Cryo-injury in algae and the implications this has to the conservation of micro-algae

As with all products and processes exploiting biological materials algal biotechnology has an absolute requirement for stable, function-fulfilling master stockcultures. Conventionally microalgae and cyanobacteria are maintained by serial transfer with inocula (2.5-25% v/v) being transferred to fresh medium and the culture being held under controlled environmental conditions. This method is satisfactory for many algal taxa, but by its nature cannot provide an absolute guarantee of phenotypic or genotypic stability. Cryopreservation at ultra-low temperatures (> -130oC) is the only methodology that can provide this level of security to master stock-cultures; however, many algae are recalcitrant to cryopreservation with low or no survival. This review explores the reasons of this cryo-recalcitrance on the application of conventional colligative, two-step cryopreservation protocols and points towards the options available to enhance postcryopreservation viability.

Effects of centrifugal stress on cell disruption and glycerol leakage from Dunaliella salina

Dunaliella salina accumulates large amounts of intracellular glycerol in response to the increases in salt concentration, thus is a potential source for producing fuel grade glycerol as an alternative to biodiesel-derived crude glycerol. D. salina lacks a cell wall; therefore the mode of harvesting Dunaliella cells is critical to avoid cell disruption caused by extreme engineering conditions. This study explored cell disruption and glycerol leakage of D. salina under various centrifugal stresses during cell harvesting. Results show a centrifugal g-force lower than 5000 g caused little cell disruption, while a g-force higher than 9000 g led to ~40% loss of the intact cells and glycerol yields from the recovered algal pellets. Theoretical calculations of the centrifugal stresses that could rupture Dunaliella cells were in agreement with the experimental results, indicating optimisation of centrifugation conditions is important for recovering intact cells of D. salina enriched in glycerol.

Feasibility of lipid mechanical extraction from viable Monoraphidium minutum

Background. Mechanical stress was investigated as a mean to harvest microalgal lipids without affecting algal cells’ viability. Monoraphidium minutum was cultivated in laboratory-scale photobioreactors and suspension cultures were submitted to mechanical stress to compare a cyclone, a centrifuge and a homogenizer. Lipid content within the extracellular medium was analyzed prior to and after treatment, and the amount of released lipids was quantified. Algal cell viability was also evaluated before and after treatment.Results. After mechanical-stress treatments, 7.0 to 12.7% of the intracellular lipids of Monoraphidium minutum were released and found in the extracellular medium, while recovered algal cells presented low levels of disruption after treatments.Conclusions. To the best of our knowledge, this is the first proof-of-concept demonstration on the use of mechanical stress for lipid extraction from viable microalgae. Certain level of centrifugation proved to make algae release around 10% of their lipids to the extracellular medium. This mostly exploratory work calls for deeper investigation, paving the way for a biofuel production based on continuous lipid recovery and microalgae reuses.