Nannochloropsis is a genus of alga within the heterokont line of eukaryotes, that is being investigated for biofuel production. One marine Nannochloropsis species has been shown to be suitable for algal biofuel production due to its ease of growth and high oil content (28.7% of dry weight), mainly unsaturated fatty acids and a significant percentage of palmitic acid. It also contains enough unsaturated fatty acid linolenic acid and polyunsaturated acid (>4 double bonds) for a quality biodiesel.
Conditions that lead to oil content increase
Oil productivity is defined as the oil produced by the algae per day per liter of culture, which is dependent on both growth rate and lipid content. Growth rate indicates how rapid the algae grow and lipid content indicates the percentage of dry weight that is lipid. In most of the studies, these two factors are investigated independently. Under normal growth conditions, Nannochloropsis does not reach its optimal oil production. Several conditions, including stress conditions, have been reported to increase oil content in Nannochloropsis.
Nitrogen deprivation
Nitrogen is essential for algal growth. Within a cell, nitrogen is involved in synthesizing amino acids, nucleic acids, chlorophyll, and other nitrogen-containing organic compounds. In a study in which 30 different microalgal strains were screened, one Nannochloropsis strain was shown to obtain 60% lipid content after nitrogen deprivation, up from 30% under normal growth conditions. This strain was selected for further scale-up experiments in a photobioreactor under natural sunlight. Lipid productivity increased to 204 milligram per liter per day(mg/L/day) under nitrogen starvation conditions, almost twice as much as the 117 mg/L/day under sufficient nutrition conditions. Based on these results, a two-phase cultivation process, with a nutrient sufficient phase to rapidly increase number of cells prior to a nitrogen deprived phase to boost lipid content, was found to produce more than 90 kg of lipid per hectare per day in outdoor cultures. I, depending on sun light conditions.
Carbon dioxide aeration
Algae play an important role in earth's carbon cycle. Algae generate large deposits of carbonate minerals and organic compounds that are resistant to microbial breakdown, thereby contributing to the reduction of CO2 level in the atmosphere, making the earth more habitable for other organisms. The CO2 concentration also has an effect on algae growth and lipid content. In Nannochloropsis oculata, the effect of CO2 concentration on biomass production and lipid accumulation was investigated. The results showed that the lipid content of N. oculata increased from 30.8% to 50.4% upon 2% CO2 aeration. Thus, this algal strain is recommended to be grown with 2% CO2 to maximize lipid production.
Blue light and Ultraviolet A (UV-A) A light-acquisition problem exists for aquatic algae since submergence can reduce light intensity and dampen photosynthesis. For land plants, full-spectrum of sunlight, from blue to red light, is available for chlorophyll absorption. However, red light is absorbed in the few meters of water closest to the surface of an aquatic environment and the light environment beneath these few meters is mainly blue-green in quality. Algal cells are likely to be transported to such depth of water, and many have evolved a mechanism to better absorb blue-green light. A Nannochloropsis species isolated from Singapore's coastal water was investigated under different light wavelengths (red, green, blue, and white) and intensities to determine the optimal condition for biomass productivity and lipid production. The maximum fatty acid yield was achieved for both phototrophic (sunlight is the only energy source) and mixotrophic (utilize both sunlight and energy from carbon source) cultures at 55.15 and 111.96 mg/L, respectively, under cell exposure to blue light (470 nm). The biomass productivity of the algae also peaked under blue light for both cultures.
In another study, UV-A (320 - 400 nm) was added to the photosynthetically active light spectrum (400 - 700 nm) to culture Nannochloropsis in order to study the effect of UV-A on growth and lipid accumulation. The results showed that modulated UV-A usage can lead to an increase in growth rate.