Predominance of heavily calcified coccolithophores at low CaCO ₃ saturation during winter in the Bay of Biscay

Abstract

Coccolithophores are an important component of the Earth system, and, as calcifiers, their possible susceptibility to ocean acidification is of major concern. Laboratory studies at enhanced pCO 2 levels have produced divergent results without overall consensus. However, it has been predicted from these studies that, although calcification may not be depressed in all species, acidification will produce "a transition in dominance from more to less heavily calcified coccolithophores" [Ridgwell A, et al., (2009) Biogeosciences 6:2611-2623. A recent observational study [Beaufort L, et al., (2011) Nature 476:80-83] also suggested that coccolithophores are less calcified in more acidic conditions. We present the results of a large observational study of coccolithophore morphology in the Bay of Biscay. Samples were collected once a month for over a year, along a 1,000-km-long transect. Our data clearly show that there is a pronounced seasonality in the morphotypes of Emiliania huxleyi, the most abundant coccolithophore species. Whereas pH and CaCO 3 saturation are lowest in winter, the E. huxleyi population shifts from <10% (summer) to >90% (winter) of the heavily calcified form. However, it is unlikely that the shifts in carbonate chemistry alone caused the morphotype shift. Our finding that the most heavily calcified morphotype dominates when conditions are most acidic is contrary to the earlier predictions and raises further questions about the fate of coccolithophores in a high-CO 2 world.phytoplankton | North Atlantic | climate change C occolithophores contribute between ∼1% and 10% of marine primary production (1), dominate the pelagic calcium carbonate flux (2), and alter ocean albedo (3). Model predictions suggest that, if CO 2 emissions continue unabated, global surface ocean pH will decrease by 0.3-0.5 units by 2100, leading to a halving of the carbonate ion concentration (4). Along with other calcifiers, coccolithophores such as Emiliania huxleyi are considered susceptible to this ocean acidification (OA). This hypothesis is contentious, however, with diverse calcification responses reported for culture experiments. Many experiments on E. huxleyi (the most common coccolithophore) have found depressed calcification at elevated CO 2 concentration and the associated low pH and low CaCO 3 saturation state (Ω) (5-11), whereas others have found elevated calcification (12, 13) or no trend (10). An in-depth discussion on the reasons behind the contrasting results of Riebesell et al. (5) and can be found in refs. 14 and 15. In a recent study, four different strains of E. huxleyi cultured under identical environmental conditions exhibited varying responses to elevated CO 2 (16), as was also found between coccolithophore species (17).Laboratory studies are unrealistic in many respects and, because of their typically short timescales, preclude the possibility of evolutionary adaptation to the imposed change, a key uncertainty in OA research (17-19). It is therefore vital to complement laboratory experiments with observ...

Coccolithophores on the north-west European shelf: calcification rates and environmental controls

“…This contrasts with other studies where variability in coccolithophore dynamics across pH or C gradients (e.g. Charalampopoulou et al, 2011;Smith et al, 2012;Poulton et al, 2011Poulton et al, , 2013 are associated with co-varying gradients in growth-limiting factors such as temperature, nutrient concentrations and light availability. This contrast in coccolithophore response to pH or C , between gradients where carbonate chemistry covaries with other environmental parameters and gradients where there is no co-variability implies that any correlation between pH or C and coccolithophore dynamics along environmental gradients should be viewed with caution and in the context of any naturally occurring co-correlation with nutrient and light availability.…”

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

“…Smith et al, 2012;Meier et al, 2014). SEM analysis of the sediment trap samples revealed that only morphotype B/C, as described by Young et al (2003), thrives in the AZ-S waters south of Tasmania.…”

“…On the other hand, ballasting by carbonates appears to increase the transfer of organic carbon to the ocean interior (Armstrong et al, 2002;Klaas and Archer, 2002). On seasonal timescales the counter pump contribution dominates (Boyd and Trull, 2007), but more complex interactions can occur over longer timescales as a result of changing extents of carbonate dissolution in sediments, including the possibility that enhanced calcite dissolution in the Southern Ocean contributed to lower atmospheric CO 2 levels during glacial maxima (Archer and Maier-Reimer, 1994;Sigman and Boyle, 2000;Ridgwell and Zeebe, 2005).…”

“…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. Therefore, the possible effect of light intensity on coccolith weight in our traps is plausible but not demonstrable with our current data.…”

Decline in Coccolithophore Diversity and Impact on Coccolith Morphogenesis Along a Natural CO2 Gradient

“…However, because salinity, temperature, and light vary with carbonate saturation in these two seas it is difficult to disentangle the effects of these potentially confounding factors. Other studies have highlighted the complexity of decoupling multiple environmental changes on coccolithophore calcification (e.g., Smith et al, 2009;Horigome et al, 2014).…”