Abstract
Abstract. Within the context of the UK Ocean Acidification project, Emiliania huxleyi (type A) coccolith morphology was examined from samples collected during cruise D366. In particular, a morphometric study of coccolith size and degree of calcification was made on scanning electron microscope images of samples from shipboard CO2 perturbation experiments and from a set of environmental samples with significant variation in calcite saturation state (Ωcalcite). One bioassay in particular (E4 from the southern North Sea) yielded unambiguous results – in this bioassay exponential growth from a low initial cell density occurred with no nutrient enrichment and coccosphere numbers increased tenfold during the experiment. The samples with elevated CO2 saw significantly reduced coccolithophore growth. However, coccolithophore morphology was not significantly affected by the changing CO2 conditions even under the highest levels of perturbation (1000 μatm CO2). Environmental samples similarly showed no correlation of coccolithophore morphology with calcite saturation state. Some variation in coccolith size and degree of calcification does occur but this seems to be predominantly due to genotypic differentiation between populations on the shelf and in the open ocean.
Highlights
Coccolithophores are one of the most abundant and widespread groups of calcifying plankton and so have attracted extensive study in terms of their likely response to ocean acidification
They did, note that other effects such as growth rate and cell size changes could confuse this response, that the response was often muted and that it was important to look at changes in the ratio of calcification to photosynthetic carbon fixation and at calcification rates per cell. Building on these initial indications of a distinct influence of carbonate chemistry on coccolithophores, several ecological studies suggested that variations in carbonate saturation state might influence aspects of the distribution of modern coccolithophores, such as timing of blooms (Merico et al, 2006) and absence of coccolithophores from parts of the Antarctic Ocean (Cubillos et al, 2007) and from the Baltic Sea (Tyrrell et al, 2008)
Most strikingly it has been suggested that coccolith mass in Emiliania huxleyi and closely related species is controlled by saturation state in both the modern ocean and the late Quaternary fossil record (Beaufort et al, 2011)
Summary
Coccolithophores are one of the most abundant and widespread groups of calcifying plankton and so have attracted extensive study in terms of their likely response to ocean acidification. Experimental work with laboratory cultures and large-scale semi-enclosed field cultures, mesocosms, suggested that there was a clear reduction in calcification rates with increasing pCO2 (Riebesell et al, 2000; Riebesell, 2004; Zondervan et al, 2002; Engel et al, 2005) They did, note that other effects such as growth rate and cell size changes could confuse this response, that the response was often muted ( shown by Fiorini et al, 2011) and that it was important to look at changes in the ratio of calcification to photosynthetic carbon fixation and at calcification rates per cell. These images were used for morphometric work and for counts of the numbers of loose coccoliths
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