Spirulina platensis is a unique photosynthetic cyanobacterium that is used as a commercial feed for animals and a dietary supplement for humans. S. platensis has significant auto-flotation activity due to its gas vesicles. The natural flotation activity of these cells provides an effective method for harvesting of Spirulina biomass. The present research found that hydrodynamic pressure, light level and bicarbonate concentration affected the natural flotation of S. platensis. In particular, auto-flotation increased as light level decreased (5.5-times greater at no supplemental light than at 300 μ㏖ photons/㎡/s), as bicarbonate level increased (5-times higher at 0.6 than 0.2 M) and as hydrodynamic pressure decreased (indicated by depth of the culture). These results should be considered when designing photobioreactors for direct harvesting by auto-flotation.

Microalgae and cyanobacteria are photosynthetic microorganism used as human nutrients. Light is the most important factor affecting the biomass accumulation and production of bioactive compound by photosynthetic microorganisms. However, excessive light can suppress their growth by damaging the photosynthetic machinery such as Photosystem II (PS2). Therefore, an LED-based illumination system that is used for cultivation of photosynthetic cells needs a device to appropriately regulate the light level, but the addition of an electronic dimmer increases the initial costs. Spirulina is one of the promising and representative photosynthetic microorganisms. In the present study, we describe the use of a new semicylindrical column photobioreactor for cultivation of Spirulina. The amount of incident light reaching the photobioreactor was easily adjusted by changing angle of the LED in relation to the flat surface of the bioreactor. Arrangement of the LED bar at a 45° angle led to the highest growth and chlorophyll content, and led to no retardation of cell growth during the initial growth phase.