A simple model for describing coccolithophorid cellular rate responses to ocean change

Chair: Heidi Pethybridge

Natasha A. Gafar (1)*, Bradley Eyre (1), Kai G. Schulz (1)

1 Centre for Coastal Biogeochemistry, School of Environment Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia

Background
Coccolithophores are an important component of the marine phytoplankton assemblage, which have been influencing Earth’s carbon cycling for over 200 million years. Human driven increases in atmospheric CO2 are expected to result in significant changes in ocean carbonate chemistry, temperature and light availability. Work on developing and fitting coccolithophore responses into a model framework of these future changes is ongoing (i.e. Bach et al. 2011, 2015), but predominantly focused on the effects of ocean acidification. This study constructs a physiologically-inspired model describing coccolithophore responses to the combination of all three stressors of changing carbonate chemistry, temperature and light.

Methods
Published data were obtained on the effects of carbonate chemistry speciation, temperature and light on Gephyrocapsa oceanica.

The basic physiological irradiance-photosynthesis model developed by Megard et al. (1984) was modified to include changing carbonate chemistry and temperature, to fit the response of calcification, photosynthesis and growth rates. The ability of the model to explain changes in these rates was assessed using nonlinear regression.

Findings
The model was able to explain up to 81% of the variability in measured rates across a broad carbonate chemistry (25-4000 μatm), light (50-800 μmol quanta m-2 s-1) and temperature (15-25 °C) range.

Furthermore, it was able to capture known physiological features like light inhibition and minimum and maximum temperature boundaries outside the range of the available data.

Conclusions
This was the first effort to create a predictive model which incorporates the interactive effects of carbonate chemistry, temperature and light on metabolic rates in coccolithophores. Preliminary results indicate that this model can be easily adapted to other coccolithophores, such as the well-studied Emiliania huxleyi. Furthermore, it can be incorporated into larger efforts to project the effects of climate change on marine organisms