Relative distribution of heterococcolithophores and holococcolithophores in the Mediterranean Sea during the MedSeA and Meteor M84/3 cruises (May 2013, April 2011)

Coccolithophores are unicellular pelagic algae, capable of calcification. In the Mediterranean Sea, several species have a well-known haplo-diploid life cycle, alternating the production of different types of calcite plates, the holo- and hetero-coccoliths. The environmental triggers of haplo-diploid transformations in coccolithophores are still uncertain and studies in the natural environment are fundamental to reveal their ecology. We analyzed the distribution of both phases along a W-E Mediterranean transect during April 2011 and May 2013 (spring season), following strong environmental gradients. The proportion of holococcolithophores:heterococcolithophores of selected species varies not only vertically along the water column, but also longitudinally, following the overall biogeochemical gradients. Based on the environmental affinities of the coccolithophore life phases, we conclude that a dimorphic life cycle might provide additional adaptability to the south-eastern (SE) Mediterranean environment (particularly rich in carbonate ions, warm, stratified and nutrient limited) and support the survival of species whose diploid phases are in contrast adapted to Atlantic or south-western (SW) Mediterranean conditions. These mechanisms can alter the total coccolithophore response to ongoing climate changes.