Particulate inorganic to organic carbon production as a predictor for coccolithophorid sensitivity to ongoing ocean acidification

Abstract

Ocean acidification (OA) can induce shifts in plankton community composition, with coccolithophores being mostly negatively impacted. This is likely to change particulate inorganic and organic carbon (PIC and POC, respectively) production, with impacts on the biological carbon pump. Hence, assessing and, most importantly, understanding species‐specific sensitivities of coccolithophores is paramount. In a multispecies comparison, spanning more than two orders of magnitude in terms of POC and PIC production rates, among Calcidiscus leptoporus, Coccolithus pelagicus subsp. braarudii, Emiliania huxleyi, Gephyrocapsa oceanica, and Scyphosphaera apsteinii, we found that cellular PIC : POC was a good predictor for a species’ OA sensitivity. This is likely related to the need for cellular pH homeostasis, which is challenged by the process of calcification producing protons internally, especially when seawater pH decreases in an OA scenario. With higher PIC : POC, species and strains being more sensitive to OA coccolithophores may shift toward less calcified varieties in the future.

A 15-million-year-long record of phenotypic evolution in the heavily calcified coccolithophore <i>Helicosphaera</i> and its biogeochemical implications

“…Across the modern representatives of the Helicosphaera genus, coccolith dimensions and cell size are correlated. For example, the smallest coccolith and coccosphere sizes are found in H. pavimentum (coccolith 4-6 µm, coccosphere 9-16 µm) and in H. hyalina (coccolith 5-8 µm, coccosphere 11-16 µm), whereas the largest coccolith and coccosphere sizes are found in H. carteri (coccolith 7-12 µm, coccosphere 15-25 µm) and H. wallichii (coccolith 7-12 µm, coccosphere 19-27 µm; http://www.mikrotax.org/Nannotax3/index.php?dir= Coccolithophores, last access: 4 May 2020; Geisen et al, 2004). It can therefore be argued with confidence that these traits followed the same principle in extinct fossil representatives, meaning that H. sellii had smaller coccolith and cell size than H. carteri, while H. granulata had the largest cell size of the three species.…”

“…One possible explanation for different adaptive strategies can be related to the distinction between obligate calcifiers, including Coccolithus and Helicosphaera, and nonobligate calcifiers, such as Emiliania huxleyi (Durak et al, 2016;Walker et al, 2018). The phenotypic plasticity in obligate calcifiers could be more restricted when it comes to morphological innovation and biogeochemical output compared to nonobligate calcifiers, who can regulate their calcification rates (and thus PIC : POC ratio) to zero and overcome carbon limitation or other environmental stressors such as ocean acidification (Gafar et al, 2019). Obligate calcifiers, such as Coccolithus, Calcidiscus and Helicosphaera, combined were relatively more abundant than reticulofenestrids until ∼ 7 Ma when their fluxes decreased significantly at both sites (Fig.…”

“…Earlier experimental studies have found a range of strain-specific physiological traits in H. carteri, likely related to local ecophysiological adaptation (Šupraha et al, 2015). In addition, this species is considered highly susceptible to ocean acidification due to its heavily calcified coccoliths and high PIC : POC ratio (Gafar et al, 2019). Finally, both modern and fossil Helicosphaera morphospecies are well-defined and identifiable under a light microscope, which allows for long-term tracking of speciation events, community dynamics and phenotypic evolution at the morphospecies level.…”

A 15 million-year long record of phenotypic evolution in the heavily calcified coccolithophore <i>Helicosphaera</i> and its biogeochemical implications

“…The phenotypic plasticity in obligate calcifiers could be more restricted when it comes to morphological innovation and biogeochemical performance compared to non-obligate calcifiers, who can regulate their calcification rates (and thus PIC:POC ratio) to zero and overcome carbon limitation or other environmental stressors such as ocean acidification (Gafar et al, 2019).…”

“…Earlier experimental studies have found a range of strain-specific physiological traits in H. carteri, likely related to local eco-physiological adaptation (Šupraha et al, 2015). In addition, this species is considered highly susceptible to ocean acidification due to its heavily calcified coccoliths and high PIC:POC ratio (Gafar et al, 2019). Finally, both modern and fossil Helicosphaera morphospecies are well-defined and identifiable under a light microscope, which allows for long-term tracking of speciation events, community dynamics and phenotypic evolution at the morphospecies-level.…”