Combined influence of light and temperature on growth rates of Nannochloropsis oceanica:: linking cellular responses to large-scale biomass production

The interaction effects between irradiance and temperature on growth rates of Nannochloropsis oceanica were determined in both laboratory cultures and large-scale tubular photobioreactors. Growth responses were investigated in 48 batch cultures subjected to crossing light/temperature gradients ranging from 34 - 80 mu mol photons m(-2) s(-1) and 14.5 - 35.7 degrees C respectively. Comparisons were made to growth responses observed in production systems (200 L biofences) operated in climate-regulated greenhouses with controlled temperature and artificial light gradients. Cellular responses showed increasing specific growth rates as a function of temperature, with a peak at 25 - 29 degrees C, after which the growth became increasingly unstable. The optimum temperature for growth increased with higher light intensities up to approximately 28 degrees C at 80 mu mol photons m(-2) s(-1). At low light intensities the specific growth rate was less affected by temperature. The maximum daily production measured in the biofence systems increased proportionally with irradiation and reached approximately 0.7 g L-1 d(-1) at 1030 mu mol photons m(-2) s(-1) average daily radiation for a culture temperature of 24 degrees C. This corresponds to a daily yield of 140 g per day in a 200 L biofence system. When specific growth rates for the biofence cultures were measured at different densities and plotted against temperature, results showed a peak with the 24 degrees C temperature treatment. This peak became less pronounced as the density increased in the cultures. This is consistent with the laboratory results; increasing cell density in the biofence cultures resulted in less average light cell(-1), which produced the same temperature dependent response as seen by reducing the external irradiance exposure for the dilute laboratory cultures.