UC San Diego

Microalgae are single-celled, free-living aquatic organisms responsible for half of global primary productivity that are used for biofuels as a renewable source of energy. Physiological rates of all ectotherms such as respiration and photosynthesis are affected by temperature differently. Experiments show that algal growth often increases with temperature up to an optimum, ranging from 10°C to 30°C. However, these experiments often maintained constant day and night temperatures. Few studies have focused on the effects of nightly drops in temperature on algal growth. High nightly temperatures may increase respiration and therefore reduce algal growth. To investigate how day and night temperatures affect algal growth, I grew Nannochloropsis oceanica under six different temperature regimes consisting of day and nighttime temperatures of 5°C, 15°C and 25°C. Algae exposed to a daytime temperature of 25°C and nighttime temp of 15°C and the 15°C treatment of a 15°C daytime and nighttime temp had the greatest growth of all. Constant temperatures of 25°C or 5°C during the day and night resulted in slower growth. Therefore, a nightly drop in temperature has a positive effect on the growth curve of the algae if the night temperature is 15°C or greater. This indicates that optimal temperatures for growth of Nannochloropsis are lower at night than during the day. Therefore, to predict the effects of climate warming on primary productivity and identify optimal growth conditions for algae used for natural products, we must consider the effects of different day and night temperatures.