Temperature and light intensity are the main factors affecting microalgae growth in unlimited nutrient conditions. The approach of assuming temperature and light intensity as two independent factors in photosynthesis models is generally accepted— however, a weak coupling between both variables is already observed at low light intensities, while limited data about the joint effect of temperature and light intensity are available under photoinhibiting light intensities.
At low light intensities, the photosynthetic rate is limited by the rate of photon supply, which is considered to be independent of temperature. At high light intensities, photosynthesis is no longer limited by photon supply, while temperature, on the other hand, can affect the rate of photosynthesis dark reactions, causing a potential imbalance between energy supply and energy consumption and affecting the threshold for saturation. A stronger coupling between light and temperature could therefore be expected at higher light intensities. Besides the short-term effects of temperature on growth rate, long-term effects may as well occur (acclimation to temperature) as well as changes in cell morphology.
An ongoing project at University of Padova is aiming to study the coupled effect of light and temperature at photosaturating light intensities on the microalga Scenedesmus obliquus. Figure 1 shows two small scale photobioreactors (PBRs) that are being used to test different combinations of light and temperature on acclimated steady state continuous microalgae cultures. Reducing the scale of PBRs can accelerate the process of data acquisition while easing the control of the manipulated variables and reducing the experimental effort.
Figure 1. Milli-photobioreactors used for the experiments (volume = 45 mL; thickness = 15 mm).
Around 48h after inoculating the reactors with the cells (in batch conditions), exponential growth phase is reached, and the reactors are connected in continuous operation. Optical density at 750nm (OD750) is measured every day to track stability. After 3 days with a stable biomass concentration, dry weight, cell count, pigment analysis, and image analysis are performed to characterise every sample.
Provisional results seem to indicate that the coupling between light and temperature is actually not very strong even at photosaturating light intensities; nevertheless, fitting the obtained data with an uncoupled model (i.e. a model that considers temperature and light intensity to be two independent factors) would be the final test to confirm this hypothesis.