Effect of continuous and daytime mixing on Nannochloropsis growth in raceway ponds

Turbulence mixing is critical for generating high microalgal biomass productivity. Mixing also represents a major operational cost in large-scale microalgal production. The integration of photovoltaic cells with microalgae cultivation systems has been shown to eliminate the requirement of conventional grid-supplied electricity in rural areas. However, through such systems, the availability of electricity and the operation of equipment would solely depend on available sunlight and limited to only daytime. In accordance, in this study, we evaluated the effect of continuous paddle wheel mixing (24 h) and daytime mixing (12 h) on the growth, productivity and photosynthesis of Nannochloropsis sp. grown in outdoor paddle wheel driven raceway ponds operated at different depths (15 and 25 cm). Specific growth rates, volumetric and aerial biomass productivities were found to be significantly higher in ponds with 24 hour mixing compared to ponds with only 12 hour mixing operated at the same depth. The depth of the cultures did not affect the growth rate and volumetric productivity of cultures in both mixing conditions. Photosynthetic performance of cultures evaluated through chlorophyll a fluorescence measurements of photosystem II trended higher in ponds with 24 hour mixing compared to ponds subjected to 12 hour mixing. Our results clearly indicate the importance of continuous mixing to achieve high biomass productivity in cultures of Nannochloropsis sp.