Along with light intensity, temperature is one of the most difficult parameters to optimize in large-scale outdoor culture systems. Fluctuations in temperature, both daily and seasonally, can lead to significant decreases in productivity. The optimal growth temperature for microalgae is species specific, but often in the region of 20°C to 30°C (Chisti, 2008). Many algal species can tolerate temperatures of up to 15°C lower than their optimum, with reduced growth rates, but a temperature of only a few degrees higher than optimal can lead to cell death (Mata et al., 2010). The net effi­ciency of photosynthesis declines at high temperature as the rate of respiration rises significantly, while the increased flux through the Calvin cycle is moderate. This effect is worsened by the fact that CO2 becomes less soluble at elevated temperatures, more rapidly than O2 (Pulz, 2001).

Low seasonal, morning, and evening temperatures can lead to significant losses in productivity, although low nighttime temperatures are potentially advantageous due to a reduction in the respiration rate. As much as 25% of the biomass produced during daylight hours can be lost at night due to respiration (Chisti, 2007). Cool nighttime temperatures can minimize this loss.

Closed reactor systems almost always require some form of temperature control. They often suffer from overheating during hot days when temperatures inside the reactor can reach in excess of 50°C. Heat exchangers or evaporative water-cooling systems may be employed to counteract this (Mata et al., 2010). The culture system can also be placed inside a greenhouse, or contacted with water to minimize tem­perature fluctuations (Chisti, 2007). Closed PBRs are sometimes floated, either whole or just the solar collector, in a temperature-modulating water bath. Double­walled reactors with part of the liquid volume used for heating and cooling have been devised (Ugwu et al., 2008), although all such modifications add to the cost of production.

There is a relationship between temperature and light availability. Exposure to a rapid increase in light intensity when the temperature is below optimum (as occurs in the early morning in outdoor cultures) can lead to photo-inhibitory stress as cells are too cold to process incoming photons, thereby reducing photosynthetic efficiency for a good part of the morning (Vonshak, 1997). Low temperatures are therefore particularly suboptimal in the early morning, and any efforts to employ heat reactors should be concentrated just before dawn.