Abstract: A method of cultivating biomass by photosynthesis. The method includes providing a fluid growth medium in which biomass is dispersed and which light can penetrate, exposing the growth medium to a source of incident light characterised in that the incident light is lensed using lensing means so as to form one or more elongate light foci in the growth medium, and transporting growth medium through said foci in a direction transverse or oblique with respect to a longitudinal foci axis so as to provide temporal photo-modulation in the growth medium.