Predicting optimum growth and lipid accumulation of the microalgae Nannochloropsis salina and minimizing invading organisms with a response surface model

Background/Question/Methods

Despite the potential of algal biofuel to replace fossil fuels, mass production of microalgae is currently limited by existing cultivation strategies, which rely heavily on open cultivation systems.  Enhanced cultivation strategies that promote algae growth and lipid production in these systems while minimizing the invasion of non-target algae and predators are sought to improve the economic viability of algal biofuel. Optimization of marine microalgae Nannochloropsis salina cultivation was undertaken using the response surface method. We manipulated basic environmental parameters to promote algal growth and lipid accumulation, while limit invading organisms. A central composite design was applied to study the effects over a range of temperature (11˚-29˚ C), pH (6.3-9.7), and salinity (14.5 – 41.5 psu) values on the microalga (and invading organisms). The experiment was conducted in a greenhouse, where open-faced aquaria were subjected to natural light and temperature conditions. We monitored growth and lipid accumulation of the marine microalga Nannochloropsis salina and invasion of algal competitors and predators in open cultures. Together, this work demonstrates an ecological approach to the current limitations of cultivation strategies.

Results/Conclusions

Our model calculated the optimum conditions for algae growth and lipid accumulation, while reducing rate of invasion and growth of undesired organisms. The final model obtained was used to clarify the effects of each factor and their interactions on the growth of Nannochloropsis salina. The optimum growth conditions are suggested to be 20˚ C, pH 8 and 28 psu salinity. Lipid accumulation was greatest at higher salinities, while invading organisms were most affected by extreme pH levels. These conditions were tested and validated experimentally since the maximum growth achieved with these parameters is the best in this study. With the estimated model, the effects of temperature, pH, and salinity on Nannochloropsis salina and the interactions between these variables were clarified. The model proved to be efficient in defining optimal conditions of N. salina. For optimum biodiesel production in an outdoor raceway, we recommend growing N. salina at the stated optimum conditions during growth phase and then increasing salinity and pH levels when steady state is reached to further maximize lipid accumulation and limit invading organisms.