Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Plants, Soils, and Climate

Committee Chair(s)

Bruce Bugbee


Bruce Bugbee


Jeanette Norton


Lance Seefeldt


William Doucette


Ralph Whitesides


Microalgae are single celled plants that inhabit aquatic and terrestrial environments across the planet. Many species are oleaginous, which means they are capable of producing oils, similar to many higher plants we are familiar with like canola, safflower and coconut. Different from higher plants, however, algae have simple structures that allow them to grow at very high rates. Due to these characteristics—oil production and rapid growth rates—algae are considered a promising future source of oil. Algal oils could be useful for production of food for people, feed for animals, biodiesel, detergents, and many other applications.

Algae have not been heavily used to this point as a source of lipids for a variety of reasons. One primary reason is that algal lipid formation generally is prompted by stress, such as nutrient deficiency. Nutrient deficiencies reduce growth, resulting in a difficult tradeoff between elevated cellular lipids and abundant cell division. This tradeoff is not well understood. We also have a poor understanding of what happens in the cell physiologically in response to nutrient deficiency that drives this lipid formation. To make algae useful as lipid producers on commercial scales, research is needed.

This dissertation is a report on three sets of research: 1) An assessment of species differences in growth and lipid content tradeoffs with high and low level nitrogen deficiency; 2) Investigation of physiological drivers of lipid formation, by mass balance accounting of cellular nitrogen pools with progressing deficiency; 3) Examination of the effects of sodium chloride and silicon on lipid production in a marine diatom.

1) Nitrogen deficiency typically had disproportionate effects on growth and lipid content, with profound differences among species. Optimally balancing the tradeoff required a wide range in nitrogen supply among species. Some species grew first and then accumulated lipids, while other species grew and accumulated lipids concurrently— a characteristic that increased lipid productivity. High lipid content generally resulted from a response to minimal stress.

2) Commonalities among species in cellular nitrogen at the initiation of lipid accumulation provided insight into the physiological drivers for lipid accumulation in nitrogen deficient algae. Total nitrogen uptake and retention differed widely among species, but the ratio of minimum retained nitrogen to nitrogen at the initiation of lipid accumulation was consistent among species at 0.5 ± 0.04. This suggests that lipid accumulation was signaled by a common magnitude of nitrogen deficiency. Among the cellular pools of nitrogen at the initiation of lipid accumulation, the concentration of RNA and the protein to RNA ratio were most similar among species with averages of 3.2 ± 0.26 g L-1 (8.2% variation) and 16 ± 1.5 (9.2% variation), respectively. This implicates vii critical levels of these parameters as potential signals initiating the accumulation of lipids.

3) In a marine diatom, low levels of either sodium chloride or silicon resulted in at least 50% increases in lipid content. The synergy of simultaneous, moderate sodium chloride and silicon stress resulted in lipid content up to 73%. There was a strong sodium chloride/silicon interaction in total and ash-free dry mass densities that arose because low sodium chloride was inhibitory to growth, but the inhibition was overcome with excessive silicon supply. This suggests that low sodium chloride may have affected metabolism of silicon.

These studies give insight into how nutrient deficiency can be used effectively to enhance lipid production in oleaginous algae.