Modeling maximum lipid productivity of microalgae: Review and next step

Microalgae are fast growing organisms and have the ability to accumulate lipid, which can be converted to biofuels. Here, we review specific growth rate, population productivity, and lipid productivity based on 192 publications of the marine microalgae Nannochloropsis. Specific growth rate was reported by thirty publications often using exponential growth equations, and fourteen publications stated biomass productivity. However, direct comparison among productivity estimates is impossible due to differences in calculations or omission of equations. Less than 5% of the publications directly reported lipid productivity, the key parameter for biofuels. We extracted growth data from 30 publications using Plot Digitizer software and tested best fit with exponential and logistic equations. The logistic equation often represents growth data better than the exponential one. Furthermore, we argue that maximum sustainable yield (MSY) is a more useful measure for harvest rates than specific growth rates. Interestingly, MSY displayed closer linear relationships with carrying capacity measures (r2=0.780, p<0.001 and r2=0.552, p<0.001 for algae density and biomass, respectively) than growth rate (r2=0.297, p<0.001 and r2=0.095, p=0.090). We propose to apply concepts of logistic growth, carrying capacity and MSY calculations to estimates of maximum lipid productivity, similar to commonly used density and biomass calculations.