Keywords

5.1 Introduction

Ocean acidification is a global threat to the world’s oceans, estuaries, and rivers. It is projected to grow as carbon dioxide (CO2) and continues to be emitted into the atmosphere at record-high levels. The oceans take up CO2 from the atmosphere and are responsible for absorbing around a third of the CO2 emitted by fossil fuel burning, deforestation, and cement production since the industrial revolution (Sabine et al. 2004). While this is beneficial in terms of limiting the rise in atmospheric CO2 concentrations and hence greenhouse warming due to this CO2, there are direct consequences for ocean chemistry. Ocean acidification describes the lowering of seawater pH and carbonate saturation that result from increasing atmospheric CO2 concentrations. There are also indirect and potentially adverse biological and ecological consequences of the chemical changes taking place in the ocean now and as projected into the future (Ridgwell and Schmidt 2010).

Climate affects diatoms in complex ways. As the planet warms due to the increase in carbon dioxide, scientists predict that diatoms will decrease compared to other planktons such as coccolithophores and cyanobacteria (Tatters et al. 2013). In lakes and rivers, a changing climate alters river flow in many parts of the world. The frequency and severity of droughts and floods is changing, which influences diatom species and where they grow. Furthermore, climate controls circulation patterns and thermal stratification of lakes and oceans, which alter diatom species composition (Bach and Taucher 2019). Diatoms affect climate on a global scale. As diatoms photosynthesize, they absorb carbon dioxide from the atmosphere and release oxygen. Although diatoms are very small, they live in the vast oceans, the world over. The effect of fixation of carbon by diatoms and release of oxygen alters the chemistry of the atmosphere. It is well-known that diatoms play a vital role in pelagic food webs and elemental cycling in the oceans (Smetacek et al. 2012). Therefore, understanding the effects of global warming on diatom community structure is essential for considering the sensitivity of biogeochemical cycles and ecosystem services in the world oceans.

5.2 Ocean Acidification

Ocean acidification is the ongoing decrease in the pH value of the Earth’s oceans, caused by the uptake of CO2 from the atmosphere. Friedlingstein et al. (2020) stated that the rate of atmospheric CO2 levels has persisted to increase and nearly tripled between the 1960s and 2010s. Oceans have mitigated this growth by absorbing about a quarter of CO2 emissions between 1850 and 2019. As the amount of carbon dioxide in the atmosphere increases, the amount of carbon dioxide absorbed by the ocean also increases. This leads to a series of chemical reactions in the seawater which has a negative impact on marine life and ecosystem functioning (Vargas et al. 2022). As the ocean acidifies, the concentration of carbonate ions decreases. Calcifying organisms such as mussels, corals, and various plankton species need exactly these molecules to build their shells and skeletons (Fig. 5.1). Also, other marine organisms that do not have calcium carbonate shells or skeletons need to spend more energy to regulate their bodily functions in acidifying waters.

Fig. 5.1
A diagram demonstrates a cycle that converts atmospheric carbon dioxide into hydrogen and carbonate ions. Acidification of calcifying organisms leads to deformed corals.

Ocean acidification

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5.3 Effects of Ocean Acidification on Marine Diatoms Community

Diatoms are among the most important and prolific microalgae in terms of both abundance and ecological functionality in the ocean. They are within the division of Bacillariophyta and serve directly or indirectly as food for many animals with an assessed contribution of almost 25% to global primary production (Tréguer and De La Rocha 2013). There are at least 30,000 of diatom species which differ in size and ranging from below 3 μm up to a few millimeters (Mann and Vanormelingen 2013). Diatoms are found as single cells or in chains in pelagic and/or benthic habitats and take free-living, surface-attached, symbiotic, or parasitic lifestyles (Mann and Vanormelingen 2013).

Diatom community composition could be affected by different environmental stressors in different ocean areas due to their massive relevance for the Earth system (Tréguer et al. 2018). There have been a number of research articles (Table 5.1) that studied the effects of future CO2 on marine diatom communities in short-term incubations (Bach et al. 2019; Feng et al. 2009, 2010; Hare et al. 2007; Kim et al. 2006; Tortell et al. 2002, 2008). Also, short-term ocean acidification experiments were done with single species of cultured diatoms (Chen and Gao 2003; Li et al. 2012; Sobrino et al. 2008; Sun et al. 2011; Wu et al. 2010). Few others (Crawfurd et al. 2011; Tatters et al. 2012) have used experimental plans in which isolated diatoms were exposed to different CO2 conditions for longer periods (more than three months). Most studies indicated that elevated CO2 led to a measurable increase in phytoplankton productivity, promoting the growth of larger chain-forming diatom. Future studies will be required to evaluate whether this is also the case for other types of algal communities from other marine systems (Table 5.1).

Table 5.1 Effects of ocean acidification reported on marine diatoms community
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5.4 Impacts of Ocean Acidification on the Growth of Diatoms

Diatoms in different waters suffer from variations of light and temperature as well as fluctuations in seawater carbonate chemistry. It is predictable that the growing partial pressure of CO2 (pCO2) in seawater due to ocean acidification will reduce the cellular requirement of diatoms for energy and resources, therefore stimulating diatom growth and carbon (C) fixation (Tortell et al. 2008). Diatoms show diversified responses to ocean acidification; higher CO2 concentrations are displayed to enhance (Gao et al. 2012b; Kim et al. 2006; King et al. 2011), have no effect (Boelen et al. 2011) or even inhibit (Li and Campbell 2013; Low-Décarie et al. 2011; McCarthy et al. 2012; Sugie and Yoshimura 2013) growth rates of diatom species. However, raised CO2 in the ocean increases its availability to algae; the reduced pH can affect the acid-base balance of cells (Flynn et al. 2012). In addition, the higher CO2 and reduced pH levels can interact with solar radiation and temperature, showing synergistic, antagonistic, or balanced effects (Gao et al. 2012a). Therefore, the mechanisms involved in the responses to ocean acidification of diatoms need to be further explored.

5.5 The Physiological Response of Marine Diatoms to Ocean Acidification

The responses of marine diatoms to ocean acidification are highly variable and species-specific as shown in Table 5.2. Diatoms work greatly efficient CO2 concentrating mechanisms (CCMs) to reach a high ratio of carboxylation to oxygenation (Raven et al. 2011). They are resistant to high levels of UV radiation (Wu et al. 2012), preferable a low sensitivity to photoinactivation of PSII compared with other phytoplanktons (Key et al. 2010 and Wu et al. 2011), and positively exploit variable light (Lavaud et al. 2007).

Table 5.2 Physiological response reported of marine diatoms
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Li et al. (2012) estimated the combined effects of ocean acidification, UV radiation, and temperature on the diatom Phaeodactylum tricornutum and grew it under two CO2 concentrations (390 and 1000 μatm); growth at the higher CO2 concentration increased non-photochemical quenching (NPQ) of cells and partially responded the damage to PS II (photosystem II) produced by UV-A and UV-B. The ratio of repair to UV-B-induced damage decreased with increased NPQ, reflecting induction of NPQ when repair dropped behind the damage, and it was higher under the ocean acidification condition, showing that the increased pCO2 and lowered pH counteracted UV-B-induced harm. As for photosynthetic carbon fixation rate which increased with increasing temperature from 15 to 25 °C, the elevated CO2 and temperature levels synergistically interacted to reduce the inhibition caused by UV-B and thus increase the carbon fixation.

5.6 Conclusions and Future Perspectives

Ocean acidification is known to reduce calcification of many calcifying organisms. Different diatom species may have entirely diverse responses to ocean acidification, mostly because of variances in species or phenotypes. The ocean acidification made changes in diatom competitiveness, and assemblage structure may change key ecosystem services; therefore, monitoring community abundance of diatoms over longer timescales is important to gain information on their responses to environmental changes.