Emiliania huxleyi coccolith calcite mass modulation by morphological changes and ecology in the Mediterranean Sea

Abstract

To understand the response of marine calcifying organisms under high CO2 scenarios, it is critical to study their calcification patterns in the natural environment. This paper focuses on a major calcifying phytoplankton group, the coccolithophores, through the analysis of water samples collected along a W-E Mediterranean transect during two research cruises, in April 2011 (Meteor cruise M84/3) and May 2013 (MedSeA cruise 2013). The Mediterranean Sea is a marginal sea characterized by large biogeochemical gradients. Currently, it is undergoing both warming and ocean acidification, processes which are rapidly modifying species distribution and calcification. The species Emiliania huxleyi largely dominates the total coccolithophore production in present day oceans and marine basins, including the Mediterranean Sea. A series of morphometric measurements were performed on the coccoliths of this species to estimate their mass, length and calculate a calcification index (proxy for the size-normalized calcification degree). The most abundant morphotype of E. huxleyi in the Mediterranean Sea is Type A. Coccoliths of this morphotype were additionally analyzed based on scanning electron microscopy images: four calcification varieties were quantified, according to the relationship between slit length-tube width, and the state of the central area (open or closed). The average E. huxleyi coccolith mass along the Mediterranean oceanographic transect depended strongly on both the average coccolith length and calcification index. The variability in average coccolith length and calcification index across samples reflected oscillations in the relative abundance of the calcification varieties. We also demonstrated that the distribution of the calcification varieties followed the main environmental gradients (carbonate chemistry, salinity, temperature, nutrient concentrations). Hence, shifts in the distribution of the calcification varieties and of the average E. huxleyi coccolith mass are to be expected in the Mediterranean Sea under climate change. These physiological and ecological responses will modulate the net coccolithophore calcification and, ultimately, the regional carbonate export to the seafloor.

Full annual monitoring of Subantarctic Emiliania huxleyi populations reveals highly calcified morphotypes in high-CO2 winter conditions

“…B/C) 21,55,57 . Moreover, our results are in agreement with recent findings in the Mediterranean Sea, where highest E. huxleyi coccolith mass was positively correlated with relative abundance of Type A o/c along an environmental gradient 58 . Taken together, all the above mentioned studies and our results underscore the crucial role of E. huxleyi morphotype composition in the control, both geographically and seasonally, on coccolith size and mass variability in the Southern Ocean.…”

Coccolith mass and morphology of different Emiliania huxleyi morphotypes: A critical examination using Canary Islands material

“…Type A central tube width have been linked to different degrees of calcification in E. huxleyi [30,40]. In the present study CT:L ratio followed Type A coccolith mass, as both decreased together from January to September at both sites.…”

“…The number of components/morphotypes in the model reflects the number of morphotypes counted in the SEM, with the exception of very rare morphotypes (<1% relative abundance). Previous studies have reported several different morphotypes within both Type A [40] and Group B (e.g. [34,35]), and the presence of different sub-varieties of these morphotypes may invalidate this assumption.…”

“…Other studies on morphotype-specific differences in E. huxleyi coccolith mass have combined relative abundance estimates with coccolith mass estimates obtained from their birefringence in a polarized light microscope (e.g. [8,40]). These studies did not use MCMC analysis or other tools to estimate coccolith mass per morphotype, but rather correlated changes in E. huxleyi (or Noelrhabdaceae) coccolith mass with changes in morphotype relative abundance.…”

“…[33,37]) and regional (e.g. [8,15,32,[38][39][40]) gradients, and might furthermore differ in their sensitivity to ocean acidification [41]. E. huxleyi morphotypes differ in robustness of distal shield elements and degree of central area closure, which is often interpreted as differences in degrees of calcite production or differences in coccolith mass between the morphotypes.…”