Microalgae culture method and application thereof

Abstract

The invention provides a microalgae culture method and application thereof, in the microalgae culture method, a nitrogen source comprises industrial ammonium sulfate; the nitrogen source is continuously added into the culture medium at a fixed rate; alkali liquor wrapped by a semipermeable membrane is arranged in the culture medium. According to the method provided by the invention, industrial ammonium sulfate is adopted as a nitrogen source, so that the culture cost is remarkably reduced, and the problems of culture medium acidification and excessive heavy metal caused by one-time feeding of ammonium sulfate are relieved while sufficient nitrogen source is ensured by adopting a specific adding mode; the semi-permeable membrane coated alkali liquor dynamically responds to H < + > release caused by microalgae ammonium sulfate assimilation, so that the process steps are simplified, the process automation level is improved, the yield of microalgae carbon sequestration biomass is improved by 30% compared with sodium nitrate culture, the nutritive salt cost is reduced by 70%, and the quality of algae powder, including protein content, heavy metal components and the like, meets the standard; the operation cost of the microalgae carbon sequestration industry is greatly reduced, and the development of the microalgae carbon sequestration industry is promoted.

Description

Technical Field

[0001] This invention belongs to the field of microalgae carbon fixation technology, specifically relating to a microalgae cultivation method and its application. Background Technology

[0002] Microalgae, as the oldest organisms on Earth, possess advantages such as high photosynthetic efficiency (converting solar energy quantum efficiency as high as 2-10%, compared to <1% for terrestrial plants), extremely fast reproduction rate (daily biomass growth reaches 1-3 times), strong environmental adaptability, and easy integration with other engineering technologies. They can not only efficiently fix carbon dioxide (each ton of algae can fix 1.83 tons of carbon dioxide), purify wastewater and flue gas, but also fix nutrients such as carbon, nitrogen, and phosphorus by coupling with flue gas, industrial wastewater, and domestic sewage treatment systems of coal-fired power plants and coal chemical plants. Furthermore, they can convert proteins and polysaccharides synthesized by microalgae biometabolism into biofuels or other high-value-added products, possessing enormous economic value and market prospects, and are widely used in various fields and industries.

[0003] A major reason limiting the large-scale industrialization of microalgal carbon fixation is the excessively high cost of nutrients, with nitrogen being the most crucial element in microalgal protein synthesis. Currently, many microalgal carbon fixation demonstration projects use sodium nitrate or urea as the nitrogen source for cultivating microalgae. In terms of price, sodium nitrate costs approximately 2600 yuan per ton, and urea approximately 2650 yuan per ton, significantly higher than the 400 yuan per ton price of industrial ammonium sulfate. Furthermore, multiple studies have shown that in the nitrogen metabolism pathway, different forms of nitrogen in microalgae require complex biochemical reactions to generate ammonium nitrogen before it can be synthesized into various amino acids by glutamine synthase and glutamate synthase. This indicates that microalgal cells require the least energy to metabolize ammonium nitrogen, and the reaction has the highest priority. Within a certain concentration range, ammonium nitrogen does indeed promote microalgal growth; however, when the concentration of ammonium nitrogen exceeds a certain range, it becomes toxic to microalgal cells, inhibiting their growth. Simultaneously, microalgae produce equimolar amounts of H₂ during ammonia nitrogen assimilation. + Acidification of the culture medium is also a technical challenge.

[0004] WANG (WANG J, ZHOU W, CHEN H, et al. Ammonium Nitrogen Tolerant Chlorella Strain Screening and Its Damaging Effects on Photosynthesis [J]. Sec. Microbiotechnology, 2018, 9) et al., in screening ammonium nitrogen-tolerant Chlorella strains, found that high concentrations of ammonium nitrogen are toxic to algae, manifested in their destructive effect on photosynthesis. Ammonium nitrogen directly induces photodamage to photosynthetic system II (PSII), rather than affecting the repair of photodamaged PSII. The toxicity of ammonium nitrogen to photosynthesis is complex because this compound affects not only PSII, but also PSI, the electron transport chain, and the oxygen evolution complex (OEC), with the OEC being the main site of damage. Domestic and international scholars have conducted in-depth research on how to efficiently utilize ammonium nitrogen. LI (LI S, SONG C, LI M, et al. Effect of Different Nitrogen Ratio on the Performance of CO2 Absorption and Microalgae Conversion (CAMC) Hybrid System [J]. Bioresource Technology, 2020, 306:123126.) found that when the ratio of ammonium bicarbonate to sodium nitrate was set at 1:4, the carbon utilization rate of the microalgae-converted carbon dioxide absorption hybridization technology (CAMC) could reach 40.45%, which is higher than that of the traditional microalgae carbon dioxide fixation process (about 10%-30%). This provides the optimal ratio for subsequent research on ammonium nitrogen culture of microalgae. Li Pengcheng (Li Pengcheng, Li Meidi, Yin Qingrong, et al. Effects of batch addition of NH4HCO3 on chemical absorption-microalgal conversion coupling system [J]. Clean Coal Technology, 2022, 28(9), 69-74.) found that batch addition of ammonium bicarbonate can alleviate the toxicity of high ammonium nitrogen to Spirulina and improve the nitrogen fixation rate. Furthermore, different batch addition methods can affect the formation of late-stage products of Spirulina in the coupling system.SONG (SONG C, LIU J, XIE M, et al. Intensification of a Novel Absorption-microalgae Hybrid CO2 Utilization Process via Fed-batch Mode Optimization[J]. International Journal of Greenhouse Gas Control, 2019, 82: 1-7.) optimized the batch feeding mode of ammonium bicarbonate in the absorption-microalgae hybridization process. Compared with uniform feeding and reduced feeding methods, the initial excess of ammonium bicarbonate was more conducive to the growth of Chlorella. It can adjust the pH to maintain a slightly alkaline state while reducing the ammonia escape rate, weakening the toxicity of ammonia to microalgae, and promoting biotransformation.

[0005] To address the acidification of culture media caused by ammonium nitrogen assimilation, alkaline substances are often added directly to the water to ensure microalgae growth under suitable conditions. This approach is cumbersome and increases the complexity of the process, or expensive biochemical buffers like HEPES are used. It has been observed that combining multiple nitrogen sources and adding them in batches helps mitigate the toxicity of ammonium nitrogen to microalgae, essentially by reducing the ammonium nitrogen concentration in the culture medium. However, these methods do not match the dynamic growth process of microalgae, and there is a lack of literature reports on microalgae cultivation using only ammonium nitrogen.

[0006] Therefore, how to provide a method that can effectively prevent culture medium acidification while having simple process steps has become an urgent problem to be solved. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a microalgae cultivation method and its application. The method provided by this invention uses industrial ammonium sulfate as a nitrogen source, significantly reducing cultivation costs. By employing a specific addition method, it ensures a sufficient nitrogen source while mitigating the problems of culture medium acidification and heavy metal contamination caused by a single ammonium sulfate addition. Furthermore, it utilizes a semi-permeable membrane to encapsulate the alkaline solution, dynamically responding to the H+ ions generated by the microalgae's ammonium sulfate assimilation. + The release process simplifies the steps, improves the level of automation, increases the yield of microalgae carbon fixation biomass by 30% compared to sodium nitrate cultivation, reduces nutrient costs by 70%, and ensures that the quality of algae powder, including protein content and heavy metal content, meets the standards. This significantly reduces the operating costs of the microalgae carbon fixation industry and promotes its development.

[0008] To achieve this objective, the present invention adopts the following technical solution:

[0009] On the one hand, the present invention provides a method for cultivating microalgae, wherein the nitrogen source in the method includes industrial ammonium sulfate;

[0010] The nitrogen source is added to the culture medium continuously at a fixed rate;

[0011] The culture medium contains an alkaline solution encapsulated by a semi-permeable membrane.

[0012] The above method uses industrial ammonium sulfate instead of sodium nitrate as the sole nitrogen source in conventional methods, significantly reducing cultivation costs. By employing a specific addition method to continuously pump the ammonium sulfate solution into the microalgae culture medium at a constant rate, sufficient nitrogen source is ensured while mitigating the problems of culture medium acidification and heavy metal contamination caused by a single ammonium sulfate addition. Furthermore, a semi-permeable membrane encapsulates the alkaline solution to dynamically respond to the H+ ions generated by the microalgae's ammonium sulfate assimilation. + The release process simplifies the steps, improves the level of automation, increases the yield of microalgae carbon fixation biomass by 30% compared to sodium nitrate cultivation, reduces nutrient costs by 70%, and ensures that the quality of algae powder, including protein content and heavy metal content, meets the standards. This significantly reduces the operating costs of the microalgae carbon fixation industry and promotes its development.

[0013] Preferably, the culture medium for the microalgae cultivation method includes BG11 medium.

[0014] Preferably, the ammonium sulfate is added to the culture medium in solution form, and the concentration of the ammonium sulfate solution is 30-35 g / L, such as 30 g / L, 31 g / L, 32 g / L, 33 g / L, 34 g / L or 35 g / L, but not limited to the values ​​listed above. Other values ​​not listed in the above range are also applicable.

[0015] Preferably, the nitrogen source is added to the culture medium at a rate of 0.1-0.5 mL / h, such as 0.1 mL / h, 0.2 mL / h, 0.3 mL / h, 0.4 mL / h or 0.5 mL / h, but not limited to the values ​​listed above. Other unlisted values ​​within the above range are also applicable.

[0016] Preferably, the product of the concentration of the ammonium sulfate solution, the rate at which the nitrogen source is added to the culture medium, and the culture period is 240-260 mg / L, such as 240 mg / L, 245 mg / L, 250 mg / L, 255 mg / L, or 260 mg / L, but not limited to the values ​​listed above. Other unlisted values ​​within the above range are also applicable.

[0017] The above-mentioned specific parameter control can both ensure the maintenance of microalgae culture and control NH4 in the culture medium. + The concentration is maintained at an extremely low level to reduce NH4 inside and outside the microalgal cells. + Concentration gradient weakens the activity of ammonia transporters and reduces intracellular NH4 in microalgal cells. +The toxicity problem caused by excess can also reduce the acidification of the culture medium and the problem of excessive heavy metals caused by one-time addition, and significantly reduce the problems caused by heavy metal components in industrial ammonium sulfate.

[0018] Preferably, the alkaline solution comprises a calcium hydroxide suspension.

[0019] The above-mentioned alkaline solution uses a calcium hydroxide suspension, which, compared to other alkaline solutions, can effectively counteract the excess H+ produced during the assimilation of ammonium sulfate by microalgae. + It controls the pH of the culture medium and can also play a role in OH. - The long-term release effect eliminates the need for real-time pH monitoring and adjustment of culture parameters during the culture cycle, simplifying the process and improving the level of automation.

[0020] Preferably, the concentration of the calcium hydroxide suspension is 0.8-1.5 kg / L, such as 0.8 kg / L, 0.9 kg / L, 1 kg / L, 1.1 kg / L, 1.2 kg / L, 0.3 kg / L, 1.4 kg / L or 1.5 kg / L, but not limited to the values ​​listed above. Other unlisted values ​​within the above range are also applicable.

[0021] Preferably, the volume of the calcium hydroxide suspension is 0.08-0.15% of the volume of the culture medium, such as 0.08%, 0.09%, 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, or 0.15%, but not limited to the values ​​listed above. Other unlisted values ​​within the above range are also applicable.

[0022] Preferably, the molecular flux of the semipermeable membrane is 80-120 Da, such as 80 Da, 90 Da, 100 Da, 110 Da or 120 Da, but not limited to the values ​​listed above. Other unlisted values ​​within the above range are also applicable.

[0023] Preferably, the semipermeable membrane comprises a regenerated cellulose membrane.

[0024] The other conditions of the microalgae cultivation method in this invention, such as light and temperature, are all conventional conditions in the field, and this invention does not impose too many limitations on them.

[0025] On the other hand, the present invention also provides the application of the microalgae cultivation method described above in the microalgae carbon fixation process.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] This invention provides a microalgae cultivation method that uses industrial ammonium sulfate instead of sodium nitrate as the sole nitrogen source in conventional methods, significantly reducing cultivation costs. By employing a specific addition method to continuously pump ammonium sulfate solution into the microalgae culture medium at a constant rate, sufficient nitrogen supply is ensured while mitigating the problems of medium acidification and heavy metal contamination caused by a single ammonium sulfate addition. Furthermore, a semi-permeable membrane encapsulates the alkaline solution to dynamically respond to the H+ ions generated by the microalgae's ammonium sulfate assimilation. + The release process simplifies the steps, improves the level of automation, increases the yield of microalgae carbon fixation biomass by 30% compared to sodium nitrate cultivation, reduces nutrient costs by 70%, and ensures that the quality of algae powder, including protein content and heavy metal content, meets the standards. This significantly reduces the operating costs of the microalgae carbon fixation industry and promotes its development. Detailed Implementation

[0028] To further illustrate the technical means and effects of the present invention, the following describes the technical solution of the present invention in conjunction with preferred embodiments of the present invention. However, the present invention is not limited to the scope of the embodiments.

[0029] In the following example, the microalgae was the FACHB-9 strain of Chlorella Pyrenoidosa (purchased from the Freshwater Algae Bank of the Chinese Academy of Sciences (Wuhan), catalog number FACHB-9), cultured in a container. A binary gas mixture of 15% CO2 and the remainder N2 was introduced at a flow rate of 5 mL / min using a mass flow meter to provide the carbon source for the growth and carbon fixation of Chlorella. The culture medium used was BG11 medium.

[0030] In the experiment, 10 mL algal samples were collected periodically for biomass determination: the absorbance (OD) of Chlorella at a characteristic wavelength of 680 nm was measured using an ultraviolet spectrophotometer. 680 The sample was then centrifuged at 5000 rpm for 10 min, followed by washing the algal sludge three times with deionized water. The sludge was then transferred to a vacuum drying oven at 80℃ and dried for 24 hours until the weight remained constant. The final dry weight of the biomass was then determined. The dry weight and OD of *Chlorella vulgaris* biomass were obtained through formula fitting. 680 The linear regression equation is as follows. Microalgae samples were taken every 24 hours in the experiment, and the daily biomass dry weight was tested and calculated.

[0031] Biomass dry weight (g / L) = 0.280 × OD 680

[0032] The biomass dry weight at the time of microalgae inoculation and cultivation is denoted as m1, and the dry weight at harvest is denoted as m2. The growth time is t days, and α is the carbon content in the algal cells. The measured value of α in Chlorella cells is 50%. Then, the microalgal growth rate λ and the microalgal CO2 fixation rate V... CO2-fixationThe calculation formulas are as follows:

[0033] λ(g / L / d) = (m2-m1) / t

[0034] V CO2-fixation (g / L / d) =(λ×α×44) / 12

[0035] The pH value of the algal solution was measured using a pH meter.

[0036] Example 1

[0037] This embodiment provides a method for microalgae cultivation, the specific steps of which are as follows:

[0038] Microalgae were cultured in 500 mL Erlenmeyer flasks. The pumping rate of 32.36 g / L ammonium sulfate solution was set at 7.2 mL / h / L per liter of algal solution. The light intensity was 8000 lux, and the culture temperature was 25℃. *Chlorella proteoglycans* was selected as the algal species. The experimental algal solution volume was 300 mL, and the initial microalgal biomass density was 0.5 g / L. The experimental period was 5 days. In the experimental group, 3 mL of a 1 kg / L calcium hydroxide suspension was placed in the algal solution using a semi-permeable membrane. In the control group, the same semi-permeable membrane was placed in the algal solution without adding calcium hydroxide suspension. At the end of the experiment, the biomass density of the experimental group was 3.64 g / L, the average carbon fixation rate was 1.15 g / L / d, and the pH of the algal solution was 6.77. Meanwhile, in the control group, the final biomass yield of microalgae was only 1.12 g / L, the average carbon fixation rate was 0.23 g / L / d, the pH of the algal solution was 3.34, the algal solution was severely acidified, and the growth of microalgae was severely inhibited.

[0039] Example 2

[0040] This embodiment provides a method for microalgae cultivation, the specific steps of which are as follows:

[0041] Microalgae were cultured in 3000 mL Erlenmeyer flasks. The pumping rate of 32.36 g / L ammonium sulfate solution was set at 14 mL / h / L per liter of algal solution. The light intensity was 8000 lux, and the culture temperature was 25℃. *Chlorella proteoglycans* was selected as the algal species. The experimental algal solution volume was 2500 mL, and the initial microalgal biomass density was 0.5 g / L. The experimental period was 7 days. In the experimental group, 1.5 mL of a 1 kg / L calcium hydroxide suspension was placed in the algal solution using a semi-permeable membrane. In the control group, the same semi-permeable membrane was placed in the algal solution without adding calcium hydroxide suspension. At the end of the experiment, the biomass density of the experimental group was 2.44 g / L, the average carbon fixation rate was 0.51 g / L / d, and the pH of the algal solution was 4.57. Meanwhile, in the control group, the final biomass yield of microalgae was only 1.07 g / L, the average carbon fixation rate was 0.15 g / L / d, the pH of the algal solution was 3.21, the algal solution was severely acidified, and the growth of microalgae was severely inhibited.

[0042] Example 3

[0043] This embodiment provides a method for microalgae cultivation, the specific steps of which are as follows:

[0044] Microalgae were cultured in 1000 mL Erlenmeyer flasks. The pumping rate of 32.36 g / L ammonium sulfate solution was set at 5 mL / h / L per liter of algal solution. The light intensity was 8000 lux, and the culture temperature was 25℃. *Chlorella proteoglycans* was selected as the algal species. The experimental algal solution volume was 750 mL, and the initial microalgal biomass density was 0.5 g / L. The experimental period was 5 days. In the experimental group, 1 mL of a 1 kg / L calcium hydroxide suspension was placed in the algal solution using a semi-permeable membrane. In the control group, the same semi-permeable membrane was placed in the algal solution without adding calcium hydroxide suspension. At the end of the experiment, the biomass density of the experimental group was 3.57 g / L, the average carbon fixation rate was 1.12 g / L / d, and the pH of the algal solution was 7.14. Meanwhile, in the control group, the final biomass yield of microalgae was only 1.17 g / L, the average carbon fixation rate was 0.25 g / L / d, the pH of the algal solution was 3.45, the algal solution was severely acidified, and the growth of microalgae was severely inhibited.

[0045] Example 4

[0046] This embodiment provides a method for microalgae cultivation, the specific steps of which are as follows:

[0047] Microalgae were cultured in 500 mL Erlenmeyer flasks. The ammonium sulfate solution pumping rate was set at 7.2 mL / h / L per liter of algal solution. The light intensity was 8000 lux, and the culture temperature was 25℃. *Chlorella proteoglycans* was selected as the algal species. The experimental algal solution volume was 300 mL, and the initial microalgal biomass density was 0.5 g / L. The experimental period was 5 days. In the experimental group, 3 mL of 1.08 kg / L sodium hydroxide solution was placed in the algal solution using a semi-permeable membrane. In the control group, the same semi-permeable membrane was placed in the algal solution without adding sodium hydroxide solution. At the end of the experiment, the biomass density in the experimental group was 1.05 g / L, the average carbon fixation rate was 0.309 g / L / d, and the pH of the algal solution was 10.28, indicating severe alkalinization and relative inhibition of microalgal growth. Meanwhile, in the control group, the final biomass yield of microalgae was only 1.12 g / L, the average carbon fixation rate was 0.23 g / L / d, the pH of the algal solution was 3.34, the algal solution was severely acidified, and the growth of microalgae was severely inhibited.

[0048] Comparative Example 1

[0049] This comparative example provides a method for microalgae cultivation, the specific steps of which are as follows:

[0050] Microalgae were cultured in 500 mL Erlenmeyer flasks. The ammonium sulfate solution pumping rate was set at 7.2 mL / h / L per liter of algal solution, the light intensity was 8000 lux, the culture temperature was 25℃, the algal species selected was *Chlorella protozoa*, the experimental algal solution volume was 300 mL, the initial microalgal biomass density was 0.5 g / L, and the experimental period was 5 days. In the experimental group, 3 mL of 1 kg / L calcium hydroxide suspension was placed in the algal solution using a semi-permeable membrane. In the control group, ammonium sulfate was used as the nitrogen source to maintain the total nitrogen source concentration at 247 mg / L (added once) consistent with the continuous flow conditions, and calcium hydroxide was placed in the algal solution using the same semi-permeable membrane. At the end of the experiment, the biomass density of the experimental group was 3.64 g / L, the average carbon fixation rate was 1.15 g / L / d, and the pH of the algal solution was 6.77. Meanwhile, the final biomass yield of microalgae in the control group was 3.15 g / L, the average carbon fixation rate was 0.95 g / L / d, and the pH of the algal solution was 6.82. The biomass yield of the control group was reduced by approximately 13.4% compared to the experimental group.

[0051] Comparative Example 2

[0052] This comparative example provides a method for microalgae cultivation, the specific steps of which are as follows:

[0053] Microalgae were cultured in 500 mL Erlenmeyer flasks. The ammonium sulfate solution pumping rate was set at 7.2 mL / h / L per liter of algal solution. The light intensity was 8000 lux, and the culture temperature was 25℃. *Chlorella proteoglycans* was selected as the algal species. The experimental algal solution volume was 300 mL, and the initial microalgal biomass density was 0.5 g / L. The experimental period was 5 days. In the experimental group, 3 mL of a 1 kg / L calcium hydroxide suspension was placed in the algal solution using a semi-permeable membrane. In the control group, sodium nitrate was used as the nitrogen source, and the same semi-permeable membrane was used to encapsulate the algal solution. At the end of the experiment, the biomass density of the experimental group was 3.64 g / L, the average carbon fixation rate was 1.15 g / L / d, and the pH of the algal solution was 6.77. Meanwhile, in the control group, the final biomass constant of microalgae was 2.83 g / L, the average carbon fixation rate was 0.23 g / L / d, and the pH of the algal solution was 7.65. The experimental group showed an increase in microalgae biomass yield of approximately 28.6% compared to the control group. In microalgae cultivation, nitrogen source costs account for about 85% of the main nutrient costs. In this invention, ammonium sulfate was added at 1.165 g / L, while sodium nitrate in conventional BG11 medium is added at 1.5 g / L. Considering the price, ammonium sulfate costs approximately 500 yuan per ton, and sodium nitrate costs approximately 2000 yuan per ton, resulting in an overall nutrient cost reduction of over 70%.

[0054] As can be seen from the above, the method provided by the present invention effectively simplifies the process steps, improves the level of process automation, and significantly reduces the cultivation cost. Comparing Examples 1-4 and Comparative Example 1, it can be found that the present invention uses a specific nitrogen source addition method, which can both maintain microalgae cultivation and eliminate NH4+ in the culture medium. + The concentration is maintained at an extremely low level to reduce NH4 inside and outside the microalgal cells. + Concentration gradient weakens the activity of ammonia transporters and reduces intracellular NH4 in microalgal cells. + The toxicity problem caused by excess can be mitigated, and the acidification of the culture medium and excessive heavy metals caused by single-use addition can be reduced. This significantly reduces the problems caused by heavy metals in industrial ammonium sulfate, thus playing a role in OH... - The long-term release effect eliminates the need for real-time pH monitoring and adjustment of culture parameters during the culture cycle, simplifying the process and improving the level of automation.

[0055] The applicant declares that the microalgae cultivation method and its application are illustrated by the above embodiments, but the present invention is not limited to the above embodiments, that is, it does not mean that the present invention must rely on the above embodiments to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0056] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0057] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

Claims

1. A method for cultivating microalgae, characterized in that, In the microalgae cultivation method, the nitrogen source includes industrial ammonium sulfate; The nitrogen source is added to the culture medium continuously at a fixed rate; The culture medium contains an alkaline solution encapsulated by a semi-permeable membrane.

2. The microalgae cultivation method according to claim 1, characterized in that, The culture medium used in the microalgae cultivation method includes BG11 medium.

3. The microalgae cultivation method according to claim 1 or 2, characterized in that, The ammonium sulfate is added to the culture medium in solution form, and the concentration of the ammonium sulfate solution is 30-35 g / L.

4. The microalgae cultivation method according to any one of claims 1-3, characterized in that, The nitrogen source is added to the culture medium at a rate of 0.1-0.5 mL / h.

5. The microalgae cultivation method according to any one of claims 1-4, characterized in that, The product of the concentration of the ammonium sulfate solution, the rate at which the nitrogen source is added to the culture medium, and the culture period is 240-260 mg / L.

6. The microalgae cultivation method according to any one of claims 1-5, characterized in that, The alkaline solution includes a calcium hydroxide suspension.

7. The microalgae cultivation method according to claim 6, characterized in that, The concentration of the calcium hydroxide suspension is 0.8-1.5 kg / L.

8. The microalgae cultivation method according to claim 6 or 7, characterized in that, The volume of the calcium hydroxide suspension is 0.08-0.15% of the volume of the culture medium.

9. The microalgae cultivation method according to any one of claims 1-8, characterized in that, The molecular flux of the semipermeable membrane is 80-120 Da.

10. The application of a microalgae cultivation method according to any one of claims 1-9 in a microalgae carbon fixation process.