NO MECHANISTIC DEPENDENCE OF PHOTOSYNTHESIS ON CALCIFICATION IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (HAPTOPHYTA)¹

Abstract

There is still considerable uncertainty about the relationship between calcification and photosynthesis. It has been suggested that since calcification in coccolithophorids is an intracellular process that releases CO2 , it enhances photosynthesis in a manner analogous to a carbon-concentrating mechanism (CCM). The ubiquitous, bloom-forming, and numerically abundant coccolithophorid Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler was studied in nutrient-replete, pH and [CO2 ] controlled, continuous cultures (turbidostats) under a range of [Ca(2+) ] from 0 to 9 mM. We examined the long-term, fully acclimated photosynthesis-light responses and analyzed the crystalline structure of the coccoliths using SEM. The E. huxleyi cells completely lost their coccosphere when grown in 0 [Ca(2+) ], while thin, undercalcified and brittle coccoliths were evident at 1 mM [Ca(2+) ]. Coccoliths showed increasing levels of calcification with increasing [Ca(2+) ]. More robust coccoliths were noted, with no discernable differences in coccolith morphology when the cells were grown in either 5 or 9 mM (ambient seawater) [Ca(2+) ]. In contrast to calcification, photosynthesis was not affected by the [Ca(2+) ] in the media. Cells showed no correlation of their light-dependent O2 evolution with [Ca(2+) ], and in all [Ca(2+) ]-containing turbidostats, there were no significant differences in growth rate. The results show unequivocally that as a process, photosynthesis in E. huxleyi is mechanistically independent from calcification.

Photosynthetic responses of <i>Emiliania huxleyi</i> to UV radiation and elevated temperature: roles of calcified coccoliths

“…Emiliania huxleyi depends on Ca 2+ to form carbonate crystals that build up the coccolith shell that shelters the cells. Ca 2+ availability is also associated to other process, as seen in other studies (Trimborn et al, 2007;Leonardos et al, 2009) that demonstrated that cells grown under low (0.1 mM Ca 2+ ) or without Ca 2+ decreased their growth rates as compared to those cultured under higher Ca 2+ concentrations (e.g., 1-10 mM). Similarly, in our study we determined a significant decrease in the growth rates of E. huxleyi cells after being transferred from 10 mM Ca 2+ to 0.1 mM Ca 2+ , but only during the initial 12 generations in 9 days (data not shown).…”

“…Similarly, in our study we determined a significant decrease in the growth rates of E. huxleyi cells after being transferred from 10 mM Ca 2+ to 0.1 mM Ca 2+ , but only during the initial 12 generations in 9 days (data not shown). After 100 days (148 generations), there were no significant differences in growth rates between the two Ca 2+ concentrations (Table 2), clearly suggesting that acclimation to the lowered Ca 2+ level took longer time than expected (Herfort et al, 2002(Herfort et al, , 2004Trimborn et al, 2007;Leonardos et al, 2009). Nevertheless, significant differences between the two Ca 2+ concentrations were observed in the amount of pigments -chl-a and carotenoids per cell ( Table 2).…”

“…5). Other studies (Herfort et al, 2002(Herfort et al, , 2004Trimborn et al, 2007;Leonardos et al, 2009) however, found that low Ca 2+ concentrations did not affect the POC production, photosynthetic carbon fixation or photosynthetic oxygen evolution. In addition, Trimborn et al (2007) using SEM analysis found naked cells but not coccoliths or even residues, and they concluded that cells did not calcify at 0.1 mM Ca 2+ concentrations.…”

“…While still is under debate, calcification in coccolithophores appears to be strongly linked to photosynthesis by favoring supply of CO 2 (Anning et al, 1996;Sikes et al, 1980;Sikes and Wilbur, 1982); moreover, Nimer et al (1996) found reduced photosynthetic rates when removing external calcium. Other studies (Herfort et al, 2002(Herfort et al, , 2004Trimborn et al, 2007;Leonardos et al, 2009) however, showed that changes in calcification under altered Ca 2+ concentrations did not bring about concomitant changes in photosynthesis.…”

Algal and aquatic plant carbon concentrating mechanisms in relation to environmental change

“…There is growing evidence that CO 2 generation during intracellular calcification in coccolithophores is not stoichiometrically involved as component of a CCM, or is otherwise necessarily involved in supplying CO 2 to Rubisco (Herfort et al 2004;Trimborn et al 2007;Leonardos and Geider 2009). This conclusion means that the variable nature of the reported effects of increased CO 2 on coccolithophore calcification (see Zondervan 2007;Iglesias-Rodriguez et al 2008;Doney et al 2009a;Muller et al 2010) does not directly impact on CO 2 supply to photosynthesis.…”