Microalgae are widely distributed in the world's major water systems, all microalgae individuals are very small, humans can not see with the naked eye, but do not look down on it because the microalgae individual is small, its position in the global ecosystem is very important.
The appearance of cyanobacteria 3.5 billion years ago changed the composition of the primordial atmosphere for the first time; 2 billion years later, higher eukaryotic algae appeared. The oxygen produced by algae forms an ozone layer that blocks ultraviolet light and provides a natural protective barrier for the evolution of life on Earth. Since then, "all things on the earth have been free." Therefore, microalgae were the first pioneers on Earth and the pathfinders for the evolution of higher life.
Confocal pictures of microalgae, the microalgae in the picture have flagella, indicating that they have strong swimming ability
Even in the 21st century, microalgae are still closely related to human life. Spirulina and chlorella have the function of food health care; microalgae can be used as food additives and high-quality open bait for aquaculture animals; EPA, DHA, pigment, glycerin and other bioactive substances extracted from microalgae also have the function of nutrient strengthening.
In recent years, major scientific and technological progress has also been made in controlling desert algae and processing microalgae into biological fertilizers and bioplastics. So far, the boundaries of microalgae applications are still expanding, where is the end point? Maybe no one will know.
<h1 class="pgc-h-arrow-right" > microalgae are indispensable "golden partners" in aquaculture</h1>
At present, there are more than 70 kinds of main farmed fish in China, of which there are more than 20 kinds of marine fish and more than 40 kinds of freshwater fish (in fact, there are more than 800 kinds of freshwater fish in China alone, of which more than 250 kinds have economic development value, but more than 80% of them are not the main breeding species at present). In addition, there are 20-30 species of shrimp and crabs mainly farmed, and there are more than 30 species of shellfish mainly farmed.
A common feature of these diverse aquaculture species is that they are inseparable from microalgae in the stage of juvenile development. China's important farmed algae are divided into 6 phylums, a total of more than 20 species. The main types are as follows:
Phylum Diatomaceous: Triangular brown finger algae, small crescentrod algae, Mou's algae, Slender algae, Middle rib stripe algae
Phylum Green Algae: Subcardiac flat algae, Tachylonella, Ducella, Chlorella microgress, Chlamydia rhein, Gravis, Phyllonorycter, Erythrococcus haphalus
Golden algae gate: ball and other whip golden algae, Zhanjiang and other whip golden algae, green paveella
Phylum Xanthophyllum: Isocoidan algae
Cyanobacteria Phylum: Blunt-tipped spirulina
Phylum Erythrocephalus: Purple bulbophyllum
▲▲Various green algae moving downstream of the microscope (seemingly large flat algae)
Take the phylum of phylum Phyllonorycter, for example, this microalgae is rich in astaxanthin, accounting for about 2%-4% of the dry weight of cells, which is also the highest known astaxanthin content of algae. In the cultivation of salmon trout, shrimp and crabs, technicians often use astaxanthin as a feed additive.
Astaxanthin is a natural antioxidant with 550 times the antioxidant capacity of vitamin E and 10 times that of β-carotene. Fish, shrimp and crabs eat feed containing astaxanthin, their own unsaturated fatty acid content will increase significantly, nutritional value, stress resistance will also be qualitatively improved.
▲▲At different growth stages, Haematococcus pyrophyllum has different colors and appearances, mature individuals are bright red, and immature ones are green.
< h1 class="pgc-h-arrow-right" > microalgae: a priority list of biofuels? </h1>
As a large number of single-celled heterotrophic organisms, the wastewater purification capacity of microalgae and its potential as an energy organism have attracted global attention in recent years. At the same time, the price of oil as a conventional fuel is skyrocketing — in 1990, the price of crude oil was $20 a barrel, and by 2008, it had risen to $140 a barrel. This has been matched by a sharp increase in global greenhouse gas emissions – from 1970 to 2004, total global emissions increased by 70 per cent.
▲▲Rising oil prices (left) and relatively stable biofuel prices (right)
The soaring price of crude oil and the increasingly serious greenhouse effect have led to the development of microalgae must be put on the agenda as soon as possible.
Researchers have studied 26 biofuels and found that the use of 21 of them can reduce global greenhouse gas emissions by more than 30% compared to gasoline.
Microalgae oil production is not only environmentally friendly, but also very efficient. It is estimated that the average algae pond can produce 100,000 liters of oil per hectare, which is 8 times that of rapeseed and peanuts. There are 1.5 billion mu of saline-alkali land in China, and as long as 15% of the area is raised with microalgae, it can meet more than 50% of China's diesel demand.
But from 1970 to 1990, the National Renewable Energy Laboratory studied microalgae conversion experiments for more than 20 years, and the researchers came to a disappointing conclusion: it was not economically feasible to use microalgae for biodiesel production (the study was proposed in 1996).
The explanation given by experts is that, even in the best cases of photosynthesis, processing microalgae into biofuels costs twice as much as current oil prices.
However, from 1996 to 2009, the price of oil and diesel more than doubled. With other factors (such as the price of fertilizers) unchanged, it should be economically feasible to produce biofuels from microalgae now (2020).
But is this really the case?
< h1 class="pgc-h-arrow-right" > lipid extraction and biosynthesis of microalgae, and still faces many technical challenges</h1>
Microalgae can be used as biofuels because their cells contain lipids, a common energy substance. 1g of fat is fully burned in the air and can release 39100 joules of thermal energy, in the same case protein releases energy of 23500 joules, while the most common sugar is only 17200 joules.
▲▲ The picture above are 8 different lipids, (a) triglycerides; (b) diacyl glycerides; (c) glycerides monoethrates; (d, e) phospholipids; (f) sterols; (g) thiaurids; (h) glycolipids; and (i) carotenoids.
Although the heat price of lipids is high, it faces various difficulties in purifying the lipids in microalgae into biofuels, mainly in the following three aspects:
(1). The fatty acids in microalgae are unstable and easily oxidized.
Lipids are made up of fatty acids, alkyl esters are components of fatty acids, and the stability of alkyl esters depends on the structure of the acyl chain. The American Society for Testing materials (ASTK) considers the most important properties of biofuels to be combustion quality, cold flow characteristics, and oxidation stability. However, microalgae are rich in unsaturated fatty acids, which are easily oxidized and reduce the performance of the fuel.
A little supplement: the content of unsaturated fatty acids in vegetable oils is very high, unstable (easy to be oxidized by air into saturated fatty acids), generally liquid, such as soybean oil, peanut oil. The stability of animal fats and oils is very strong, and its saturated fatty acid content is high, generally solid. Since the saturated fatty acids in animal fats are not conducive to human cardiovascular health, it is recommended to eat more vegetable oils in daily life and use animal oils as little as possible.
(2). The composition of microalgae lipids is greatly affected by the growth conditions.
Studies have shown that the living environment of microalgae and the change of day and night cycles directly affect the composition of lipids. Moreover, the accumulation of microalgae lipids is carried out after cell growth and division have stopped, that is, as the lipid content increases, the growth rate of microalgae will decrease significantly, and this obvious reverse relationship may have an important impact on the economy of algal biofuels.
(3). Cultivating microalgae requires consuming a lot of nutrients
On average, 1 kilogram of microalgae produced consumes 2 kilograms of carbon dioxide and 80 grams of nitrogen, and the price of nitrogen is higher than that of oil, which is obviously not cost-effective.
▲▲The high-density cultivation of microalgae requires a steady stream of nutrients, the most basic is the carbon source and nitrogen source.
It is worth noting that the tailwater produced by aquaculture in China every year is rich in nitrogen-containing compounds, which are free nitrogen sources. Tailwater that is too late to discharge will lead to eutrophication pollution in offshore or rivers, but if this part of the sewage is reused, that is, the nitrogen and phosphorus elements in it are absorbed by microalgae, wouldn't it be possible to kill two birds with one stone?
<h1 class="pgc-h-arrow-right" > said so much, what are the cultivation methods of microalgae? </h1>
The large-scale cultivation of microalgae can be traced back to the 1950s, and the main methods of cultivating microalgae include open shallow water runway culture and closed photobioreactors.
(1) Open shallow water runway training
Raceway Aquaculture in English for runway aquaculture, this breeding mode generally adopts a depth of 0.2 to 0.3 meters, and is equipped with water pushing equipment in the middle of the pond to ensure the circulation of water bodies, and the aquaculture area ranges from 0.5 to 1 ha.
The greater the density of algae, the darker the color of the water body, and the microalgae located at the bottom layer are not easy to receive sunlight and carbon dioxide, so it is necessary to stir and add carbon dioxide in time.
▲▲Microalgae runway breeding center located in Israel
(2) Closed photobioreactor
Compared with the open runway culture model, the advantages of photobioreactors are that they can avoid external contamination, receive light more uniformly from microalgae, and can produce higher culture densities. Another use of photobioreactors is in the aerospace sector – one of the most effective ways to produce oxygen in outer space is to use photobioreactors to cultivate microalgae, and the oxygen and carbohydrates produced by microalgae are then used by astronauts, which is full of technology.
The light source in the reactor is usually a fluorescent lamp or a metal halide lamp, which can produce a stable light with a wavelength of 400 to 700 nanometers, which is also the most effective source of radiation for photosynthesis. In addition, the reactor can easily transport carbon dioxide and remove the oxygen produced by photosynthesis in time, and the gas exchange system is very perfect.
A popular research direction in aerospace: photobioreactor culture method for microalgae
<h1 class="pgc-h-arrow-right" > summary: the "great future" of microalgae</h1>
In different waters, the distribution of microalgae is closely related to the content of nutrient salts, generally speaking, the abundance of microalgae in the offshore is much higher than that in the far sea, and the density of algae in inland rivers is higher than that in the offshore. At present, the most common application of algae is the cultivation of fish, shrimp and crab larvae in aquaculture, and no biological bait has been found to completely replace the nursery status of microalgae (although new micro-particulate feeds are also under development, but they are far less affordable than microalgae).
Microalgae world
Microalgae are a treasure trove of flow and have great potential for development. In today's rapid development of science and technology, microalgae, as a renewable biological resource, is likely to replace oil in the near future and become an energy-saving and environmentally friendly biofuel.
It is expected that by 2022, the cost of microalgae biodiesel can be reduced by more than 50% by the use of transgenic directional culture technology, and it is expected to gain the ability to compete with oil.
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