## 1 Introduction
None of them have been genetically adapted (by hybridization, mutation or genetic engineering) towards economically sustainable production, as is the case with all their terrestrial agricultural counterparts [38; 39]. The diatoms especially have drawn little industrial focus. Diatoms are the most important primary producers in the oceans, especially in temperate and Arctic areas and upwelling zones where the largest fisheries are [40; 41]. Diatoms are considered to be important potential sources of biofuel, pharmaceuticals and food/feed [5; 8; 42; 43]. They are also potential valuable producers of unsaturated fatty acids, especially omega-3 fatty acids [44; 45; 46; 47]. Diatoms differ from other microalgae by lacking organic cell walls but live in houses of opaline silica, i.e., they reside in transparent glass houses ornamented with detailed nanostructures that can handle and optimize light in diverse ways [48]. Microalgae are grown either in open (pond) culture systems or closed flat plate, tubular and vertical column design PBRs. Of these, due to the greater ability to control the environment, closed types are commonly considered the most promising ones [30; 49]. On the other hand, if economic analyses are included, it is the open pond type that often comes out as the most profitable type [50]. Common to the few species of microalgae that have been and are exploited in mass cultivation is that they are small [6; 51; 52; 53; 54; 55; 56; 57]. Since they are small, they share the trait that they have a higher surface to volume ratio relative to larger microalgae cells (Table 1). When cell size increases, it logically follows that the volume of the cell increases faster than the surface area, since volume is cubed while surface area is squared. This causes a large diatom like, e.g., _Cosciindiscus radiatus_, to have a volume that is more than 100,000 times larger than a commonly cultivated small cell like _Chlorella_ sp. (Table 1). Because small cells have a higher surface to volume ratio it is often reported that they possess a greater ability to absorb light and nutrients and that they therefore grow faster than large cells [58; 59]. This is an oversimplified concept since other factors such as pigment type, amount, thickness, nutrient concentration, and uptake processes also play important roles [60; 61]. This can cause large cells to outgrow small cells, especially when nutrient concentrations are high. [PERSON] et al. [62] stated that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in areas with high concentrations of inorganic nutrients. [PERSON] et al. [61] suggested that high maximum photosynthetic efficiencies of large-sized phytoplankton might be associated with a higher PSII photochemical efficiency, characteristic of certain taxonomic groups such as diatoms. Assuming that inorganic nutrient concentrations are sufficiently high (as they often are in photobioreactors), the most important stress factor is light, or more precisely the availability of light in space and time. According to Mie theory the diffuse scattering coefficient relative to volume of a sphere decreases with increased size [65]. According to [PERSON] and [PERSON] [66], suspensions with a mean particle size of 8.5 \(\upmu\)m attenuate 660-nm light 15 times more efficiently than suspensions of particles with a diameter of 48 \(\upmu\)m. [PERSON] [67] reported that biomass (fresh weight)-specific absorbance coefficients decreased as algal cell volume increased, regardless of the incoming irradiance, supporting the notion that reduced self-shading allows large algae to support both greater maximal biomass and production than smaller algae. In terms of optical science, large cells in light limited regimes can hence take advantage of the \"package\" effect. This can also be seen as a reduction in the absorption of pigmented particles relative to the absorption of the same pigments in solution [68].