Open systems for the mass production of photoautotrophic microalgae outdoors: physiological principles

The major physiological principles involved in mass production of photoautotrophic microalgae outdoors relate to sustained trapping of solar energy in as high an efficiency as possible throughout the year.The tactics that should be employed for this goal include the improvement of suitable species, as well as developing culturing devices and proper management protocol aimed to facilitate efficient exploitation of the supper saturating photon flux densities existing outdoors.The most common system used today in industry for outdoor production of microalgae is the open raceway, in which stirring is provided by a paddle wheel. This mode of production suffers usually from many weaknesses, since it does not permit a satisfactory response to the two major variables that limit productivity outdoors — i.e.- solar irradiance and ambient temperature. Sustained production of algal mass the year round requires constant monitoring of the state of the culture and adjusting imputs accordingly. The readily controllable variables relate to mineral nutrients and carbon balance as well as to turbulent streaming in the culture and to the population density.The drawbacks of the open system relate in essence to the lack of temperature control and the long light-path which dictates maintenance of disadvantageously low cell concentrations. The open raceway thus falls short of the requirements necessary to insure sustained, year round high productivity outdoors.It is thus proposed that in the future, closed reactors may become the major production mode of microalgae outdoors.This paper was presented at the Symposium on Applied Phycology at the Fourth International Phycological Congress, Duke University.