A Conceptual Model for Projecting Coccolithophorid Growth, Calcification and Photosynthetic Carbon Fixation Rates in Response to Global Ocean Change

Abstract

Temperature, light and carbonate chemistry all influence the growth, calcification and photosynthetic rates of coccolithophores to a similar degree. There have been multiple attempts to project the responses of coccolithophores to changes in carbonate chemistry, but the interaction with light and temperature remains elusive. Here we devise a simple conceptual model to derive a fit equation for coccolithophorid growth, photosynthetic and calcification rates in response to simultaneous changes in carbonate chemistry, temperature and light conditions. The fit equation is able to account for up to 88% of the variability in measured metabolic rates. Equation projections indicate that temperature, light and carbonate chemistry all have different modulating effects on both optimal growth conditions and the sensitivity of responses to extreme environmental conditions. Calculations suggest that a single extreme environmental condition (CO 2 , temperature, light) will reduce maximum rates regardless of how optimal the other environmental conditions may be. Thus, while the response of coccolithophores to ocean change depends on multiple variables, the one which is least optimal will have the most impact on overall rates. Finally, responses to ocean change are usually reported in terms of cellular rates. However, changes in cellular rates can be a poor predictor for assessing changes in production at the community level. We therefore introduce a new metric, the calcium carbonate production potential (CCPP), which combines the independent effects of changes in growth rate and cellular calcium carbonate content to assess how environmental changes will impact coccolith production. Direct comparison of CO 2 impacts on cellular CaCO 3 production rates and CCPP shows that while the former is still at 45% of its pre-industrial capacity at 1,000 µatm, the latter is reduced to 10%.

Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone

“…However, Paasche and Brubak (1994) observed that calcification is less strongly curtailed than photosynthesis under low light conditions, so perhaps high calcification relative to growth in winter could lead to heavier coccolith weights in that part of the seasonal cycle. Interestingly, this would contrast with a recent synthesis of results for another coccolithophore, Gephyrocapsa oceanica, in which optimal light for calcification was found to be slightly higher than for photosynthesis or growth (Gafar et al, 2018), emphasising that the sensitivities of these processes may be organism and possibly even strain specific. Smith et al (2012) previously documented a reduction in coccolith calcification of E. huxleyi coccospheres during the summer months in the Bay of Biscay, but advised caution in associating this with light intensity because calcification rates may not necessarily covary with the amount of calcite content per coccolith.…”

A niche comparison of <i>Emiliania huxleyi</i> and <i>Gephyrocapsa oceanica</i> and potential effects of climate change

“…Tables 1, 2 and 3. Species specific differences in response to changing carbonate chemistry, temperature and light were assessed by comparing the above fit to that recently produced for Gephyrocapsa oceanica (Gafar et al, 2018).…”

“…While our fit equation has previously explained variability in lab experiments quite well (Gafar et al, 2018), natural systems are much more complex, with the interactions of dozens of variables including temperature, light, nutrients, predation and competition all influencing productivity (Behrenfeld, 2014). As such we wanted to examine how our, relatively simple, equation…”

“…These data were then combined with a previously published data set on temperature and CO 2 interaction (Sett et al, 2014) and fitted to an analytical equation describing the combined effects of changing carbonate chemistry speciation, light and temperature. The resulting projections are then compared to those previously published for G. oceanica (Gafar et al, 2018) in an attempt to assess their individual success and potential realised niche in a changing ocean. 35 …”

“…While Eq. (2) can also be used to explain responses 20 under decoupled carbonate chemistry conditions (see Gafar et al 2018 for details), the fit obtained here is only valid for coupled CO 2 /pH changes as no data from decoupled experiments (i.e. Bach et al 2011) has been used.…”