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

Abstract

The interaction effects between irradiance and temperature on growth rates ofNannochloropsis oceanicawere 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 34–80μmol photons m⁻²s⁻¹and 14.5–35.7^∘C respectively. Comparisons were made to growth responses observed in production systems (200L 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^∘C, after which the growth became increasingly unstable. The optimum temperature for growth increased with higher light intensities up to approximately 28^∘C at 80μmol photons m⁻²s⁻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 0.7gL⁻¹d⁻¹at 1030μmol photons m⁻²s⁻¹average daily radiation for a culture temperature of 24^∘C. This corresponds to a daily yield of 140g per day in a 200L 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^∘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.