Emiliania huxleyi and climate change: a genetic and biogeographic investigation of bloom dynamics for a key phytoplankton species in the global carbon cycle

Abstract

Emiliania huxleyi is a ubiquitous coccolithophore present throughout the global ocean and capable of forming large blooms with significant effects on the global carbon cycle. Developing our understanding of E. huxleyi ecology is necessary in order to better quantify E. huxleyi’s role in the present carbon cycle, and to predict its role in the future carbon cycle under climate change scenarios. Major gaps in the understanding of E. huxleyi ecology were addressed using (1) controlled mesocosm experiments in June 2008 in Raunefjord, Norway, to map population genetics of E. huxleyi blooms in relation to ecological pressures (viruses and rapid growth), (2) biogeographic sampling of nannoplankton (2 - 20 ?m) in the SO, including E. huxleyi, to determine ecological pressures on E. huxleyi blooms in situ (environmental gradients), and (3) controlled iron (Fe) addition bioassay experiments in the SO to establish the role of Fe gradients in the nannoplankton community relative to the phytoplankton community.

During the mesocosm experiments, 279 individual E. huxleyi cells were isolated to establish clonal cultures, of which 143 were successfully genotyped using 5 microsatellite molecular markers. Both high gene diversity and two distinct genotypic populations were detected over the bloom time series and are strong evidence for a large reservoir of genetic variability within the E. huxleyi species concept, which may translate into phenotypic plasticity, such as differing levels of viral resistance. In the SO, the spatial and temporal biogeography of the three most numerous mineralizing nannoplankton groups, the coccolithophore E. huxleyi, the smaller (<20 ?m) species of the diatom genus Fragilariopsis, and chrysophytes of the genus Tetraparma were defined using scanning electron microscopy (SEM) analysis in conjunction with an array of biological, physical, and chemical variables during two successive cruises to the Scotia Sea. Multivariate statistical analyses were used to identify the most influential environmental variables controlling mineralizing nannoplankton biogeography. Sea surface temperature (SST) and salinity were identified as primary variables and removed from the analysis, leaving frontal boundaries, macronutrient, and dFe concentrations significantly associated with a northern E. huxleyi-dominated community (group I; higher nutrients) and a southern Tetraparma- and Fragilariopsisdominated community (group II; lower nutrients). Estimates of biomass indicated that the Scotia Sea mineralizing nannoplankton community formed a substantial part (on average 13%) of the total phytoplankton community. The results of bioassay Fe incubations indicated a response in medium and large diatoms and E. huxleyi, and a number of microplankton (20 – 200 ?m) diatom species. Overall, the work contributes substantially to our understanding of the molecular population structure, extent of phenotypic plasticity, and environmental parameters affecting the key phytoplankton E. huxleyi.

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