JPH05304945A - Fine alga belonging to genus chlorella for immobilizing co2 in high concentration - Google Patents

Abstract

PURPOSE:To obtain the subject fine alga useful as a feed for domestic animals, capable of stably growing at a specific CO2 concentration, usable for fixing a highly concentrated CO2 in an exhaust gas of fossil fuel combustion in thermal power stations, etc., contributable to reduction in release of green house effect gas. CONSTITUTION:1ml water or 1g soil as a separated specimen from water of lakes and marshes and water in paddy fields or soil collected from various parts is added to a large-sized test tube charged with a liquid medium, aerated with a mixed gas comprising 15% CO2, 2% O2 and 83% N2 under a fluorescent lighting (2,500 lux illuminance) at 25 deg.C and subjected to standing culture for 3 weeks. Fine algae growing under the condition are collected and cultured. The culture mixture is subcultured and these operations are repeated to give fine algae growing under a condition of even 15% CO2 concentration. The fine algae are cultured in an agar medium, the colony is subcultured and cloned to give six kinds of fine algae HA-1,2, HC-1,2 and HT-1,2 of the genus Chlorella capable of stably growing at 5-20vol.% CO2 concentration.

Description

Detailed Description of the Invention

[0001]

The present invention relates to C under high CO ₂ concentration conditions.
Regarding Chlorella microalgae that fixes O ₂ , it can be used to fix high-concentration CO ₂ in fossil fuel combustion exhaust gas at thermal power plants, etc., and can reduce the emission of greenhouse gases derived from human activities. It relates to microalgae of the genus Chlorella that can contribute.

[0002]

2. Description of the Related Art Algae is a general term for lower plants that grow in water and undergo photosynthesis. Among them, microalgae consisting mainly of single cells are called microalgae, but the types are extremely large. Among microalgae, chlorella, Senedesmus, and spirulina are currently being used and industrialized as health foods, but acetic acid and inorganic carbonates are mainly used as carbon sources for their culture, and microalgae are used. There have been few attempts to positively utilize the inherent CO ₂ fixing ability. As described above, there is no example in which microalgae are used to fix CO ₂ in fossil fuel combustion exhaust gas such as industrial CO ₂ and exhaust gas from thermal power plants. In addition, CO ₂ fixed products obtained by fixing CO ₂ in exhaust gas are used for feed and industrial SCP (Single Cell Protein, microbial protein).
There is currently no industry to use as. In the past, Tokugawa Biological Research Institute has researched examples of culturing microalgae with exhaust gas from burning city gas, but the amount of CO ₂ fixed was small,
Moreover, the fuel cost is too high and has not been industrialized.

In order to industrially immobilize CO ₂ derived from fossil fuel combustion gas, which is discharged from thermal power plants in large quantities, by utilizing the CO ₂ immobilizing ability of microalgae, the exhaust gas is directly fed to the culture tank. The important key is to use microalgae that function efficiently even under the introduced conditions. However, in the conventionally known strains of microalgae, the concentration of CO _{2 that} can be enriched in the air blown into the culture tank was about 1 to 5% by volume (hereinafter,% means volume% unless otherwise specified). .. Exhaust gas produced by burning fossil fuel varies greatly depending on the combustion conditions, but the average CO ₂ concentration in the exhaust gas of thermal power plants in Japan is currently around 16% for coal-fired power and 15% for oil-fired power. Before and after, about 11% in LNG thermal power (all are dry gas base). Thus, the CO ₂ concentration in the exhaust gas of a thermal power plant is around 10 to 15%. When these exhaust gases are blown directly into the culture tank,
The CO ₂ concentration is so high that it cannot grow on ordinary microalgae. Further, when used by diluting with air to obtain the optimum CO ₂ condition, the cost of the exhaust gas dilution system becomes a major factor of raising the CO ₂ fixed cost. From the above, in order to introduce the exhaust gas directly into the culture tank to culture the microalgae and fix the CO ₂ , a high concentration of C
It is necessary to search for and use microalgae strains that function even under O ₂ conditions. Those that grow under relatively high CO ₂ conditions include Nannochloris (Nannochris) as a marine microalgae.
loris) NANNO2 strain [M. Negro et al., Appl. Biochem.
Biotechnol., 28/29, 877-886 (1991), N. Nishikawa
Etc., Energy conversion and management, Vol. 33, 553
-560 (1992)], Chlamydomonas as freshwater microalgae
(Chlamydomonas) genus MGA-131 strain [Yamada et al., Journal of Japan Society for Agricultural Chemistry, Volume 164, No. 3, page 659 (1990)
], Chlorella vulgaris 21
1 / 8k strain [HJ Silva et al., J. Gen. Microbiol., 13
0, 2833-2838 (1984)] etc. have been reported. Regarding the microalgae used to fix CO ₂ in the exhaust gas, the marine biotechnology research institute found 10%.
The marine microalga Chlorococcum littorale, which can grow even at a CO ₂ concentration of ₂ ), is only known (August 31, 1990, Nihon Keizai Shimbun).

[0004] In addition, about half of the substances constituting the body of microalgae are proteins, and there is a possibility that they can be used as industrial proteins or their raw materials, and they can also be used as highly nutritious feed. In order to prevent global warming due to an increase in greenhouse gases in the atmosphere, it is important to suppress the release of greenhouse gases, but if a CO ₂ fixation product of microalgae is used as feed or industrial protein. The indirect greenhouse gas emission reduction effect described below is recognized. In other words, currently, feed grains used as livestock feed in the world account for 40% or more of the total grain production,
During the production of the feed grain, a strong greenhouse gas such as methane generation accompanying composting of the residue of the grain crop and nitrous oxide accompanying the decomposition of the fertilizer is released. IP of 1990
According to the CC (Intergovernmental Panel on Climate Change) conference report, the greenhouse effect per unit weight is about 60 times that of CO ₂ and about 270 times that of nitrous oxide, considering the scale of the next 20 years. To be powerful. However, when SCP is produced by culturing microalgae, these greenhouse gases are hardly generated. If the production increase of feed grains in the future is replaced by the production of SCP feed, it can contribute to the significant reduction of greenhouse gases generated when producing feed.
In addition, globally, the arable land of forests has been extensively converted to increase the production of feed grains, and the carbon accumulated in the forests becomes CO.
_Although it is released into the atmosphere as ₂ , the production of microalgae SCP does not accompany the cultivation of forests and is expected to have the effect of suppressing the cultivation of forests. As described above, by fixing CO ₂ in the exhaust gas and culturing the microalgae, the effect of indirectly suppressing the release of greenhouse effect gas due to deforestation can be expected.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in consideration of such a situation, and has the above-mentioned high concentration of C.
A microalga capable of culturing using an exhaust gas containing O ₂ , a CO ₂ fixing method using the exhaust gas from the microalga, and a feed or industrial protein comprising the CO ₂ fixing product of the microalga. The challenge is to provide.

[0006]

Means for Solving the Problems As a result of various studies, the present inventor has isolated and purified specific microalgae that fix and grow CO ₂ in culture aerated with a high CO ₂ concentration in the atmosphere. It was found that the culture aerated with a gas simulating the exhaust gas does not promote the growth of the microalgae or bring about a decrease in the CO ₂ fixing ability.
_{2 It} was found that the protein content in the immobilized product was as high as 40% by weight or more, and the present invention was completed.

The present inventor has paid attention to the following points in order to search for strains applicable to CO ₂ fixation in exhaust gas. The CO ₂ concentration in the exhaust gas is three orders of magnitude higher than the concentration in the atmosphere, but among the places where natural algae inhabit, there are places such as soil where the CO ₂ concentration increases depending on the time and conditions. Therefore, search samples from the natural world were collected from water such as lakes and marshes, as well as paddy water and soil of paddy fields having such a high CO ₂ concentration.

That is, the present invention is 5 to 20% by volume.
(Chlorel) that can grow stably at various CO ₂ concentrations
la) concerning microalgae of the genus. In the present invention, “stable growth” means that the growth is equivalent to or higher than the growth under normal atmospheric aeration conditions.

The present inventor uses lake water, rice field water, soil and the like from various sampling sites as a separation source, and after pre-culturing under weak light, a specific mixed gas (CO ₂ = 15%, O ₂ = 2)
%, N ₂ = 83%), aerated and cultivated, and then cloned and sterilized by a separation / purification method using a plate agar medium to obtain 6 new pure isolates of microalgae. -1 strain, HA-2 strain, HC-1 strain, HC-2
The strain was named strain HT-1 and strain HT-2. These are strains that can grow even when aerated with a gas having a high CO ₂ concentration such as exhaust gas from a thermal power plant. The morphology and life cycle of these isolates were observed using an optical microscope and genus-level classification was performed.The morphology was similar, and systematic differentiation was single cell, cell morphology was spherical, and size. Is about 2 to 7 μm, proliferates by asexual reproduction that forms autospores in the body, and the number of chloroplasts in the cell is 1
The individual had no flagella and was not motile. From the above characteristics,
All six new isolates were determined to belong to the genus Chlorella. At present, the Institute of Microbial Science and Technology of the Agency of Industrial Science and Technology does not deposit algae, so none of the above isolates of the present invention has been deposited.

Further, the microalgae of the present invention since it is able to grow without special carbon source such as acetate or carbonate in addition to CO _2, it is capable of fixing and assimilate CO _2. Therefore, the present invention relates to a method for fixing CO ₂ using the microalgae according to the present invention. In this method, it is preferable to aerate the culture medium with a gas having a CO ₂ concentration of 5 to 20%. As such a gas, for example, various exhaust gases generated when a fossil fuel is burned may be used, and in particular, thermal power station exhaust gas is used. As a specific example, the above-mentioned microalga Chlorella HA-1 strain shows the maximum growth in the culture aerated with air having a CO ₂ concentration of 10%, and a growth promoting effect due to CO ₂ enrichment is recognized. At 15% CO ₂ , inhibition was observed due to an increase in CO ₂ concentration compared to 10%, but the proliferative capacity remained at a high level. This characteristic indicates that if the exhaust gas is directly introduced into the culture tank and subjected to aeration culture, the culture can be performed without promoting the growth of the HA-1 strain or reducing the CO ₂ fixing ability.

The protein content of the microalgae, which is a CO ₂ -fixing product, is 42 to 44% of the dry matter weight.
There is no great difference between the algal cells cultured under the _two conditions. The fact that about half of the algal bodies of the microalgae of the present invention including the HA-1 strain and the like are proteins indicates that they can be used as highly nutritive feeds or industrial proteins (or raw materials thereof). Therefore, the present invention relates to a feed or industrial protein comprising the CO ₂ fixing product of microalgae according to the present invention.

[0012]

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1 Separation, purification, and classification of microalgae Lake water, paddy water or soil (38 types in total) from various sampling sites, 1 m for water as a separation sample
l, in the case of soil, 1 g of wet weight was added to a large test tube containing 20 ml of MBM liquid medium (Table 1), and static culture was carried out at 25 ° C for 3 weeks under fluorescent light (illuminance 2500 lux). The light irradiation cycle was 12 hours for bright conditions (light irradiation) and 12 hours for dark conditions (light non-irradiation).

[Table 1] (Footnote) * Composition of A-5 solution is as _{follows: H 3 BO 3 286mg MnSO 4} · 7H 2 O 250mg ZnSO 4 · 7H 2 O 22.2mg CuSO 4 · 5H 2 O 7.9mg Na 2 MoO ₄ 2.1 mg distilled water 1 liter Next, mixed gas (CO ₂ = 15%, O ₂ = 2%, N ₂ =
(83%) was allowed to flow in a test tube with an aeration rate of 60 ml / min or more, and microalgae growing under these conditions were accumulated and cultured. Light was emitted using an incandescent lamp for photography (illuminance: about 11,000 lux). The culture was carried out at a temperature of 25 ° C. for 10 hours under light conditions and 14 hours under dark conditions for 5 days. The culture solution was subcultured in an MBM medium so that the absorbance was about 0.03 to 0.05 at OD = 660 nm, and the cells were further integrated and cultured for 3 days under the above conditions. As a result of further repeating subculture and enrichment culture, CO ₂ concentration of 15%
An enriched culture sample containing microalgae that could grow even under the conditions described above was obtained. Microalgae were separated from the obtained integrated culture sample by the following method. After serially diluting the accumulated culture sample with sterile water,
Apply the dilution to Detmer plate agar (Table 2),
The cells were statically cultured at 25 ° C for 2 weeks under a fluorescent lamp (illuminance: 2500 lux). Light irradiation cycle is 12 hours in light and 1 in dark
It was 2 hours. The grown colonies were subcultured on MBM agar medium (MBM liquid medium in Table 1 to which 1.5% agar was added) and MBM liquid medium.

TABLE 2 Composition of Detmer agar plate medium _{─────────────────────── Ca (NO 3) 2 ·} 4H 2 O 1.0g KCl 0.25g _{MgSO 4 · 7H 2 O 0.25g KH} 2 PO 4 0.25g FeCl 3 0.002g agar 15g water 1 liter of water ─────────────────────── The subcultured samples were confirmed to be contaminated by bacteria and the like on a nutrient agar medium, etc., and the separated samples were confirmed to be clones free from other microorganisms. Furthermore, the purified isolate was subjected to the above growth test under the enrichment culture condition again to confirm the growth ability under the enrichment culture condition. In this way, 6 strains of microalgae that grow with a high concentration of CO ₂ fixed at 15% were separated, and these strains were designated as HA-1 strain and HA-2 strain.
Strain, HC-1 strain, HC-2 strain, HT-1 strain and HT-
Two strains were named. The morphology and life cycle of these isolates were observed with an optical microscope, and all were judged to belong to the genus Chlorella. Table 3 shows the collection sites and morphology of the separation sources of the above-mentioned isolates. Further, micrographs of these isolates are shown in FIGS. 1 to 6, and respective color photographs are shown in reference photos 1 to 6. In addition, in the samples other than the above 6 isolates, microalgae strains that can grow under the conditions could not be isolated. In addition, the following 9 species of the National Institute for Environmental Studies strain-preserving microalgae strain: Chloralla pyrenoidosa NIES
-226, Chloralla vulgaris NIES-227, Scenedesmus acu
minatus NIES-92, Scenedesmus acutusNIES-94, Chlamy
domonas augustae NIES-158, Chlamydomonas pulsatill
a NIES-122, Oscillatoria agardhii NIES-204, Spirul
ina platensis NIES-45, Spirulina subsalsa NIES-27
When the growth was examined under the above-mentioned enrichment culture conditions, clear growth was observed in Chloralla pyrenoid.
With osa NIES-226 only, the other strains became white and died or barely grew.

[Table 3] Location and characteristics of isolates ^* ──────────────────────────────────── Isolate collection Ground (separated sample) Cell size Genus name ─────────────────────────────────── HA-1 strain Miho village, Ibaraki prefecture (low soil of lotus field) 2-7 μm Chlorella HA-2 strain Ibaraki prefecture Kitaura Shirahama (lake water) 2-8 μm Chlorella HC-1 strain Saga prefecture Chikugo river downstream (tamen water) 2-6 μm Chlorella HC-2 Co., Ltd. Lower Chikugo River, Saga (Tamizu) 2-7 μm Chlorella HT-1 Co., Ltd. Ayagami Town, Kagawa (reservoir water) 2-6 μm Chlorella HT-2 Co., Ltd. Zentsuji City, Kagawa (reservoir water) 2-7 μm Chlorella ──── ─────────────────────────────── (Footnote) * All six isolates are single-celled, spherical, and autologous. Formation by growing by asexual reproduction. It has only one chloroplast and has no flagella and is not motile. It is a green alga.

Example 2 Culture Characteristics of Microalgae Isolates (a) Growth Ratio Six isolates of the genus Chlorella described in Example 1 and the control strain Ch
For loralla pyrenoidosaNIES-226 (National Institute for Environmental Studies strain preservation microalgae), was examined the relationship CO ₂ concentration and growth rate. The culture device used in this example is a culture tank,
It consisted of a constant temperature bath, a light irradiation system and a CO ₂ enriched air supply device, and the light irradiation system and the CO ₂ enriched air supply device were time-controlled by a timer. The culture tank was a cylindrical acrylic container having a height of 30 cm and an inner diameter of 8 cm, and the outer wall was coated with white paint to prevent light from entering from the side surface. The light irradiation system includes a parallel light flux type high brightness light source device UI-501C.
(Manufactured by Ushio Denki) was used. In this device, ultraviolet rays and infrared rays are attenuated using a lens type filter and a cold mirror, respectively, and in terms of wavelength characteristics, visible light (400 to 700 nm) that can be used for photosynthesis is generated in a well-balanced manner. Can be simulated. In the main culture device,
Light is emitted from the upper part of the culture tank, and the light receiving area of the culture solution is 5
It was 0 cm ² . The average illuminance on the light-receiving surface measured by an illuminometer is 55,000 lux, which is equivalent to 1100 μE / m ² / sec, which is about half of the summer daytime illuminance in the Kanto region. The CO ₂ enriched air supply device is composed of a pure CO ₂ gas cylinder, an air pump, a gas flow meter, and a solenoid valve,
The air pump and solenoid valve are linked to the timer, and air and CO
The mixing of ₂ can be controlled. Adjusted C
The O ₂ enriched air was supplied as fine bubbles from the bottom of the culture solution through a glass filter (Kinoshita Rika Co., Ltd.). The CO ₂ concentration in the CO ₂ -enriched air was periodically confirmed by an infrared absorption type CO ₂ analyzer LX-710 (made by Iijima Electronics Industry Co., Ltd.). The temperature of the culture tank was kept constant by a temperature controller (manufactured by Taiyo Co., Ltd.). The culture experiment was performed as follows. Four sets of the above-mentioned culture devices are installed, and each device has air, air containing 5% CO ₂ , air containing 10% CO ₂ and 15% CO _2.
Containing air was aerated at a flow rate of 0.25 l / min.
As a culture medium, M4N medium (Table 4) having a higher nutrient concentration than the above MBM liquid medium and more suitable for high density culture was used. As for the inoculum, 70% of each isolate and control strain were cultivated in M4N medium with aeration of air.
It was prepared by centrifugation at 00 rpm for 30 minutes, washing with distilled water, further centrifugation, and resuspension in distilled water. 950 ml of M4N medium was placed in the culture tank of each culture device, and 50 ml of the inoculum was inoculated. The amount of algal bodies in the inoculum was the same in a series of experiments for each strain, and the inoculum amount for each strain was about 80.
To 110 mg. After inoculation, each device was cultured at 25 ° C. for 1 week while aerating air containing CO ₂ at each concentration. The light irradiation cycle was 12 hours for light conditions and 12 hours for dark conditions, and ventilation was performed only during the light conditions. The culture was performed in an open system rather than aseptic conditions. After culturing, the entire amount of the culture solution was centrifuged, washed with distilled water, further centrifuged, and resuspended in distilled water. 1 suspension
It was dried at 05 ° C. and the dry matter produced was measured. The dry matter sample was further measured for carbon content and nitrogen content using a C / N coder NC-800 type (manufactured by Sumitomo Chemical Co., Ltd.).

[Table 4] The results are shown in FIG. 7 to FIG. 13. As an index showing the growth of microalgae, the growth ratio, that is, the amount grown in one week of culture (the amount obtained by subtracting the inoculated dry matter weight from the final dry matter weight) was defined as the inoculated dry matter weight. The divided value was used. Therefore, the value obtained by adding 1 to the growth ratio shows how many times the amount of chlorella inoculated was increased in one week. Under the conditions of the present experiment, the rate of increase in dry weight of chlorella with time was almost linear, so it was judged that the growth ratio could be used as an indicator of the growth rate. according to this,
The growth ratio of the chlorella HA-1 strain of the present invention (FIG. 7) is 5%.
CO ₂ and 10% CO ₂ are almost the same, and 15% CO ₂ slightly decreases but maintains a high value. HA-2 strain (FIG. 8), HC-1 strain (FIG. 9) and HT-2 Stock (Fig. 1
The 3 strains of 2) showed a substantially constant high growth ratio from 5% CO ₂ to 15% CO ₂ , and the HT-1 strain (FIG. 11) also increased the growth ratio as the CO ₂ concentration increased. Therefore, it is clear that these 5 strains promote growth at a CO ₂ concentration similar to that of a 10% to 15% CO ₂ concentration of a thermal power plant exhaust gas. Regarding the CO ₂ concentration, the thermal power plant exhaust gas is directly used. It is possible to cultivate the cells without any need for dilution with air. Moreover, HC-2 strain (Fig. 10) Although slightly inferior to the 5 strains, growth ratio is highest at 10% CO ₂ concentration, shows the cultural characteristics not found in microalgae of the genus Chlorella far is there. In contrast, the control strain Chlora
In the case of lla pyrenoidosa NIES-226 (Fig. 13), the growth ratio was about 3 times, and a clear growth promoting effect was recognized under the condition that 5% CO ₂ enriched air was aerated compared to the case where air was aerated. But 10% CO ₂ and 15% CO ₂
Growth was observed under This indicates that when the algae are used to cultivate the exhaust gas from a thermal power plant, the exhaust gas cannot be efficiently cultured unless the exhaust gas is diluted with air.

(B) Growth rate, CO ₂ fixation rate, photosynthetic efficiency Next, from the experimental results of the previous section (a), how much solar energy quantity was irradiated, how much dry matter quantity, and how much carbon quantity was fixed carbon quantity were irradiated. Table 5 collectively shows whether or not the conversion has been performed. In Table 5, the growth ratio of each isolate at the optimum CO ₂ concentration [see (a) above], growth rate, CO ₂ fixation rate and photosynthetic efficiency (PE).
The growth rate was converted from the amount of dry matter produced into the rate per day of the light irradiation area (unit: g dry matter / m ² / day), and C
The O ₂ fixation rate was calculated by multiplying the dry matter produced by the amount of carbon obtained by the measurement with a C / N coder (unit: g carbon / m 2.
² / day), and the photosynthetic efficiency was calculated as the fixed energy amount from the amount of carbon taken up by the alga body of Chlorella and expressed as a percentage of the irradiation light energy amount (that is, the chemical energy per 1 g carbon of microalga is 11.4K
Since it is cal, the amount of carbon was multiplied by this value and divided by the irradiation energy of light; the amount of irradiation light energy under the present experimental conditions = 2450 Kcal / m ² / day-culture tank).

[Table 5] Growth rate, CO ₂ fixation rate, photosynthetic efficiency of chlorella isolates ───────────────────────────────── Test strain Optimal CO ₂ growth ratio Growth rate CO ₂ photosynthesis Concentration Fixed rate Efficiency ──────────────────────────────── HA -1 10 5.90 17.1 7.80 3.62 HA-2 5 4.63 10.6 4.44 2.06 HC-1 10 4.86 14.0 6.29 2.92 HC-2 10 3.61 11.7 4.98 2.31 HT-1 15 5.74 12.9 6.10 2.83 HT-2 10 5.52 12.6 5.69 2.64 Control 5 5 4.74 15.4 6.83 3.17 ──────────────────────────────── Based on the results shown in Table 5 The control of Chloralla pyrenoid
The osa NIES-226 strain had a growth rate and CO under 5% CO ₂ condition.
₂ Fixed speed is fast, E. Is also high. However, as indicated in the previous section (a), 57% compared to 10% CO ₂ in 5% CO _2, at high concentrations CO ₂ since the growth ratio is decreased to 15% in CO ₂ 40% Growth promotion effect cannot be expected. In contrast, if isolates of the present invention, HA-1 strain with high-concentration CO ₂ conditions, growth rate, CO ₂ fixed speed and P. E. Both showed values higher than the value under the control of 5% CO ₂ condition, and HC-1 strain and HT-1 strain also showed values close thereto, and high productivity was 10% to 15% CO 2.
It can be obtained even at ₂ concentrations.

Example 3 Culture Characteristics of HA-1 Strain on Culture Conditions The culture characteristics of the HA-6 strain of the genus Chlorella having the highest growth rate among the above-mentioned 6 isolates of the present invention were examined. The main factors affecting the growth of microalgae are light, medium composition, aeration CO ₂ concentration, aeration amount (stirring speed), temperature and medium pH.
₂ conditions of 4 kinds of culture factors such as concentration, aeration rate, temperature and medium pH were examined under different conditions. The culture was carried out according to Example 2 except that the above conditions were changed. The results are shown in FIGS.
7 shows. According to this, a CO ₂ concentration of 10% is optimum for growth, and even at 20%, a considerably high growth ratio is shown, and thereafter, as the CO ₂ concentration increases, the growth ratio also decreases to 100%.
Almost no growth occurred under CO ₂ aeration conditions (Fig. 1
4). In the experiment on the air flow rate shown in FIG.
Growth was slightly better at 5 liters / minute and 0.5 liters / minute, but 0.10 liters / minute and 1.0 liters / minute
The difference from the minute was small. In the experiment in which the temperature shown in FIG. 16 was examined, the growth ratio was low at 20 ° C.
It was found that the HA-1 strain was almost constant from 0 ° C to 35 ° C, and that the HA-1 strain was a mesophilic strain with a wide temperature adaptation range. The experiment on medium pH shown in FIG. 17 showed that the optimum pH (initial medium) was about 4 to 5. However, along with the culture, the pH of the medium increased by about 0.6 after the culture for 1 week. When the pH was 3, it could not grow and died. However, this strain actively grows even at pH 4, indicating that it is a strain that functions even at low pH. Since the pH of the medium has a strong influence on the growth of Chlorella as well as the CO ₂ concentration, NO contained in the exhaust gas
Considering the possibility of the influence of acidic substances such as _X , low pH tolerance of the HA-1 strain is a useful trait. From the above results, in the case of culturing using flue gas, the chlorella HA-1 strain can be efficiently cultivated without strict control of exhaust gas concentration, flow rate, temperature and medium pH, which are important control items. It is possible. Considering a large-scale culture system, the culture system needs to be simple and easy to maintain. Therefore, it can be said that the HA-1 strain of the genus Chlorella is suitable for a simple CO ₂ -fixing culture system.

Example 4 Protein Content of Microalgae The protein content of microalgae, which is a CO ₂ -fixing product of the HA-1 strain of Chlorella obtained in Example 1, was analyzed. The analysis was performed according to the following procedure. First, the algal cells are sonicated for 15 minutes to be crushed, and 1N NaOH is further added, followed by heating at 100 ° C. for 10 minutes to extract proteins,
Albumin was measured using a protein assay kit from Bio-Rad as a standard. The results are shown in Table 6.
There was no great difference in the protein content of the algal cells cultured under each CO ₂ condition, and 42 to 44% of the dry weight was protein.
This result indicates that nearly half of the HA-1 strain algal bodies are proteins and can be used as highly nutritious feeds or industrial proteins or their raw materials.

[Table 6] Protein content of HA-1 strain algal cells aerated under various CO ₂ concentration conditions ──────────────────────────── ─────Aeration CO ₂ concentration (%) ────────────────────── 0.035 5 10 15 (air) ──────── ───────────────────────── Protein content (% of dry matter) 44.2 44.4 42.0 43.2 ─────── ───────────────────────────

[0018]

As described in detail above, the present invention has five advantages.
Stable growth at high CO ₂ concentration of 20% to 20%
It is an isolation of a microalga of the genus Chlorella that fixes O ₂ . Therefore, the microalgae of the present invention can be used as CO in a thermal power plant or the like.
_{Even when the} exhaust gas having a high concentration of ₂ is directly introduced into the culture tank, CO ₂ in the exhaust gas is fixed and stable growth is possible. Furthermore, the CO ₂ fixing product of the microalgae of the present invention has a high protein content and can be effectively used as a feed for livestock and a raw material for industrial proteins. As described above, by fixing CO _{2 in} exhaust gas using the microalgae of the present invention and effectively utilizing the fixed product, prevention of global warming due to greenhouse gases such as CO ₂ and increase of population and living level can be achieved. The present invention greatly contributes to the solution of global environmental problems because it solves the food problem caused by the rise and prevents destruction of tropical forests and desertification.

[Brief description of drawings]

FIG. 1 is a micrograph showing the morphology of the organism of the chlorella HA-1 strain of the present invention.

FIG. 2 is a micrograph showing the morphology of the organism of the chlorella HA-2 strain of the present invention.

FIG. 3 is a micrograph showing the morphology of the organism of the chlorella HC-1 strain of the present invention.

FIG. 4 is a micrograph showing the morphology of organisms of the Chlorella strain HC-2 of the present invention.

FIG. 5 is a photomicrograph showing the morphology of the organism of the Chlorella sp. HT-1 strain of the present invention.

FIG. 6 is a micrograph showing the morphology of the organism of the chlorella HT-2 strain of the present invention.

FIG. 7 is a graph showing growth in the case of culturing the HA-1 strain with aeration of air having different CO ₂ concentrations.

FIG. 8 is a graph showing growth in the case of culturing the HA-2 strain with aeration of air having different CO ₂ concentrations.

FIG. 9 is a graph showing growth in the case of culturing the HC-1 strain with aeration of air having different CO ₂ concentrations.

FIG. 10: HC-2 by aeration with air having different CO ₂ concentrations
It is a graph which shows growth at the time of culturing a strain.

FIG. 11: HT-1 was produced by aerating air with different CO ₂ concentrations.
It is a graph which shows growth at the time of culturing a strain.

FIG. 12: HT-2 by ventilating air with different CO ₂ concentrations
It is a graph which shows growth at the time of culturing a strain.

FIG. 13 is a graph showing the growth when a control strain (Chloralla pyrenoidosa NIES-226) was cultured by aerating air with different CO ₂ concentrations.

FIG. 14 is a graph showing the relationship between CO ₂ concentration and growth of strain HA-1.

FIG. 15 is a graph showing the relationship between the amount of aeration and the growth of strain HA-1.

FIG. 16 is a graph showing the relationship between temperature and growth of HA-1 strain.

FIG. 17 is a graph showing the relationship between medium pH and growth of HA-1 strain.

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Claims (11)

[Claims] 1. A microalga of the genus Chlorella capable of stable growth at a CO ₂ concentration of 5 to 20% by volume. 2. The microalga according to claim 1, which is a Chlorella HA-1 strain. 3. The microalga according to claim 1, which is a strain of Chlorella HA-2. 4. The microalga according to claim 1, which is a Chlorella sp. HC-1 strain. 5. The microalgae according to claim 1, which is a chlorella HC-2 strain. 6. The microalgae according to claim 1, which is an HT-1 strain of Chlorella sp. 7. The microalgae according to claim 1, which is an HT-2 strain of the genus Chlorella. 8. A method of fixing CO ₂ using the microalgae according to claim 1. 9. The method according to claim 8, which comprises aerating the culture medium with a gas having a CO ₂ concentration of 5 to 20% by volume. 10. The method according to claim 9, wherein the gas to be aerated is exhaust gas from a thermal power plant. 11. A feed or industrial protein comprising the CO ₂ -fixing product of the microalga according to any one of claims 1 to 7.