uptake.discharging the protons from calcification into their boundarylayers (2) and Figure 1, photosynthetic organisms can maintainor even elevate CO2 concentrations despite photosyntheticCO2 uptake (3). This enhances carboxylation efficiencies(McConnaughey and Whelan, 1997; Pomar and Hallock, 2008).Ca++ +HCO−3 −> CaCO3 +H+ (2)H+ +HCO−3 −> CH2O+O2 (3)Adding reactions (2) and (3) one obtains a 1:1 ratio ofcalcification to photosynthesis in which both calcification andorganic matter are part of the same system.Frontiers in Environmental Science | www.frontiersin.org 2 November 2017 | Volume 5 | Article 74Brandano et al. Reply: Commentary.Seagrass CO2Ca++ + 2HCO−3 −> CaCO3 + CH2O+O2 (4)The CaCO3 accumulates in the lithosphere, being the limestonesand dolostones a permanent storage form of CO2. The sameapplies to the skeletons of calcifying organisms. The only processreleasing the CO2 trapped in the carbonate rocks is the recyclingof the lithosphere in the subduction zones and the associatedvolcanism.When Macreadie et al. (2017) use the reaction (5) to illustratecalcification as a source of CO2, they ignore the stoichiometry ofthe complete system (6):Ca++ + 2HCO−3 −> CaCO3 +H2O+ CO2 (5)2CO2 + 2H2O <−> 2H2CO3 <−> 2H++ 2HCO−3 (6)As it is obvious, to release one CO2 from carbonate precipitation(5), two CO2 are needed to produce the two requiredbicarbonates (6). So, the balance of the carbonate precipitation isa net CO2 sink. This is well illustrated in the different calcificationprocesses (Figure 1)Macreadie et al. (2017) opinion that calcification representsa global CO2 source to the atmosphere, and thereforeseagrass meadows could represent a significant net CO2source is just the vision of one half of the processes, andcannot be sustained when looking at the entire carboncycle.As result of these biased misconceptions, the authorsenter in continuous contradictions. Their estimations fromMediterranean case histories made them to conclude thatcalcification in P. oceanica meadows could be responsible forthe emission of 0.7 to 4.2 Tg C yr−1 to the atmosphere (Mateoand Serrano, 2012). So why are people promoting to restoreand protect P. oceanica meadows, in order to fulfill their keyrole in carbon cycle and storage by adopting efficient regulationof human pressures such as fishing? Some of the coauthorsof the Macreadie et al. (2017) commentary (e.g., Fourqureanet al., 2012) state that the rapid loss of seagrasses resulted in asubstantial decrease in C sequestration by seagrass ecosystems of6 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 sourceof CO2? If they conclude seagrasses are a source, then we shouldreview the entire evolution of Earth and Life.AUTHOR CONTRIBUTIONSMB, LP, and GM led the paper. All authors contributed to thediscussion.ACKNOWLEDGMENTSCriticisms by reviewer Hildegard Westphal and Associate EditorCody Sheik greatly improved the manuscript.REFERENCESBrandano, M., Cuffaro, M., Gaglianone, G., Pettricca, P., Stagno, V., and Mateu-Vicens, G. (2016). Evaluating the role of seagrass in Cenozoic CO2 variations.Front. Environ. Sci. 4:72. doi: 10.3389/fenvs.2016.00072Fourqurean, J. W., Duarte, C. M., Kennedy, H., Marbà, N., Holmer, M., Mateo,M. A., et al. (2012). Seagrass ecosystems as a globally significant carbon stock.Nature Geosci. 5: 505–509. doi: 10.1038/ngeo1477Macreadie, P. I., Baird, M. E., Trevathan-Tackett, S. M., Larkum, A. W. D.,and Ralph, P. J. (2014). Quantifying and modelling the carbon sequestrationcapacity of seagrass meadows: a critical assessment. Mar. Pollut. Bull. 83,430–439. doi: 10.1016/j.marpolbul.2013.07.038Macreadie, P. I., Serrano, O., Duarte, C. M., 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.2017.00055Mateo, M. A., and, O., Serrano (2012). The Carbon Sink Associated to Posidoniaoceanica. Switzerland and Málaga: IUCN, Gland.McConnaughey, T. A., and Whelan, J. F. (1997). Calcification generates protonsfor nutrient and bicarbonate uptake. Earth Sci. Rev. 42, 95–117.Pomar, L., and Hallock, P. (2008). Carbonate factories: aconundrum in sedimentary geology. Earth Sci. Rev. 87, 134–169.doi: 10.1016/j.earscirev.2007.12.002Conflict of Interest Statement: The authors declare that the research wasconducted in the absence of any commercial or financial relationships that couldbe construed as a potential conflict of interest.Copyright © 2017 Brandano, Pomar, Mateu-Vicens, Cuffaro, Gaglianone, Petriccaand Stagno. This is an open-access article distributed under the terms of the CreativeCommons Attribution License (CC BY). The use, distribution or reproduction inother forums is permitted, provided the original author(s) or licensor are creditedand that the original publication in this journal is cited, in accordance with acceptedacademic practice. No use, distribution or reproduction is permitted which does notcomply with these terms.Frontiers in Environmental Science | www.frontiersin.org 3 November 2017 | Volume 5 | Article 74

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