Nitrogen-starvation of algae to produce more lipids
“The purpose of this experiment is to explore the possibility of naturally depleting the nitrogen of algae’s media to boost its lipid yields,” said Gregory. “This is an improvement of the normal methods that involve harvesting the algae and physically transferring it into a nitrogen depleted environment. With my technique it should be possible to bypass the transferring step and grow the culture in a single media. This reduces the labor and material costs of growing algae for its lipid.”
Natural nitrogen depletion is a process where nitrogen is depleted naturally through the algae’s consumption. This method allows the algae to stay in one media while consuming the nitrogen, eventually depleting it. This process is an improvement over normal methods because it improves lipid content – and thus the amount of biofuel that can be produced. These higher lipid yields are expected because nitrogen depletion has already been proven to increase the lipid in algae – and this is an improved way of doing it.
As noted in the text, the idea that depletion of nitrogen nutrients causes an increase in lipid content is known.
For example, from K.K. Sharma, High Lipid Induction in Microalgae for Biodiesel Production, Energies 2012, 5, 1532-1553; :
Nitrogen is the single most critical nutrient affecting lipid metabolism in algae. A general trend
towards accumulation of lipids, particularly TAG, in response to nitrogen deficiency has been
observed in numerous species or strains of various microalgae [24–26]. Hu et al.  conducted a
study on nitrogen stress responses of several green microalgae, diatoms and cyanobacteria and all
tested species showed a significant rise in lipid production. A detailed and large-scale model of lipid induction by nutrient starvation (nitrogen, phosphorus) on several diatoms, green algae, red algae, prymnesiophytes and eustimatophytes is presented in a study carried out by Rodolfi et al. .
A problem is that nitrogen starvation can lead to the production of less biomass; from published US patent application 20120322157 (to Sapphire):
 Much research has been conducted over the last few decades regarding using microalgae as an alternative and renewable source of lipid-rich biomass feedstock for bio fuels. Microalgae are an attractive model in that they are capable of producing substantial amounts of lipids such as TAGs and DAGs under stress conditions, such as nitrogen starvation. However, a decrease in growth of the microalgae under nitrogen starvation makes it harder to use microalgae in the large scale production of biofuels.
Bryan Willson noted in US 20080160591 :
In other cases, the algae may be subjected to environmental stress conditions designed to enhance lipid production, such as nitrogen starvation or other nutrient deficient conditions. The amount of nitrogen in depleted media may vary from 0 to 75% of the normal amount, depending on the medium used and the type of algae to be stressed.
As to amounts, from US 20140162330 :
 Under optimal growth conditions, diatoms and other microalgae synthesize fatty acids primarily for esterification into glycerol-based membrane lipids, which constitute about 5-20% of their dry cell weight. However, under unfavorable environmental conditions, such as during nitrogen deprivation, many algae shift their lipid profile towards the formation and accumulation of neutral lipids, principally in the form of triacylglycerol. Under such unfavorable growth conditions, the total lipid composition of certain microalgae can increase to above 50% of the algae's dry cell weight.
One issue is that the desired target is increasing total lipid produced (say per unit time and/or per surface area), not simply increasing lipid per biomass. Another issue is that the organic phase from the biomass (obtained for example through hydrothermal liquefaction) still contains nitrogen, which is undesirable for biofuels.