Review
Two decades of seawater acidification experiments on tropical scleractinian corals: Overview, meta-analysis and perspectives

https://doi.org/10.1016/j.marpolbul.2022.113552Get rights and content

Highlights

  • Seawater acidification experiments on tropical scleractinians are relatively recent.

  • Seawater acidification mainly affects calcification and reproduction capacities.

  • Knowledge is biased with a majority of work on adult and shallow-water corals.

  • Geographical bias: acidification experiments performed in only 18% of coral ecoregions.

  • We recommend a list of actions and priorities for future research in this field.

Abstract

Ocean acidification has emerged as a major concern in the last fifteen years and studies on the impacts of seawater acidification on marine organisms have multiplied accordingly. This review aimed at synthesizing the literature on the effects of seawater acidification on tropical scleractinians under laboratory-controlled conditions. We identified 141 articles (published between 1999 and 2021) and separated endpoints into 22 biological categories to identify global trends for mitigation and gaps in knowledge and research priorities for future investigators. The relative number of affected endpoints increased with pH intensity (particularly for endpoints associated to calcification and reproduction). When exposed to pH 7.6–7.8 (compared to higher pH), 49% of endpoints were affected. The diversity in experimental designs prevented deciphering the modulating role of coral life stages, genera or duration of exposure. Finally, important bias in research efforts included most experiments on adult corals (68.5%), in 27 out of 150 (18%) coral ecoregions and exclusively from shallow-waters.

Introduction

Due to the increasing emissions of carbon dioxide into the atmosphere, the Ocean is warming and getting more acidic. Ocean acidification (OA) has been identified as a global environmental threat and included as the United Nations' Sustainable Development Goal 14.3, as well as one of the nine planetary boundaries (Jagers et al. 2019; Rockström et al., 2009). OA impacts on marine species and ecosystems are well documented, including effect on marine calcifiers, threatening coral reefs as well as broader marine ecosystems (Doney et al., 2009; Feely, 2004; Gattuso and Hansson, 2011; Hendriks et al., 2010; Jiang et al., 2019; Kroeker et al., 2010, Kroeker et al., 2013; Orr et al., 2005).

Most of the available knowledge on the effects of OA on marine organisms comes from short-term laboratory experiments on isolated organisms (Kroeker et al., 2010). Fixed-term studies have the disadvantage of potentially under/overestimate the effects of OA, as some taxa may show vulnerability/acclimatization in the long-term (Fantazzini et al., 2015). Similarly, other life stages than those considered may show different vulnerability and controlled conditions does not consider the indirect effects due to OA-driven ecological changes (Fabricius et al., 2011). In this context, knowledge acquired through field experiments taking advantage of organisms that are naturally exposed to OA (e.g. CO2 vent systems) are very complementary as they account for the life-long acclimatization of organisms. These studies already showed that OA may change reef community composition and metabolism (Biscéré et al., 2019; Noonan et al., 2018). They also reported effects on skeletal porosity (Prada et al., 2021) and a variety of responses to OA on calcification rate, suggesting species-specific acclimatization to OA (Strahl et al., 2015).

Coral reefs have received particular attention as they are among the most severely threatened ecosystem on Earth (Pandolfi, 2003; Raven, 2005). The effects of seawater acidification on corals have been extensively studied (Chan and Connolly, 2013; Erez et al., 2011), but responses often differ between studies depending on parameters such as tested populations, species and life-cycle stages (Kawahata et al., 2019). These apparent conflicting results in the literature could also be attributed to variations in experimental designs and methodologies. In this study, we reviewed the literature that examined the effects of decreased pH on tropical scleractinian corals under laboratory-controlled conditions. A meta-analysis based on 169 experiments conducted in 141 peer-reviewed articles published since 1999 was performed, with four main objectives:

  • (1)

    To provide a semi-quantitative description of the evolution of research efforts testing the effects of seawater acidification on tropical scleractinian corals,

  • (2)

    To evaluate the effect of seawater acidification on coral biological functions,

  • (3)

    To investigate the modulating influence of exposure duration, coral life stages and coral genera on sensitivity to seawater acidification and,

  • (4)

    To highlight research gaps and bias.

Based on this review, we provide recommendations and perspectives for future research and an updated baseline for scientists starting in this field of research.

Section snippets

Material and methods

The code used in this manuscript was based on Jacob et al. (2020): https://doi.org/10.5281/zenodo.3694955.

Literature trend

In our database, we identified 169 experiments in 141 articles (listed in Supplementary material) that have evaluated the effects seawater acidification under laboratory-controlled conditions on tropical scleractinian corals. The six best studied regions account for 69% of the studies: Australia (15%), Japan (14%), French Polynesia (14%), Hawaii (10%), the USA (excluding Hawaii, 8%) and Taiwan (8%; Fig. 2A). Six percent of the studies were carried out on corals that have been maintained and

Research gaps and perspectives

Based on all aforementioned points, we selected a list of priorities and actions that should be addressed for future experimental works investigating the effects of seawater acidification on tropical scleractinians (Table 3).

Funding

This work was supported by the “Fondation de France” for a project called “ACID REEFS”, by the “Ministère de la Transition Ecologique et Solidaire” and the Foundation for Research on Biodiversity (FRB) for a project entitled “ACID REEFS2”.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Jacob et al. (2020) for making their R scripts available and Dr. Moreau C. for his help in making Fig. 1. This work was funded by the Fondation de France for a project called “ACID REEFS”, by Ministère de la Transition écologique et Solidaire and the Foundation for Research on Biodiversity (FRB) for a project entitled “ACID REEFS”. The IAEA is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco.

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