Nannochloropsis and biofuels
Encyclopedia
Nannochloropsis
Nannochloropsis is a genus of alga comprising approximately 6 species. The genus in the current taxonomic classification was first termed by Hibberd . The species have mostly been known from the marine environment but also occur in fresh and brackish water...
is a genus of alga within the heterokont
Heterokont
The heterokonts or stramenopiles are a major line of eukaryotes currently containing more than 100,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...
line of eukaryotes, that is being investigated for biofuel
Biofuel
Biofuel is a type of fuel whose energy is derived from biological carbon fixation. Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases...
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
Biodiesel
Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol....
.
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
NitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
is essential for algal growth. Within a cell, nitrogen is involved in sythesizing amino acids, nucleic acids, chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
, 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 cycleCarbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
. 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 isoloated from Singapore's costal 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.
Alginate oligosaccharides
Alginate, or alginic acidAlginic acid
Alginic acid, also called algin or alginate, is an anionic polysaccharide distributed widely in the cell walls of brown algae, where it, through binding water, forms a viscous gum. In extracted form it absorbs water quickly; it is capable of absorbing 200-300 times its own weight in water. Its...
, is a natural acidic linear polysaccharide derived from seaweed. It is composed of α-L-guluronate and β-D-mannuronate. Bulk alginate is widely used in the food industry and for medical purposes due to its unique characteristics such as high viscosity in aqueous solution and gel-forming property in the presence of calcium ions. Previous studies have also shown that alginate oligosaccharides may act as growth promoting agents on some plant cells. The effect of an alginate oligosaccharide mixture (AOM) on N. oculata was studied. The growth rate of this alga was significantly increased by AOM. Moreover, AOM appeared to alleviate the algicidal effect of Cu2+ significantly. These results suggests that AOM can be used a growth promoting supplement for N. oculata culture.
Temperature
Temp has significant impact on algal growth rate, cell size, and biochemical composition. Either in natural habitats of algae or in controlled growth systems, temperature will vary. In a study on the effect of temperature on growth rate and lipid content, temperature showed no significant relation with Nannochloropsis sp. growth rate between 15 °C and 30 °C. However, another algal species in the same study, Isochrysis galbana, showed increased growth rate as the temperature increased from 15 °C to 30 °C. In many algal species, increased lipid content is also observed under increased temperature.Culture technologies
Different culture technologies are being tested with Nannochloropsis to determine most cost-effective culture methods.Helical-tubular photobioreactor
Tubular systems are the most widely used commercial culture systems. They are usually made of polypropylene acrylic or polyvinylchloride pipes which have small internal diameters and an air pump that generates bubbles to mix and agitate the culture. They usually use artificial light but some models use natural light. The major disadvantages of this type of system, varying among individual systems, are high space requirements, cleaning, low efficiency, low gas transfer, and hydrodynamic stress. Several other problems also occur, including growth of the algae on the tube wall leading to blockage of light, high oxygen concentration inhibition of growth, and limits on the length of the tube in single run. Coiled systems were developed mainly to improve space utilization. The main advantages are: 1) large ratio of culture volume to surface area and optimized light penetration depth; 2) easy control over temperature and contaminants; 3) easy spatial distribution of fresh air and CO2; 4) better CO2 transfer through the culture; and 5) automated sensor providing cell concentration reads.Open pond and flat-plate photobioreactor
Raceway ponds are shallow ponds between 10 and 50 cm deep. They are less expensive to build compared to photobioreactors, and have low-energy-consuming paddlewheels to mix the circulate the culture. The culture is open to the atmosphere, thus allowing liquid evaporation to stabilize the temperature. They are widely used to culture several algae and cynobacteria. However, only limited types of microalgae can be grown in open ponds. Other disadvantages include large area required, low efficiency of light utilization, poor gas/liquid transfer, no temperature control, high risk of contamination, and low final density of culture.Flat-plates are a closed system such as helical-tubular photobioreactor. They have a flat surface screen made of glass or optical light film for the even reception of light. A study compared the cultures of Nannochloropsis sp. in open ponds and photobioreactors, tubular and flat-plate. Horizontal tubular photobioreactor was not shown to be economically viable. Both open pond and flat-plate photoreactors were proven to be feasible, given that the lipid content of biomass could be increased to 60%. However, neither system is competitive due to the low cost of petroleum.
Fed-batch culture
Some algae can grow faster under mixotrophic conditions rather than under photoautotrophic conditions. Under mixotrophic conditions, both light and a carbon substrate such as sugars can provide energy for cell growth. Although adding glucose increased the growth rate of the algal culture, it also adds extra cost to algal lipid production. This needs to be further studied to determine economical feasibility.Flue gas method
A unique bio-technology-based environmental system can utilize flue gasFlue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...
from coal burning power plants. This method is reported to decrease the cost of algae production significantly. It also absorbs excessive CO2, thus alleviating greenhouse effect.