Emiliania huxleyi: a pretty hardy microbe!

The oceans are an important source of atmospheric trace gases, such as Dimethyl sulfide (DMS), which originates from the enzymatic breakdown of the secondary algal metabolite Dimethylsulfoniopropionate (DMSP). DMS is believed to promote the formation of cloud- condensation nuclei (CCN) in the marine boundary layer, which may increase the earth’s albedo resulting in cooling also known as the CLAW hypothesis. The direct biological control of DMS over CCN has been questioned (Levasseur, 2013) but nevertheless DMS is still of central importance to atmospheric chemistry. Understanding the consequences of anthropogenic climate change, such as ocean acidification, on marine DMS production and its sea-to–air transfer is of principal importance. This study provides the first insight into the synergistic effects of elevated pCO₂ (associated with ocean acidification) and temperature on DMSP/DMS production and growth in Emiliania huxleyi – a globally important phytoplankton coccolithophore.

Emiliania huxleyi strain CCMP 373 was cultured in a laboratory, which formed exponentially growing, semi- continuous cultures of the high- DMS- producing haptophyte. Cultures were exposed to four different treatments: ‘ambient’, ‘future CO₂’ (+CO₂), ‘future temperature’ (+T) and ‘greenhouse’ (future CO₂ and temperature; +TCO₂). Temperature was increased from 17 (ambient) to 21°C and pCO₂ concentration was increased from 385μatm (ambient) to 1000 μatm which presents the possible increases by the year 2100. Cell growth, DMS and intracellular DMSP production was then quantified.

When exposed to 21°C, cell diameter decreased by 1.6%, but interestingly caused a 2.9% increase in growth rate based on cell number. Previous studies exposing different E. huxleyi strains to elevated CO₂ show varied responses, which highlight the high phenotypic plasticity in the species. An example of the effects this could have is shown in a study when a natural plankton assemblage from the Equatorial Pacific preferred larger diatoms to smaller Phaeocystis sp. which demonstrates the changes in nutrient cycling and competition which may result if E. huxleyi cell size changes, this is of concern as E. huxleyi is such an important food source to many zooplankton.

The results of the +CO₂ experiment suggest that DMS production may decrease 50% under future CO_2, which is an alarming decrease in the ‘climate-cooling’ DMS. However predicting the direction of future DMS production cannot be assessed from experiments that manipulate a single parameter in isolation so this result I do not think is of much importance. The results of the ‘greenhouse’ (+TCO₂) treatment suggest that future DMS production could be the same as under ambient conditions in this certain strain of E. huxleyi. This is interesting that there is no effects, but the response of E. huxleyi to changes in pCO₂ and temperature could be strain specific and further work is necessary to assess the roles of DMSP in the physiological response of algae to temperature and pCO₂ stress.

Intracellular DMSP concentrations were not significantly higher in the +CO₂ treatment, but the temperature increase from 17 to 21°C resulted in a significant increase in mean intracellular DMSP. Showing that temperature is probably the may influencing force in this experiment.

Another implication that I found interesting was that elevated pCO₂ could lessen oxidative stress, in turn decreasing DMS production. This is expected as it has been previously shown that limited pCO₂ increases oxidative stress in E. huxleyi and other phytoplankton which then leads to an increase in intracellular DMSP and DMS. Future studies to investigate the oxidative stress and DMS relationship, I feel, is the next step.

 Arnold, H. E., Kerrison, P., & Steinke, M. (2013). Interacting effects of ocean acidification and warming on growth and DMS‐production in the haptophyte coccolithophore Emiliania huxleyi. Global change biology, 19(4), 1007-1016.

Very interesting review: Levasseur, M. (2013). Impact of Arctic meltdown on the microbial cycling of sulphur. Nature Geoscience, 6(9), 691-700.

Can be found via: http://onlinelibrary.wiley.com/store/10.1111/gcb.12105/asset/gcb12105.pdf?v=1&t=i27f0r64&s=2ca0e8f1dad274c0d0e6314f97bd4c8cbbf585ec