On-line monitoring of large cultivations of microalgae and cyanobacteria

•Cultivation of microalgae requires on-line monitoring for high productivity.•Technologies for on-line, in situ monitoring of process variables are reviewed.•On-line measurement options for biological parameters are very limited.•New methods such as IR spectroscopy that have on-line potential are introduced.

Large cultivations of microalgae will benefit from on-line monitoring to achieve process control and improved productivity. This monitoring requires reliable sensors for on-line, in situ measurement of both physicochemical and biological process variables. Although standard industrial sensors can be used for many physicochemical variables, monitoring methods for most biological quantities rely on sensors that are currently suitable only for laboratory scale or off-line use. Here, we review these methods and discuss new approaches that could be adapted. We suggest that these new methods should be noninvasive and based on approaches that have already been applied to other bioprocesses; examples discussed here are in situ microscopy, flow cytometry (FC), IR spectroscopy, and software sensors.