Ocean acidification and marine microorganisms : responses and consequences

Oceanologia (2015) 57, 349—361 Available online at www.sciencedirect.com ScienceDirect j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / o c e a n o ORIGINAL RESEARCH ARTICLE Ocean acidification and marine microorganisms: responses and consequences§ Surajit Das *, Neelam Mangwani Department of Life Science, National Institute of Technology, Rourkela, India Received 22 May 2014; accepted 1 July 2015 Available online 4 August 2015 KEYWORDS Ocean acidification; Climate change; Marine microorganisms; Ecosystem; Mesocosm Summary Ocean acidification (OA) is one of the global issues caused by rising atmospheric CO2. The rising pCO2 and resulting pH decrease has altered ocean carbonate chemistry. Microbes are key components of marine environments involved in nutrient cycles and carbon flow in marine ecosystems. However, these marine microbes and the microbial processes are sensitive to ocean pH shift. Thus, OA affects the microbial diversity, primary productivity and trace gases emission in oceans. Apart from that, it can also manipulate the microbial activities such as quorum sensing, extracellular enzyme activity and nitrogen cycling. Short-term laboratory experiments, mesocosm studies and changing marine diversity scenarios have illustrated undesirable effects of OA on marine microorganisms and ecosystems. However, from the microbial perspective, the current understanding on effect of OA is based mainly on limited experimental studies. It is challenging to predict response of marine microbes based on such experiments for this complex process. To study the response of marine microbes towards OA, multiple approaches should be implemented by using functional genomics, new generation microscopy, small-scale interaction among organisms and/or between organic matter and organisms. This review focuses on the response of marine microorganisms to OA and the experimental approaches to investigate the effect of changing ocean carbonate chemistry on microbial mediated processes. # 2015 Institute of Oceanology of the Polish Academy of Sciences. Production and hosting by Elsevier Sp. z o.o. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). § N.M. gratefully acknowledges the research fellowship from Ministry of Human Resource Development, Government of India for doctoral research. * Corresponding author at: Laboratory of Environmental Microbiology and Ecology (LEnME), Department of Life Science, National Institute of Technology, Rourkela 769 008, Odisha, India. Tel.: +91 661 246 2684; fax: +91 661 246 2022. E-mail addresses: , (S. Das). Peer review under the responsibility of Institute of Oceanology of the Polish Academy of Sciences. http://dx.doi.org/10.1016/j.oceano.2015.07.003 0078-3234/# 2015 Institute of Oceanology of the Polish Academy of Sciences. Production and hosting by Elsevier Sp. z o.o. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 350 1. Introduction Human activities such as burning of fossil fuels and industrialization have resulted in rising atmospheric CO2 concentration. Emission of CO2 causing increasing concentration of CO2 in the atmosphere is one of the major drivers of global warming as well as seawater carbonate chemistry. Oceans play a very important role in the global carbon cycle and Earth's climate system (Chavez et al., 2011). The uptake of anthropogenic CO2 from the atmosphere by ocean physics and biology has already led to substantial changes in the ocean carbon cycle, with potentially larger changes looming ahead (Reid et al., 2009; Takahashi et al., 2012). Oceans act as a reservoir for CO2 and there is a flux of CO2 across the interface between the atmosphere and ocean surface. Uptake of CO2 by the ocean is an essential buffering process of seawater, however, it also alters the chemistry of the seawater at a fundamental level. Increase of CO2 in the ocean and a decline in ocean pH, thus, promoting one of the most critical events known as ocean acidification (OA) (Raven, 2005). The average ocean surface water pH has fallen by approximately 0.1 unit over about the past 200 years (Raven, 2005) and is expected to decrease a further 0.3—0.4 unit if atmospheric CO2 concentrations reach 800 ppmv (Orr et al., 2005) against the present concentration of 397 ppmv. The subsequent impact of OA on marine life has become one of the most important issues. The chemical changes that occur when CO2 is absorbed by the ocean result in formation of carbonic acid which decreases seawater pH, carbonate ion concentration and calcium carbonate saturation (Lohbeck et al., 2012). Maintenance of appropriate carbonate ion saturation is essential for the formation of calcium carbonate, which is the basic building block of skeletons and shells of a large number of marine organisms, including corals, shellfish and plankton (Doney et al., 2009a; Hoegh-Guldberg et al., 2007). The marine environment covers more than 70% of the total earth surface, which encompasses a diverse set of habitats ranging from tropical, shallow water coral-reef to deep ocean trenches. Within these habitats, millions of organisms survive which include many autotrophs, animals and both autotrophic and heterotrophic microorganisms (Dash et al., 2013; Pomeroy et al., 2007). The impact of OA has been extensively studied in calcifying marine organisms to understand the impact on calcification process. However, the consequences of OA also affect the marine microorganisms that are responsible for the net productivity of the ocean. Microbes are key component of marine biogeochemical cycles which are involved in nutrient cycles, organic matter decomposition and carbon flow in the marine ecosystem (Arnosti, 2011; Azam and Malfatti, 2007). So far, deleterious effects of OA have not been studied in marine microbes in detail, although, OA is modulating many crucial activities of microbes (such as N2 fixation, primary production, trace gas emission and extracellular enzyme activities). This review discusses the effect of OA on ocean carbonate chemistry and marine microbially-mediated processes. It also covers the experimental models and approaches that can be used to study the response of marine microbes towards changing ocean chemistry. S. Das, N. Mangwani 2. Chemistry of ocean acidification: ocean carbonate system A series of chemical reactions control seawater carbonate chemistry, which in turn interacts with atmospheric pCO2. The ocean acts as a sink of CO2 and once dissolved in seawater, CO2 reacts with water to form carbonic acid (H2CO3). However, ocean stores CO2 as dissolved inorganic carbon (DIC) which remains in the form of dissolved CO2 and H2CO3 (1%) while the rest is in the form of HCO3
(90%) and CO32
(9%) (Brewer, 1997; Rost et al., 2008; Wolf-Gladrow et al., 1999). Adding CO2 to seawater, thus increase HCO3
that bring about a decrease in ocean wate (...truncated)