## 5 Cultivation Systems  
Phototrophic microalgal cultivation systems can generally be classified into one of two classes, closed (for instance vertical or horizontal tubular, flat panel or other photobioreactor [PBR]) or open (for instance, open ponds, paddle-wheel driven raceways, and cascade raceways). Open ponds or raceways are used mainly for wastewater treatment or food and feed production because they are generally cheaper to set-up (capital expenditure--CAPEX) and run (operating expenditure--OPEX), but offer limited control over light, temperature, and CO\({}_{2}\) conditions; the biomass is prone to external contamination or crashes due to predation and is not suitable for pharmaceutical applications. Closed PBRs offer improved culture stability and biomass densities and overall lower chances for contamination, but they have higher CAPEX and OPEX. Due to the generally considered requirement for contained cultivation, closed systems are the de facto systems of choice for GM microalga at present.  
The open culturing systems in use at the moment at large scales produce low density cultures (0.8-3.6 g/L) [67] and one of the main limiting factors in autotrophic growth is the light penetration into the culture [68]. Despite the obvious commercial risks discussed previously, academic interest has, nevertheless, focused on improving photosynthetic efficiencies by a range of approaches including GM to manipulate the light harvesting complex (typically via a decrease in antenna size) in order to increase the culture density [69; 70; 71]. To date the improvements have been low, and more effective results have simply been achieved by improvements to the cultivation systems such as simply increased mixing or decreasing the depth of the culture and thereby increasing the relative surface area used for cultivation [72].  
Cascade raceways are a more recently developed innovation in open production systems [72], where the system design creates a shallow moving flow of culture maintaining an extremely small light path length and achieving substantially higher biomass densities (e.g., 15-35 g/L of _Chlorella_ spp. has been reported [73]). Compared with tubular PBRs, flat panel PBRs seem to be the most productive choice for closed cultivation (24 g/L of biomass (_Chlorella sorokiniana_)). However these systems suffer from very high costs and problems with scale up [74]. A new closed system in course of evaluation is the foam-bed PBR, where the algae are trapped in bubbles with a foaming agent and bovine serum albumin suspension to stabilise them; lower gas pressure is needed for mixing but the addition of a foam-breaking system is required so that overall operating costs are potentially higher and the yields are not specified as yet [75].  
For all of the above production scenarios, results are still a far cry from heterotrophic production systems that can be used to produce some microalgal strains at industrial scales, where biomass densities of 100 g/L and above dry weight can be achieved, at which point the biomass does not need to be dewatered but can just be dried, disrupted, or extracted, eliminating one energy intensive step of the downstream process.