Elevated CO2 Leads to Enhanced Photosynthesis but Decreased Growth in Early Life Stages of Reef Building Coralline Algae

Alexandra Ordoñez,Michael Kühl,Niclas Heidelberg Lyndby,Daniel Wangpraseurt,Guillermo Diaz-Pulido

doi:10.3389/fmars.2018.00495

Abstract

Crustose coralline algae (CCA) are key organisms in coral reef ecosystems, where they contribute to reef building and substrate stabilization. While ocean acidification due to increasing CO2 can affect the biology, physiology and ecology of fully developed CCA, the impacts of elevated CO2 on the early life stages of CCA are much less explored. We assessed the photosynthetic activity and growth of 10 days old recruits of the reef-building crustose coralline alga Porolithon c.f. onkodes exposed to ambient and enhanced CO2 seawater concentration causing a downward shift in pH of ~0.3 units. Growth of the CCA was estimated using measurements of crust thickness and marginal expansion, while photosynthetic activity was studied with O2 microsensors. We found that elevated seawater CO2 enhanced gross photosynthesis and respiration, but significantly reduced vertical and marginal growth of the early life stages of P. c.f. onkodes. Elevated CO2 stimulated photosynthesis, particularly at high irradiance, likely due to increased availability of CO2, but this increase did not translate into increased algal growth as expected, suggesting a decoupling of these two processes under ocean acidification scenarios. This study confirms the sensitivity of early stages of CCA to elevated CO2 and identifies complexities in the physiological processes underlying the decreased growth and abundance in these important coral reef builders upon ocean acidification.

Highlights

Crustose coralline algae (CCA) are abundant in tropical and temperate reef ecosystems, where they provide important ecosystem functions (Littler, 1972; Littler and Doty, 1975)
This variability in response to ocean acidification (OA) may be due to a number of reasons, including use of different experimental setups and time scales (e.g., 1 year experiment with Lithophyllum cabiochae vs. 3 months experiment with Lithophyllum incrustans Martin et al, 2013; Noisette et al, 2013), different life history stages, and flexibility of different CCA species in their use of inorganic carbon uptake strategies, i.e., use of carbon concentrating mechanisms (CCM) vs. diffusive CO2 uptake (Cornwall et al, 2012, 2015; Diaz-Pulido et al, 2016)
The impacts of ocean acidification (OA) on adult coralline algae have been relatively well documented (Hofmann and Bischof, 2014), and there is a growing body of evidence supporting the sensitivity of the early life history stages of coralline algae to OA

Summary

Introduction

Crustose coralline algae (CCA) are abundant in tropical and temperate reef ecosystems, where they provide important ecosystem functions (Littler, 1972; Littler and Doty, 1975). The available experimental evidence shows that the responses of CCA photosynthesis to OA are highly variable, including positive (Semesi et al, 2009) or negative effects (Anthony et al, 2008; Gao and Zheng, 2010; Martin et al, 2013), as well as no significant response (Noisette et al, 2013; Johnson et al, 2014; Comeau et al, 2017) This variability in response to OA may be due to a number of reasons, including use of different experimental setups and time scales (e.g., 1 year experiment with Lithophyllum cabiochae vs 3 months experiment with Lithophyllum incrustans Martin et al, 2013; Noisette et al, 2013), different life history stages (e.g., recruits vs adults), and flexibility of different CCA species in their use of inorganic carbon uptake strategies, i.e., use of carbon concentrating mechanisms (CCM) vs diffusive CO2 uptake (Cornwall et al, 2012, 2015; Diaz-Pulido et al, 2016)

Methods

Results

Conclusion