## 1 Introduction
[PERSON] and [PERSON] [58] reported that the ratio of whole-cell to disrupted-cell absorption ranged from 0.5 to 0.7 for a small cell of _Thalassioira_ sp. The consequence of this is simply that the size of the microalgae will play an important role in the utilization of light, especially in densely populated commercial photobioreactors. We mean this is an important point that often has been missed during the choice of species to cultivate in photobioreactors. This \"simple\" fact, i.e., that self-shading can depend on particle size, has been addressed earlier [69; 70]. However, it is also clear that this is modified by the fact that microalgae cells consist of several organelles of variable sizes and shapes, e.g., chloroplasts, nucleus, nucleolus, mitochondria, lipid vesicles, gas or fluid filled vacuoles and pyrenoids with varying optical properties. The cell wall can also vary (e.g., \"glass\" frustrule in diatoms), and the final level of reduced absorption and diffusion for each species will be a result of the number and quality of internal organelles and structures. The exact geometrical measure of a cell will only be a good measure of its optical properties in \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Species** & **Diameter** & **Area** & **Volume** & **Area:** & **Species:** _Chlorella_ \\  & **(\(\upmu\)m)** & **(\(\upmu\)m\({}^{3}\))** & **Volume** & **Volume** & **Volume** \\ \hline _Chlorella_ sp. & 5 & 79 & 65 & 1.22 & 1 \\ _Attheya longicornis_ & 6 & 188 & 198 & 0.95 & 3 \\ _Porosira glacialis 1_ & 40 & 6280 & 43,960 & 0.14 & 676 \\ _Porosira glacialis 2_ & 45 & 9538 & 71,505 & 0.13 & 1100 \\ _Cosciindiscus radiatus 1_ & 180 & 152,604 & 2,289,060 & 0.07 & 35,216 \\ _Cosciindiscus radiatus 2_ & 220 & 227,964 & 8,366,666 & 0.03 & 128,718 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Diameter, surface area and biovolume of the green microalgae _Chlorella_ sp. and diatoms of different sizes (_Attheya longicornis_ and strains of _Porsira glacialis_ and _Cosciindiscus radiatus_). Diatom sizes are from the authors’ own culture collections while the _Chlorella_ sp. size and volume are from Martinez-Jeronimo and Gutierrez-Valdivia [63]. Other volumes are calculated from Menden-Deuer and Lessard [64]. solution when the cell is completely full of material. What adds to this is that some diatoms can manipulate the wavelength of light [48]. Today's reactors, except from vertical column designs, are constructed to handle short light depths due to the high self-shading of the small species applied [71]. These constructions are complicated and expensive devices that take up large areas vs. volume cultured. Our cultivation initiative was therefore designed to apply large cells to allow for long light depths and low self-shading. Potentially, this approach makes it possible to use (and illuminate efficiently) simple constructed columns with large volumes [64]. Another advantage related to large cells is that the harvesting process can be more efficient. The sinking speed of a sphere is ca. proportional to the square of the radius (Stokes law with a modification for shape and density). Large cells will therefore sink faster than small species if the specific cell density is higher than water, enabling, e.g., centrifugation to be more efficient [72]. If production costs could be lowered and volumes increased substantially, the potential market for algae biomass is large, as, e.g., algal biofuels and feed [39; 73]. Although, if this shall be achieved, as stressed earlier, improvements related to biology and technology are required [74; 75; 76]. The present project was initiated in 2015 on the premise that new cost-reducing solutions were needed. When choosing species to cultivate, focus was on high photosynthetic efficiency, low self-shading, and ability to handle low temperatures. Thus, large northern/Arctic diatoms were prime candidates. The final choice of species was done after thorough literature studies as well as numerous P vs. I experiments.