Response of coccolithophorid Emiliania huxleyi to elevated partial pressure of CO2 under nitrogen limitation

Abstract

Precipitation of calcium carbonate by phytoplankton in the photic oceanic layer is an important process regulating the carbon cycling and the exchange of CO 2 at the ocean-atmosphere interface. Previous experiments have demonstrated that, under nutrient-sufficient conditions, doubling the partial pressure of CO 2 (pCO 2 ) in seawater -a likely scenario for the end of the centurycan significantly decrease both the rate of calcification by coccolithophorids and the ratio of inorganic to organic carbon production. The present work investigates the effects of high pCO 2 on calcification by the coccolithophore Emiliania huxleyi (Strain TW1) grown under nitrogen-limiting conditions, a situation that can also prevail in the ocean. Nitrogen limitation was achieved in NO 3 -limited continuous cultures renewed at the rate of 0.5 d -1 and exposed to a saturating light level. pCO 2 was increased from 400 to 700 ppm and controlled by bubbling CO 2 -rich or CO 2 -free air into the cultures. The pCO 2 shift has a rapid effect on cell physiology that occurs within 2 cell divisions subsequent to the perturbation. Net calcification rate (C ) decreased by 25% and, in contrast to previous studies with N-replete cultures, gross community production (GCP) and dark community respiration (DCR) also decreased. These results suggest that increasing pCO 2 has no noticeable effect on the calcification/photosynthesis ratio (C /P) when cells of E. huxleyi are NO 3 -limited.

Foraminifera promote calcification by elevating their intracellular pH

“…The accompanying decrease in seawater pH by an estimated 0.3-0.4 units for the coming century (39,40) may well countereffect the extra supply of inorganic carbon and result in a net decreased growth rate. For coccolithophores it has been suggested that future ocean acidification will result in a reduced net calcification (41)(42)(43), but for foraminifera data from micro-or mesocosm experiments are scarce. It has been shown that shell weights in the planktonic species Orbulina universa decrease with decreased carbonate ion concentration (44), possibly reflected by shifts in formainiferal shell weights over glacial-interglacial cycles and consequent shifts in oceanic pH (45).…”

Sensitivities of marine carbon fluxes to ocean change

“…Of relevance to the carbonate pump are the pelagic calcifying groups, in particular the coccolithophores, foraminifera, and pteropods. Most studies to date indicate a decrease in calcification of these groups with increasing seawater acidification, both in laboratory experiments (e.g., [39][40][41][42] and field studies (43,44). However, this point remains controversial (45,46).…”

Emiliania huxleyi increases calcification but not expression of calcification-related genes in long-term exposure to elevated temperature and p CO 2

“…One of the first studies on E. huxleyi revealed decreasing calcification rates under elevated pCO 2 [5]. While this response was later observed in a number of other studies [9][10][11][12][13][14][15], contrasting results have also been reported where elevated pCO 2 resulted in either no change in calcification (where in this case the term 'calcification' refers to calcium carbon quota and/or calcification rate) [15] or increased calcification [4]. Regardless of outcome, these earlier studies tested the response of E. huxleyi to elevated pCO 2 in short-term experiments (less than 20 generations).…”

“…Allowing for multiple days to pass between sampling for physiological parameters ensured that the population of cells had been replaced completely between collection points, given the high growth rate in our chemostats of 1.1 d 21 (more than one division per day). This sampling strategy is routinely used for continuous culture experiments, as cells several generations apart represent independent samples of the same population [13,24,31,36].…”