Emiliania huxleyi increases calcification but not expression of calcification-related genes in long-term exposure to elevated temperature and p CO ₂

Abstract

Increased atmospheric p CO 2 is expected to render future oceans warmer and more acidic than they are at present. Calcifying organisms such as coccolithophores that fix and export carbon into the deep sea provide feedbacks to increasing atmospheric p CO 2 . Acclimation experiments suggest negative effects of warming and acidification on coccolithophore calcification, but the ability of these organisms to adapt to future environmental conditions is not well understood. Here, we tested the combined effect of p CO 2 and temperature on the coccolithophore Emiliania huxleyi over more than 700 generations. Cells increased inorganic carbon content and calcification rate under warm and acidified conditions compared with ambient conditions, whereas organic carbon content and primary production did not show any change. In contrast to findings from short-term experiments, our results suggest that long-term acclimation or adaptation could change, or even reverse, negative calcification responses in E. huxleyi and its feedback to the global carbon cycle. Genome-wide profiles of gene expression using RNA-seq revealed that genes thought to be essential for calcification are not those that are most strongly differentially expressed under long-term exposure to future ocean conditions. Rather, differentially expressed genes observed here represent new targets to study responses to ocean acidification and warming.

Considerations for the use of transcriptomics in identifying the ‘genes that matter’ for environmental adaptation

“…Whether in the form of microarrays or more recently RNA sequencing, considerable effort has been directed toward characterizing shifts in mRNA abundance triggered by changes in key environmental variables such as temperature (Logan and Buckley, 2015), salinity (Evans and Somero, 2008), oxygen (Gracey et al, 2011) and pH (Benner et al, 2013;Evans et al, 2013). Within the field of environmental physiology, transcriptomics has been used successfully to address a broad range of questions concerning how or whether organisms can acclimate or adapt to the abiotic conditions associated with life in specific habitats (Evans and Hofmann, 2012).…”

Phytoplankton adapt to changing ocean environments

“…In addition, temperature response curves measured in the laboratory show that phytoplankton usually have the fastest growth rates at or slightly below the mean temperature of the environment they were isolated from, suggesting that natural populations are adapted to their local environment (15,22), although some species have niches that do not reflect the environmental conditions from which they were isolated (23). Evolutionary experiments in the laboratory indicate that phytoplankton species have the capacity to evolve over hundreds to thousands of generations in response to single environmental factors; specifically, changes in CO 2 concentration or temperature (24)(25)(26)(27)(28)(29). Laboratory evolution experiments do not replicate either the highly dynamic marine environment or the trajectory of climate change, so it is necessary to look to see how phytoplankton evolve in the field.…”

Biochemical adaptation to ocean acidification

“…Those two environmental drivers interactively alter the ratios of calcification and photosynthesis products of particulate inorganic and organic carbon (Lefebvre et al, 2012). Interestingly, warming seems to ameliorate the negative effects of OA under long-term culture (Benner et al, 2013). The multivariate responses to changes in temperature, P CO2 and nitrogen source remain to be examined.…”

“…The coccolithophore Emiliania huxleyi is a particularly well studied species, with remarkable diversity in how it responds to OA across genotypes (Langer et al, 2009). Studies of specific strains held under different conditions for different lengths of time illustrate the potential for coccolithophores to make plastic and adaptive responses to OA (Benner et al, 2013;Langer et al, 2009;Lefebvre et al, 2012;Lohbeck et al, 2012Lohbeck et al, , 2013Schlüter et al, 2014). E. huxleyi typically exhibited a plastic response after 8 generations at high P CO2 and an adaptive response after 500 generations under the same high P CO2 conditions .…”