Microalgae: Characteristics, Cultivation, and Applications
Microalga, in its narrowest sense, is a photoautotrophic, unicellular eukaryote which uses CO2 and gains energy from light.
In eukaryotic algae, the donor DNA is integrated into the genomic or chloroplast DNA. Only Chlamydomonas reinhardtii (single-cell green alga with 2 flagella) has a history of stable genetic modifications and subsequent cultivation of the GM-strains. Vegetative cells of C. reinhardtii are haploid. Under stress conditions, e.g., nitrogen starvation, haploid gametes develop. There are two mating types, identical in appearance and known as mt(+) and mt(-), which can fuse to form a diploid zygote.
In the ‘centric’ diatoms (a paraphyletic group of basal lineages), sexual reproduction is oogamous, i.e., fertilization occurs between small motile sperm and larger immobile eggs. Pennate diatoms, on the other hand, are usually isogamous, with similar large, non-flagellate, amoeboid gametes. In this case, there is often no differentiation into ‘male’ and ‘female’. The vegetative cells of diatoms are diploid (2N), and so meiosis can take place, producing 1N gametes, which then fuse to form the zygote. The zygote is not flagellated, and it serves as a dormant form of the strains in the soil. In the light, the zygote undergoes meiosis and releases four flagellated haploid cells that resume the vegetative life cycle.
Algae Cultivation
Cultivation of algae at natural locations includes both micro and macro-algae (seaweed). For example, the micro-algae Dunaliella salina and Haematococcus pluvialis are cultivated under high salt conditions in ponds in coastal areas for food, fertilizer, or for the extraction of alginate, agar, and carrageenan as food ingredients.
Haematococcus pluvialis is a freshwater species of Chlorophyta. This species is well known for its high content of the strong astaxanthin, which is important in aquaculture and cosmetics. The high amount of astaxanthin is present in the resting cells, which are produced and rapidly accumulated when the environmental conditions become unfavorable for normal cell growth. Examples of such conditions include bright light, high salinity, and low availability of nutrients. Their resting cysts are often responsible for the blood-red color seen in the bottom of dried-out rock pools and bird baths. This color is caused by astaxanthin, which is believed to protect the resting cysts from the detrimental effect of UV radiation when exposed to direct sunlight.
Dunaliella salina is a type of halophile green micro-algae especially found in sea salt fields. Known for its antioxidant activity because of its ability to create large amounts of carotenoids (β-carotene), it is used in cosmetics and dietary supplements. Few organisms can survive in such highly saline conditions as salt evaporation ponds. To survive, these organisms have high concentrations of β-carotene to protect against the intense light, and high concentrations of glycerol to provide protection against osmotic pressure.
Algal Metabolism
Most algae are autotrophic (using energy from light by photosynthesis), some are heterotrophic (get energy from non-photosynthetic origin also). Mixotrophic algae can use sunlight or organic carbon.
The growth of algae for industrial production also shows a wide range: from dark in steel fermenters (heterotrophic) to light in glass or plastic growth systems (phototrophic or mixotrophic), from salt (seawater), brackish to fresh water. Other factors like pH, temperature, nutrients, and aeration are of importance for optimal growth. Optimization of culture conditions is an important issue in algae research.
Specific Algae Species
The unicellular red micro-alga Galdieria sulphuraria is a eukaryote that can represent up to 90% of the biomass in extreme habitats such as hot sulfur springs with pH values of 0 to 4 and temperatures of up to 56°C. This red alga thrives autotrophically as well as heterotrophically on more than 50 different carbon sources, including a number of rare sugars and sugar alcohols. Phycobiliproteins.
Chlorella is a genus of single-cell green algae. It is spherical in shape, about 2 to 10 μm in diameter, and is without flagella. Chlorella contains the green photosynthetic pigments chlorophyll-a and -b in its chloroplast. Through photosynthesis, it multiplies rapidly, requiring only carbon dioxide, water, sunlight, and a small amount of minerals to reproduce. It can also grow in heterotrophic fed-batch cultures.
Chlorella sp. principally grows at glucose concentrations of more than 60 g/l, but residual concentrations as low as 10 g/l significantly inhibit their growth.
Scenedesmus is one of the most common freshwater genera; they are colonial and non-motile.
Phaeodactylum tricornutum is a diatom, which can exist in different morphotypes (fusiform, triradiate, and oval), and changes in cell shape can be stimulated by environmental conditions. Another peculiarity is that during asexual reproduction, the frustules do not appear to decrease in size. This allows continuous culture without the need for sexual reproduction. In fact, it is unknown if P. tricornutum is capable of sexual reproduction. Maximum product yield and maximum biomass concentration were achieved at the same cultivation temperature between 21.5 and 23ºC.
Cell densities of more than 100 g/l cell dry weight, achieved with Chlorella, Crypthecodinium, and Galdieria species, highlight the potential of heterotrophic microalgal processes.
For selectivity reasons and the resulting ease of handling, microalgae such as Galdieria sulphuraria, which perform well at 42ºC and at a pH of 2, are desirable.
Heterotrophic growth of Dunaliella sp. and Nannochloropsis sp. is possible but is not practicable due to its very slow growth.
Galdieria sulphuraria reached the highest specific growth rates at glucose concentrations of between 2 and 166 g/l, while a glucose concentration of 200 g/l was regarded as inhibiting its growth.
Customizable Biomass Composition
Customizable biomass composition (following strategies, potentially combined with fed-batch culture):
- Controlling the availability of components in the growth medium other than carbon
- Replacing a medium component with an alternative (e.g., using a different carbon or nitrogen source or exchanging sulfur with selenium)
- Adapting the culture conditions (for example, T, pO2, pH) to conditions that would typically be outside of the optimal range for biomass growth.
Biomass composition can be customized and/or product formation can be enhanced through tailoring the composition of the culture medium.