There are several therories on why stress conditions induce lipid accumulation under stress conditions. One popular hypothesis is that because the algae normal king accumulate lipid during the cell cycle prior to cell division, and stress conditions often block cell cycle progression, the entire culture becomes synchronized with increased lipid content.

Another hypothesis involves rearranging the entire cellular components. Under stress conditions there is often an observed decrease in protein content per cell, and rearrangement of membrane lipids (in particular chloroplast lipids). This can occur to the point where there is little actual increase in lipid content of the culture as a whole, however lipid content as a function of ash free dry weight is markedly increased.

The third hypothesis involves maintaining cellular redox homeostasis. Essentially some people believe that lipid biosynthesis under stress conditions acts as a sink for electrons. Essentially, the cells cannot shunt all solar energy absorbed into nonphotochemical quenching (NPQ) and therefor accumulate excess amounts of reducing energy in the form of NAdPH (reducing power). In order for the cells to not become over reduced, they increase lipid production because lipids are highly reduced molecules.

There is also the thought that they are simply storing energy for later consumption in the form of lipid, (remember that under stress conditions chlamydomonas firstly increases starch for long term storage).

Personally I believe that lipid storage in different algae is a combination of all of these theories. It depends on the strain and conditions and there is not one steadfast answer for all strains and stress condition

Hope this helps

I guess under the stress conditions, the metabolic flux to the life cycle should be inhibited to some extent. Therefore, it will create a "similar" high C/N condition for the algae, and the only way it can do is storing the extra-carbons as the form of lipids. That is why some kind of moderate stress could be beneficial to the lipid accumulation.

Oil-accumulating microalgae have the potential to enable large-scale biodiesel

production without competing for arable land or biodiverse natural landscapes. High lipid productivity of dominant, fast-growing algae is a major prerequisite for commercial production of microalgal oil-derived biodiesel. However, under optimal growth conditions, large amounts of algal biomass are produced, but with relatively low lipid contents, while species with high lipid contents are typically slow growing. Major advances in this area can be made through the induction of lipid biosynthesis, e.g., by environmental stresses. Lipids, in the form of triacylglycerides typically provide a storage function in the cell that enables microalgae to endure adverse environmental conditions. Essentially algal biomass and triacylglycerides compete for photosynthetic assimilate and a reprogramming of physiological pathways is required to stimulate lipid biosynthesis. There has been a wide range of studies carried out to identify and develop efficient lipid induction techniques in microalgae such as nutrients stress (e.g., nitrogen and/or phosphorus starvation), osmotic stress, radiation, pH, temperature, heavy metals and other chemicals. In addition, several genetic strategies for increased triacylglycerides production and inducibility are currently being developed. In this review, we discuss the potential of lipid induction techniques in microalgae and also their application at commercial scale for the production of biodiesel.

In the studies of lipid metabolism of unicellular photoautotrophic eukaryotes (microalgae), themain attention is commonly paid to polar membrane lipids and their fatty acid (FA) composition, whereasneutral lipids, represented predominantly by triacylglycerols (TAG), are insufficiently studied. As was reported recently, the role of these compounds in microalgae is not limited to their storage function. It was

found that TAG are frequently involved in adaptation to environmental conditions.

Another hypothesis revolves around algae to algae competition during the late Precambrian and Cambrian era when evolutionary explosion resulted in intense competition. Nitrogen depletion was a signal for storing more durable energy storage. Algae that could store more lipids survived the nitrogen depletion period. Please note that one can induce stress by changing ratio of carbon to nitrogen source (while keeping the absolute concentration of nitrogen the same!). As Heshmet pointed out, the accumulation tryacyl glycerol can have other environmental advantages (e.g. will be more buoyant leading to better photosynthetic rate.

Lipids is a reserve material so the cells survive through the night, raining days, shadows and etc. Other kinds of reserve materials are starch and PHB.

Environmental stresses (photo oxidative/nutritional/environmental) lead to TAG accumulation in algae. It is both cabon and energy source usually deposited inside the algal cells as lipid bodies in the cytoplasm. Unlike complex plant systems algae cant accumulate specific classes of lipid rather directly go for neutral lipid accumulation.

Microalgae accumulate neutral lipids, especially TAGs to source the extra photosynthate to a more durable molecules. Most of the TAGs have in their glycerol backbone, more monounsaturated fatty acids, so that they can be easily channelized for beta oxidation pathway of energy production and again the DAG produced are of great importance and constitute vital part of membrane  lipids for integrity maintenance and signalling as well.

Please see the articles.