Comparison of continuous and day time only mixing on Tetraselmis suecica (Chlorophyta) in outdoor raceway ponds

The mixing of microalgae bioreactors is important as it circulates cells in and out of the light zone and reduces boundary layers, benefiting photosynthesis and nutrient uptake. However, mixing is also a major economical factor, impacting on power consumption and operating costs, and it remains a barrier for those locations where no electrical supply is available. Combining solar panels with algae ponds can help to alleviate the power cost burden, but if solar panels are the sole source of generated electricity for a regional raceway pond system, the paddle wheels will only operate during day time. This study investigated the growth of halophilic green alga Tetraselmis suecica, in outdoor paddle wheel-driven open ponds under three different mixing regimes. The first experimental condition was a continuously mixed regime, while in the second and third conditions, the paddle wheels were stopped overnight and restarted 1 h after sunrise or 1 h before sunrise, respectively. Growth rate, biomass productivity and cell weight showed no statistically significant difference during the period when the culture was growing under optimal environmental conditions. Chlorophyll fluorescence measurements of photosystem II maximum quantum yield in the light (Fv′/Fm′) over a 24-h period found no significant differences between cultures, further suggesting that under favourable conditions ceasing mixing overnight may have no undesirable impact on Tetraselmis health or productivity. These findings imply a potential 37 % power saving and a 33 % reduction in energy-associated costs. During a heat stress event, Tetraselmis was negatively impacted in all ponds to the same degree; however, the continuously mixed pond recovered faster than ponds that were not mixed overnight