Upstream & bioreactors
Photobioreactors for microalgae
Why algal culture is limited by photon delivery rather than by gas transfer — Beer–Lambert attenuation, saturation and photoinhibition, oxygen accumulation in closed systems, and why open ponds survive.
An algal culture is a chemostat with an unusual substrate: photons. They cannot be stored, they cannot be concentrated in the liquid, and they are delivered through a surface rather than mixed into a volume. Everything peculiar about photobioreactor design follows from that.
The light path is the reactor
Light entering a dense suspension is absorbed and scattered, and its intensity falls approximately exponentially with depth — the Beer–Lambert relation, with the extinction coefficient set by pigment content and biomass concentration. Because the decay is exponential, the useful depth is small and gets smaller as the culture grows. Raise the density to increase volumetric productivity and you shorten the illuminated layer by the same mechanism, so the illuminated volume shrinks as fast as the cell count rises. This is why flat panels are made a centimetre or two thick and tubular systems are built from narrow-bore tube: the geometry exists to keep the light path short.
It is also why volumetric productivity, in grams per litre per day, is a misleading figure for an algal system. The photons arrive per unit of illuminated area, so the physically meaningful metric is areal productivity — grams of dry biomass per square metre of ground or collector area per day — and a reactor can improve its volumetric number simply by getting thinner without producing any more biomass per unit of sunlight.
Too much light and too little, in the same vessel
Photosynthesis saturates well below full sunlight. Beyond saturation the extra photons are not merely wasted: excess excitation damages photosystem II, principally the D1 reaction-centre protein, and although cells run a continuous repair cycle, net photoinhibition sets in when damage outruns repair. In a working reactor the surface layer is therefore often above saturation and photoinhibited while the core, centimetres away, is below the compensation point and respiring. Mixing is what reconciles them, not for homogeneity of nutrients but to cycle cells between the two zones fast enough that each sees an intermittent, time-averaged intensity — the reason turbulence and circulation rate matter far more here than in a heterotrophic fermenter.
Closed systems then create a problem open ones do not have. Photosynthesis produces oxygen, and in a long tube with no free surface dissolved oxygen rises well above air saturation. High oxygen favours the oxygenase activity of RuBisCO over carboxylation, so photorespiration cuts net fixation, and the culture is also more vulnerable to oxidative damage. Tubular designs therefore need degassing columns at intervals, which limits how long a run of tube can be, while carbon dioxide must be supplied continuously against a similar transfer constraint.
Why open ponds persist
Raceway ponds have poorer areal productivity than closed reactors, worse temperature control and no defence against grazers or competing species. They survive because they are an order of magnitude cheaper to build, need almost no energy to circulate, and are self-degassing. The contamination problem is solved biologically rather than mechanically: commercially successful open cultivation is dominated by organisms that make their own selective environment — Spirulina at high alkalinity, Dunaliella at high salinity — where few competitors can live. Where no such extremophile exists for the desired product, containment has to be paid for.