Environmental controls on the growth, photosynthetic and calcification rates of a Southern Hemisphere strain of the coccolithophore Emiliania huxleyi

Abstract

We conducted a series of diagnostic fitness response experiments on the coccolithophore, Emiliania huxleyi, isolated from the Subtropical Convergence east of New Zealand. Dose response curves (i.e., physiological rate vs. environmental driver) were constructed for growth, photosynthetic, and calcification rates of E. huxleyi relative to each of five environmental drivers (nitrate concentration, phosphate concentration, irradiance, temperature, and pCO 2 ). The relative importance of each environmental driver on E. huxleyi rate processes was then ranked using a semi-quantitative approach by comparing the percentage change caused by each environmental driver on the measured physiological metrics under the projected conditions for the year 2100, relative to those for the present day, in the Subtropical Convergence. The results reveal that the projected future decrease in nitrate concentration (33%) played the most important role in controlling the growth, photosynthetic and calcification rates of E. huxleyi, whereas raising pCO 2 to 75 Pa (750 ppm) decreased the calcification : photosynthesis ratios to the greatest degree. These findings reveal that other environmental drivers may be equally or more influential than CO 2 in regulating the physiological responses of E. huxleyi, and provide new diagnostic information to better understand how this ecologically important species will respond to the projected future changes to multiple environmental drivers.

Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment

“…For this reason we suggest that nitrate and phosphate availability is not an obvious driver of the southward decrease in coccolithophore abundances in Southern Ocean HNLC waters (i.e. these nutrients are sufficient everywhere), although these nutrients may be important in determining the success of coccolithophores in oligotrophic waters at the northern edge of the Southern Ocean, given the high half-saturation constant for nitrate uptake observed in some laboratory studies (∼ 13 µM; Feng et al, 2016) and the possibility that high-temperature and lownutrient conditions may non-linearly amplify phytoplankton stresses (Thomas et al, 2017).…”

“…Further progress in understanding the controls on coccolithophore abundances in the Southern Ocean is clearly needed. At present, temperature, light and competition with diatoms for iron appear to be the strongest candidates (at least for southward expansion (Charalampopoulou et al, 2016;Gafar et al, 2017; with nitrate a strong influence on the location of the northern oligotrophic boundary : Feng et al, 2016).…”

“…Coccolithophores, especially the most common species Emiliania huxleyi, have been studied sufficiently in the laboratory to allow possible important controls on their niches and especially their calcification rates to be proposed, including temperature, pH, pCO 2 , calcite saturation state, light, and macro-and micronutrient availability Feng et al, 2016;Mackinder et al, 2010;Muller et al, 2015;Müller et al, 2017;Schlüter et al, 2014;Schulz et al, 2007;Sett et al, 2014;Zhang et al, 2015). We collected observations of many of these properties in parallel with our PIC observations and now briefly examine whether they present correlations that might contribute to understanding why coccolithophores are found mainly in northern subantarctic waters and not further south.…”

“…Many laboratory studies have emphasized the importance of temperature on coccolithophore growth rates, as compiled recently (Feng et al, 2016), and warming has been suggested as a possible cause of decadal northward apparent range expansion in the North Atlantic (Rivero-Calle et al, 2015) and the occurrence of unusual blooms in the Bering Sea (Merico et al, 2004). To provide a brief visualization of the expected univariate response, we fit the "Norberg" thermal optimum envelope model (Norberg, 2004) to growth rate data for 5-25 • C with modern pCO 2 and nutrient-replete conditions for a Southern Ocean morphotype A strain of Emiliania huxleyi, isolated from south of Tasmania (Muller et al, 2015), with optimum temperature z = 15, thermal window w = 10 and scaling constant a, in which the exponential term represents the broad global temperature dependence of generic phytoplankton growth rates (Eppley, 1972) and produces the known skewed form of organismic thermal tolerances, following Eq.…”

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

“…Paasche, 2002;Zondervan, 2007;Langer and Benner, 2009;Feng et al, 2017). We examine each of these factors in turn, but note that all exhibit correlated seasonal cycles and thus the identification of a single driver is particularly difficult.…”