Contrasting effects of ocean acidification on tropical fleshy and calcareous algae.

Maggie Dorothy Johnson,Nichole N Price,Jennifer E Smith

doi:10.7717/peerj.411

Abstract

Despite the heightened awareness of ocean acidification (OA) effects on marine organisms, few studies empirically juxtapose biological responses to CO2 manipulations across functionally distinct primary producers, particularly benthic algae. Algal responses to OA may vary because increasing CO2 has the potential to fertilize photosynthesis but impair biomineralization. Using a series of repeated experiments on Palmyra Atoll, simulated OA effects were tested across a suite of ecologically important coral reef algae, including five fleshy and six calcareous species. Growth, calcification and photophysiology were measured for each species independently and metrics were combined from each experiment using a meta-analysis to examine overall trends across functional groups categorized as fleshy, upright calcareous, and crustose coralline algae (CCA). The magnitude of the effect of OA on algal growth response varied by species, but the direction was consistent within functional groups. Exposure to OA conditions generally enhanced growth in fleshy macroalgae, reduced net calcification in upright calcareous algae, and caused net dissolution in CCA. Additionally, three of the five fleshy seaweeds tested became reproductive upon exposure to OA conditions. There was no consistent effect of OA on algal photophysiology. Our study provides experimental evidence to support the hypothesis that OA will reduce the ability of calcareous algae to biomineralize. Further, we show that CO2 enrichment either will stimulate population or somatic growth in some species of fleshy macroalgae. Thus, our results suggest that projected OA conditions may favor non-calcifying algae and influence the relative dominance of fleshy macroalgae on reefs, perpetuating or exacerbating existing shifts in reef community structure.

Highlights

Changes in ocean chemistry associated with anthropogenic carbon dioxide emissions, a process known as ocean acidification (OA) (Kleypas et al, 1999; Orr et al, 2005), have raised widespread concern for the survival and persistence of marine biota (Kleypas et al, 1999; Hoegh-Guldberg et al, 2007)
Algae were categorized into three functional groups: fleshy macroalgae (Acanthophora spicifera, Caulerpa serrulata, Dictyota bartayresiana, Hypnea pannosa, and Avrainvillea amadelpha), upright calcareous algae (Halimeda taenicola, Halimeda opuntia, Galaxaura rugosa, and Dichotomaria marginata), and crustose coralline algae (CCA: Lithophyllum prototypum, formerly Titanoderma prototypum, and Lithophyllum sp.) (Fig. 1, Table S1)
Experimental conditions CO2 enrichment treatments effectively simulated near future seawater carbonate chemistry and OA as compared to present-day ambient air controls (Table 1)

Summary

Introduction

Changes in ocean chemistry associated with anthropogenic carbon dioxide (pCO2) emissions, a process known as ocean acidification (OA) (Kleypas et al, 1999; Orr et al, 2005), have raised widespread concern for the survival and persistence of marine biota (Kleypas et al, 1999; Hoegh-Guldberg et al, 2007). The changes in the carbonate system have important implications for marine calcifiers, namely that OA may inhibit the ability of these species to grow, develop, reproduce and sustain themselves within a community, plasticity in organismal responses indicates that some species may have wider tolerance limits (Doney et al, 2009; Kroeker et al, 2010; Kroeker et al, 2013; Johnson, Moriarty & Carpenter, 2014). Less attention has been given to the response of tropical marine primary producers to rising oceanic CO2, fleshy and calcareous benthic macroalgae which are among the most dominant constituents of the coral reefs benthos

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