Thank you for the question, nitrogen starvation does not cause any adverse effect. There are no undesirable byproducts to this process. The byproducts are the same as they are for nitrogen replete microalgae cultivation all of which can be valorized.

The previous studies (Van vooren et. al. 2012, Converti et al. 2009, Pruvost et al. 2011) have mostly been about varying parameters to see what conditions result in largest lipid productivity. Most of the parameters they varied affected the light transfer in the bioreactor which gave us a reason to believe lipid productivity and light availability and absorption are linked. Our study characterized the nitrogen starvation process in terms of the evolution of absorption and scattering cross-sections of the cells along with lipid productiivty. These radiation characteristics enable us to model the radiation field inside the photobioreactor as a function of time and optimize photobioreactors for maximum lipid production.

The significance of understanding this link is crucial to move from labscale photobioreactors to large scale outdoor ones.

There are many microalgae that accumulate lipids such as chlorella vulgaris, neochloris oleobundans, Scenedesmus obliquus. What is interesting about Nannochloropsis oculata is we have seen that during nitrogen starvation most of the accumulated lipids are in the form of triglycerides which can be converted to biodiesel by transesterification which consumes less energy compared to the conversion of fatty acids that need to be esterified.

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Yes, in fact the purpose of this characterization is to use the radiation characteristics and obtain a value for the optical thickness that results in the largest lipid productivity.

Preliminary analysis suggests that there is in fact an optimum optical thickness that results in maximum lipid productivity. This optical thickness is determined in a photobioreactor by both the cell concentration present and their absorption and scattering characteristics. The latter two along with the optical path length (thickness of the reactor in our example) are important characteristics of the reactor that can be optimized.

Therefore, given the knowledge of the radiation characteristics, the cell concentration and the optical pathlength can be adjusted to result in optimum conditions for maximum lipid production.

As a side note, scattering of light by microalgae are for the most part in the forward direction (due to their size compared to the wavelength of visible light) so it serves to diffuse the light further into the photobioreactor and is not a great source of light extinction.

Yes in fact they do. They lipid content in the cells plateaus. In addition, as they lose their pigments they lose the ability to perform photosynthesis and at some point they start to consume their lipids to maintain the biomass or die. We did not extend our experiments to this regime as is not interesting for industrial lipid production. The first week of cultivation is when the largest lipid accumulation occurs and when we expect to see highest lipid productivity.

As for the cell size, I have worked two different strains of Nannochloropsis oculata and one of them was on average 2.5 micrometer and the other (featured in this work) was approximately a micrometer larger in diameter. Each species and each strain has a distinct size but what we noticed was the size or volume increased as the lipid concentration in the cells increased. This may be due to the different densities of each of the specific components of the cell.