Alteromonas for promoting growth of chaetoceros mulleri and application thereof

Abstract

The invention provides alteromonas for promoting the growth of chaetoceros mulleri and application of the alteromonas. The preservation number of the alteromonas is CGMCC No. 35130. The provided strain can significantly promote the growth of chaetoceros mulleri in a bacterial environment, so that the problems of slow amplification speed and low efficiency of bait algae in aquaculture are solved, and efficient and stable production of the bait algae such as chaetoceros mulleri is realized. The strain provided by the invention is directly separated from the phycosphere environment of chaetoceros mulleri, belongs to indigenous bacteria, coexists with target algae for a long time, and has strong adaptability. When the chaetoceros mulleri strain is applied to culture of chaetoceros mulleri, ecological risks and potential bacteriophage pollution risks possibly caused by introduction of an exogenous unknown strain are avoided. Under the co-culture condition of chaetoceros mulleri, the strain provided by the invention can significantly improve the growth speed and biomass of the chaetoceros mulleri.

Description

Technical Field

[0001] This invention belongs to the field of functional microbial screening and application technology, specifically relating to a method for promoting the growth of Chaetoceros muelleri (… Chaetoceros muelleri Alternating monotypic bacteria and their applications. Background Technology

[0002] Microalgae, especially marine diatoms, form the foundation of the aquaculture food chain, providing crucial nutrients for various commercially farmed animals, such as shellfish and crustacean larvae. Chaetoceros muelleri ( Chaetoceros muelleri Due to its high nutritional value, moderate cell size, and rapid growth rate, *Chaetoceros muelleri* is widely recognized as a high-quality food algae and is extensively used in shellfish hatcheries. Therefore, the cultivation efficiency and yield of *Chaetoceros muelleri* directly affect the cost and profitability of shellfish hatchery production.

[0003] In natural marine environments, microalgae and bacteria coexist for a long time, forming complex micro-ecosystems. Existing research shows that some bacteria, known as "probiotics" or "algae-promoting bacteria," can provide nutrients or growth factors to microalgae by secreting vitamins, plant hormones, siderophores, or decomposing organic matter, thereby promoting photosynthesis and cell proliferation of microalgae.

[0004] However, current research on growth-promoting bacteria for food algae such as *Chaetoceros muelleri* still faces many challenges. First, the number of reported algae-promoting strains is limited, and screening and discovering new, highly efficient algae-promoting strains remains a research hotspot. Second, the screening and efficacy verification of most reported algae-promoting strains were completed under sterile laboratory conditions. These stringent conditions are severely incompatible with the large-scale, open, and non-sterile production environment of aquaculture, leading to a significant reduction or complete failure of their growth-promoting effects in practical applications. Third, the growth-promoting effects of some strains are unstable or insignificant, failing to meet the urgent need for efficiency improvements in industrial production. Furthermore, introducing exogenous strains with no symbiotic history with the target algae may pose unknown ecological risks or fail to survive and function stably in the culture system due to poor adaptability. Therefore, developing a strain that can stably and efficiently promote the growth of *Chaetoceros muelleri* under non-sterile conditions is of significant practical importance for reducing food culture costs and improving aquaculture seedling production efficiency. Summary of the Invention

[0005] The purpose of this invention is to provide an alternating monocytogenes strain that promotes the growth of Chaetoceros muelleri and its application. The provided strain can significantly promote the growth of Chaetoceros muelleri in a bacterial environment, thereby solving the problem of slow growth rate and low efficiency of food algae in aquaculture and realizing the efficient and stable production of food algae such as Chaetoceros muelleri.

[0006] This invention first provides a strain of Mycotoxin B (McClella esculenta) Alteromonas macleodiiStrain A1 was deposited on July 8, 2025, at the China General Microbiological Culture Collection Center (CGMCC) located at No. 3, Courtyard 1, Beichen West Road, Chaoyang District, Beijing, with accession number CGMCC No. 35130.

[0007] The 16S rRNA gene sequence of the provided Mycobacterium maltii strain A1 is SEQ ID NO:1; The present invention also provides an application of the aforementioned Mycotoxin A1 strain in promoting microalgae growth; The microalgae mentioned are feed microalgae used for aquaculture breeding and seedling raising; The bait microalgae, as a specific example described in this embodiment, is Chaetoceros muelleri.

[0008] Furthermore, the bait microalgae are golden algae or flat algae.

[0009] The present invention also provides a method for promoting the growth of Chaetoceros muelleri, wherein the method comprises adding the aforementioned Mycobacterium maltii A1 strain to the culture medium for culturing Chaetoceros muelleri and co-culturing. Furthermore, the co-culture is conducted at a temperature of 25±2℃, a light intensity of 3000–5000 Lux, and a light-dark cycle of 12 h:12 h; the initial bacterial inoculation density is 102. 3 -10 6 cells / μL, preferably 10 4 Cells / μL; F / 2 or Ningbo University No. 3 medium can be used.

[0010] The present invention also provides a biological agent for promoting the growth of Chaetoceros muelleri, wherein the biological agent may be a bacterial suspension, lyophilized powder or immobilized particles containing the live bacteria of the strain.

[0011] Compared with the prior art, the present invention has the following significant advantages: High ecological safety: The strain of this invention was directly isolated from the algal environment of *Chaetoceros muelleri*, making it an "indigenous bacterium" that coexists with the target algae for a long time and exhibits strong adaptability. Applying it to the cultivation of *Chaetoceros muelleri* avoids the ecological risks and potential phage contamination risks that may arise from introducing exogenous unknown strains.

[0012] Significant growth-promoting effect: Under co-culture conditions with Chaetoceros muelleri, the strain of this invention can significantly enhance the growth rate and biomass of algae. Experimental data show that on the 4th day of culture, the algal cell density in the experimental group with the strain of this invention can reach up to twice that of the control group without the strain, demonstrating a significant effect.

[0013] The application conditions are simple and it is easy to industrialize: the strain of this invention can be rapidly expanded using conventional LB medium without the need for special or expensive culture conditions. Its application method is simple co-culture, which is easy to operate and can be easily promoted in existing aquaculture farms or feed production workshops, showing good prospects for industrial application.

[0014] Rapid action: The strain of this invention can quickly adapt and proliferate in the co-culture system. In the early stage of culture, such as day 2-4, a significant promoting effect on algal growth can be observed, which helps to shorten the algal culture cycle, provide sufficient feed for the next step of feeding the shellfish seedlings more quickly, and improve the overall production efficiency.

[0015] It has universal applicability: it also promotes the growth of commonly used microalgae in production, such as Chlorella vulgaris and Platycladus orientalis, in addition to Chaetoceros muelleri. Although the strain of this invention was isolated from Chaetoceros muelleri culture, when co-cultured with Chlorella vulgaris (or Platycladus orientalis), the chlorophyll content and cell density of Chlorella vulgaris (or Platycladus orientalis) were significantly higher than those of the control group without the addition.

[0016] It can still function at low temperatures: Under the low-temperature (10°C) culture environment of Platycodon grandiflorum, the strain of the present invention can still show a significant promoting effect. Attached Figure Description

[0017] Figure 1 Photographs of Alteromonas macleodii A1 strain in LB solid medium during the early (left) and late (right) stages of colony formation. Figure 2 Pie chart showing the relative abundance of bacteria in Chaetoceros muelleri algal solution at the seedling nursery; Figure 3 Example 2: Chlorophyll concentration changes in the experimental and control groups of Chaetoceros muelleri during cultivation; Figure 4 Example 2: Changes in cell density of Chaetoceros muelleri in the experimental and control groups during cultivation; Figure 5 Example 2: Changes in bacterial cell density of A1 in the experimental and control groups during culture; Figure 6 Example 3: Chlorophyll concentration changes in the experimental and control groups of golden algae during cultivation; Figure 7 Example 3: Curves showing the changes in golden algae cell density between the experimental and control groups during cultivation; Figure 8 Example 4: Chlorophyll concentration changes in the experimental and control groups of Platycladus orientalis during cultivation; Figure 9 Example 4: Curves showing the changes in cell density of Platycladus orientalis in the experimental and control groups during cultivation; Figure 10 Example 5: Chlorophyll concentration changes in the experimental and control groups of Platycladus orientalis during cultivation; where: * indicates significant difference; ns indicates no significant difference; Figure 11 Example 5: Curves showing the changes in cell density of Platycladus orientalis in the experimental and control groups during cultivation; where * indicates a significant difference; ns indicates no significant difference. Detailed Implementation

[0018] The bacterial strain provided by this invention was isolated from the phycosphere environment of *Chaetoceros muelleri*, which has been stably cultured for many years in a shellfish nursery, and exhibits good symbiotic adaptability. Under non-sterile / open conditions, this strain can significantly improve the growth rate of *Chaetoceros muelleri*, with a significant promoting effect observed as early as day 2–4 of culture. The algal cell density can reach 1.8–2.0 times that of the control, and the chlorophyll a concentration can be increased by 60–80%. The strain can be prepared into bacterial suspension and freeze-dried powder, suitable for large-scale production of feed algae in conventional culture media such as F / 2 and aquaculture. The method is simple to operate, ecologically safe, and has good industrialization prospects.

[0019] The strain provided by this invention can rapidly proliferate in a co-culture system with Chaetoceros muelleri, significantly promoting cell density growth and chlorophyll a concentration accumulation during the logarithmic growth phase of Chaetoceros muelleri. This strain exhibits no toxic side effects on algal cells and demonstrates good symbiotic adaptability.

[0020] The golden algae and flat algae used in the embodiments of this invention are all derived from algae that have been stably cultured in a seedling nursery for a long time, but algae that can be provided in a public algae seed bank can also be used.

[0021] The present invention will now be described in detail with reference to the embodiments and accompanying drawings.

[0022] Example 1: Screening and biological identification of Alteromonas macleodii A1 1. Source of microbial strains The strain Alteromonas macleodii A1 used in this invention was isolated from a long-term stable culture (greater than 5 years) of Chaetoceros muelleri culture at the seedling farm of Ningbo Beibeile Aquaculture Co., Ltd. The isolation method is as follows: the preserved Chaetoceros muelleri culture was diluted 10-6 times with sterile seawater, spread on LB solid medium, and cultured for a sufficient time (24-48 hours) to obtain single colonies. Single strains were then isolated, purified, and verified.

[0023] The isolated strain was identified by full-length 16S rRNA sequencing (primers: 27F and 1492R) and, after comparison with closely related strains, was confirmed to belong to the genus *Alteromonas*. This strain was named *Alteromonas macleodii* A1 and is deposited at the China General Microbiological Culture Collection Center (CGMCC) with accession number CGMCC No. 35130 on July 8, 2025.

[0024] Its 16S rRNA gene sequence is shown in SEQ ID NO:1:

[0025] The biological characteristics of this strain are as follows: Gram-negative, rod-shaped, measuring (1.5-2.5) × (0.5-0.9) μm. On LB solid medium, after 24 hours of incubation at 25°C, it forms white, transparent, smooth colonies with a diameter of 1-2 mm. After 48-72 hours, the colonies gradually darken and develop wrinkles. Figure 1 The strain can reach a diameter of over 1.5 cm. It can grow stably at 20–28℃, salinity 20–40‰, and pH 7.0–8.5. In liquid LB medium, it enters the logarithmic growth phase after 3 hours of cultivation at 25℃ and 180 rpm, and reaches the stationary phase after 24 hours, with an OD600 value of 2.0–2.5.

[0026] 2. Large-scale culture of the strain The isolated bacterial strain was inoculated into LB broth and cultured in the dark at 25°C with a shaking speed of 180 rpm. After 24 hours, a high-concentration bacterial suspension was obtained. The bacterial suspension obtained from the above expansion culture was centrifuged at 1500g for 10 minutes at 4°C, and the supernatant was carefully discarded. The precipitate was resuspended in sterile F / 2 algal culture medium or other culture medium suitable for diatom growth (Ningbo University Medium No. 3), and washed with shaking. Centrifugation was repeated and the supernatant was discarded. This washing step was repeated 3 times to completely remove residual LB medium components and obtain a purified bacterial suspension.

[0027] Example 2: The promoting effect of Alteromonas macleodii A1 on the growth of Chaetoceros muelleri. 1. Co-culture with Chaetoceros muelleri (1) Experimental groups: In two 250 mL sterile Erlenmeyer flasks, add 100 mL of sterile F / 2 culture medium and 10 mL of Chaetoceros muelleri algae in the logarithmic growth phase to each flask. Experimental group: Add an additional 10 mL of the washed bacterial culture prepared in step 2 to the Erlenmeyer flask. Control group: Add an additional 10 mL of sterile F / 2 culture medium to the Erlenmeyer flask as a blank control.

[0028] (2) Co-culture conditions with Chaetoceros muelleri: Both the experimental and control groups were placed in a light incubator, and the culture conditions were controlled as follows: temperature 25℃, light intensity 4000 Lux, and light-dark cycle 12 hours:12 hours. During the culture period, the container was shaken several times daily to prevent sedimentation. Those skilled in the art will understand that within a reasonable temperature range (e.g., 20-28℃) and light range (e.g., 3000-5000 Lux), this invention can achieve effective growth promotion. These conditions are non-sterile / open: the container is in contact with the outside air during the culture period. The algal solution is in a non-sterile environment, and high-throughput sequencing of its 16S rRNA gene revealed bacteria including *Salmonella*, *Phaeodactylogyrus*, *Serratia*, *Alternaria*, *Hymenobacter*, and *Seminobacter*. Figure 2 ).

[0029] (3) Initial concentration: At the start of cultivation (day 0), the initial cell density of *Chaetoceros muelleri* in the control group was 96.75 ± 9.47 cells / μL, and the chlorophyll a concentration was 91.87 ± 1.99 μg / L. The initial cell density in the experimental group was 80.80 ± 6.61 cells / μL, and the chlorophyll a concentration was 94.07 ± 0.91 μg / L. In the experimental group, after the addition of bacteria, the initial concentration of strain *Alteromonas macleodii* A1 in the co-culture system was 11490.04 ± 506.03 cells / μL. At this point, there was no significant difference in cell density and chlorophyll concentration between the two groups.

[0030] 2. Indicator Testing and Results Starting from day 0 of the culture, samples were taken from each group every 24 hours for 7 consecutive days.

[0031] Chlorophyll a concentration detection: The concentration was detected using a PHYTO-PAM-II phytoplankton classification fluorometer. Five replicates were performed per group; statistical analysis was conducted using a t-test (two-tailed) with a significance threshold of 0.05. As shown in Figure 2, from day 3 of culture, the chlorophyll a concentration in the experimental group was significantly higher than that in the control group (P<0.05). Throughout the 7-day culture period, the chlorophyll a concentration between the two groups showed a highly significant statistical difference. This indicates that strain Alteromonas macleodii A1 can effectively promote photosynthetic pigment synthesis in *Chaetoceros muscaria*.

[0032] Algal and bacterial cell density detection: Detection was performed using a CytoFLEX LX flow cytometer. Before detection, samples were stained with fluorescein diacetate (FDA) dye. Algal cell autofluorescence was detected via channel B525 of the flow cytometer to count algal cells, while the fluorescence signal labeled with FDA dye was detected via channel B690 to count bacterial cells.

[0033] As shown in Figure 3, the growth trend of *Alteromonas macleodii* cell density in the experimental group was consistent with the change in chlorophyll a concentration. From day 3 onwards, the cell density of the experimental group significantly (P<0.05) surpassed that of the control group and maintained rapid growth throughout subsequent culture. By day 7, the cell density of the experimental group reached nearly twice that of the control group. Statistical analysis showed a highly significant difference in cell density between the two groups. This directly demonstrates that strain *Alteromonas macleodii* A1 has a strong promoting effect on the cell proliferation of *Alteromonas macleodii*.

[0034] As shown in Figure 4, in the experimental group, the cell density of strain Alteromonas macleodii A1 showed a brief decrease on day 1 after inoculation, which may be due to the adaptation period caused by environmental change. Subsequently, the bacterial density increased rapidly starting on day 2, reaching a peak of 12503.66±843.01 cells / μL on day 3, and then slowly decreased and remained at 3503.44±345.91 cells / μL.

[0035] The Alteromonas macleodii A1 strain, isolated from the Chaetoceros muelleri culture system, can rapidly proliferate under co-culture conditions with Chaetoceros muelleri and significantly promote chlorophyll a accumulation and cell density increase in Chaetoceros muelleri without adverse effects on algal cells.

[0036] Example 3: The promoting effect of Alteromonas macleodii A1 on the growth of golden algae. 1. Co-cultivation with golden algae (1) Experimental groups: In two 250 mL sterile Erlenmeyer flasks, add 100 mL of sterile F / 2 culture medium and 10 mL of golden algae in the logarithmic growth phase to each flask. Experimental group: Add an additional 10 mL of the washed bacterial culture prepared in step 2 to the Erlenmeyer flask. Control group: Add an additional 10 mL of sterile F / 2 culture medium to the Erlenmeyer flask as a blank control.

[0037] (2) Cultivation conditions: Both the experimental and control groups were placed in a light incubator, and the culture conditions were controlled as follows: temperature 25℃, light intensity 4000 Lux, and light-dark cycle 12 hours:12 hours. During the culture period, the incubator was shaken several times a day to prevent sedimentation.

[0038] (3) Initial concentration: At the start of cultivation (day 0), the initial cell density of golden algae in the control group was 213.72±25.66 cells / μL, and the chlorophyll a concentration was 98.87±2.54 μg / L. The initial cell density in the experimental group was 153.20±30.55 cells / μL, and the chlorophyll a concentration was 99.10±2.46 μg / L. In the experimental group, after the addition of bacteria, the initial concentration of strain Alteromonas macleodii A1 in the co-culture system was 20678.91±1999.62 cells / μL. At this point, there was no significant difference in cell density and chlorophyll concentration between the two groups.

[0039] 2. Indicator Testing and Results Starting from day 0 of cultivation, samples were taken from each group every 24 hours for 8 consecutive days. Chlorophyll a concentration and algal cell density were measured as described in Example 2 above.

[0040] (1) Changes in chlorophyll a concentration: such as Figure 5 As shown, from day 1 of cultivation, the chlorophyll a concentration in the experimental group was significantly higher than that in the control group (P<0.05). On day 3 of cultivation, the chlorophyll a concentration in the experimental group reached 570.43±16.78 μg / L, which was 1.8 times that of the control group. Throughout the entire 8-day cultivation period, the chlorophyll a concentration between the two groups showed a highly significant statistical difference (P<0.05). This indicates that strain Alteromonas macleodii A1 can effectively promote the photosynthetic pigment synthesis of golden algae.

[0041] (2) Changes in algal cell density: such as Figure 6 As shown, the growth trend of *Alteromonas macleodii* cell density in the experimental group was consistent with the change in chlorophyll a concentration. From day 3 onwards, the cell density of the experimental group significantly (P<0.05) exceeded that of the control group, reaching 1664.21±61.54 cells / μL, which was 1.7 times that of the control group, and continued to increase in subsequent cultures. Statistical analysis showed a significant difference in cell density between the two groups (P<0.05). This directly demonstrates that strain *Alteromonas macleodii* A1 has a significant promoting effect on the cell proliferation of golden algae.

[0042] The Alteromonas macleodii A1 strain, isolated from the Chaetoceros muelleri culture system, significantly promoted chlorophyll a accumulation and increased cell density in Chrysophytes when co-cultured with them.

[0043] Example 4: The promoting effect of Alteromonas macleodii A1 on the growth of Platyhelminthes 1. Co-culture with Platycodon grandiflorus (1) Experimental groups: In two 250 mL sterile Erlenmeyer flasks, add 100 mL of sterile F / 2 culture medium and 10 mL of *Platycodon grandiflorus* in the logarithmic growth phase to each flask. Experimental group: Add an additional 10 mL of the washed bacterial culture prepared in step 2 to the Erlenmeyer flask. Control group: Add an additional 10 mL of sterile F / 2 culture medium to the Erlenmeyer flask as a blank control.

[0044] (2) Cultivation conditions: Both the experimental and control groups were placed in a light incubator, and the culture conditions were controlled as follows: temperature 25℃, light intensity 4000 Lux, and light-dark cycle 12 hours:12 hours. During the culture period, the incubator was shaken several times a day to prevent sedimentation.

[0045] (3) Initial concentration: At the start of cultivation (day 0), the initial cell density of *Alteromonas macleodii* in the control group was 20.83 ± 1.10 cells / μL, and the chlorophyll a concentration was 206.27 ± 5.53 μg / L. The initial cell density in the experimental group was 15.83 ± 5.34 cells / μL, and the chlorophyll a concentration was 198.30 ± 21.72 μg / L. In the experimental group, after the addition of bacteria, the initial concentration of strain *Alteromonas macleodii* A1 in the co-culture system was 19084.56 ± 814.45 cells / μL. At this point, there was no significant difference in cell density and chlorophyll concentration between the two groups.

[0046] 2. Indicator Testing and Results Starting from day 0 of cultivation, samples were taken from each group every 24 hours for 8 consecutive days. Chlorophyll a concentration and algal cell density were measured as described in Example 2 above.

[0047] (1) Changes in chlorophyll a concentration: such as Figure 7 As shown, starting from day 4 of cultivation, the chlorophyll a concentration in the experimental group was significantly higher than that in the control group (P<0.05), reaching 633.57±121.19 μg / L, which was 1.6 times that of the control group. Throughout the entire 8-day cultivation period, the chlorophyll a concentration between the two groups showed a highly significant statistical difference (P<0.05). This indicates that strain Alteromonas macleodii A1 can effectively promote the photosynthetic pigment synthesis of Platycladus orientalis.

[0048] (2) Changes in algal cell density: such as Figure 8As shown, the cell density growth trend of *Alteromonas macleodii* in the experimental group was consistent with the changes in chlorophyll a concentration. From day 4, the cell density of the experimental group significantly (P<0.05) exceeded that of the control group, reaching 95.27±16.25 cells / μL, which was 1.6 times that of the control group. This density continued to increase during subsequent culture. Statistical analysis showed a significant difference in cell density between the two groups (P<0.05). This directly demonstrates that strain *Alteromonas macleodii* A1 has a significant promoting effect on the cell proliferation of *Alteromonas macleodii*.

[0049] The Alteromonas macleodii A1 strain, isolated from the Chaetoceros muelleri culture system, significantly promoted chlorophyll a accumulation and increased cell density in golden algae when co-cultured with Platycladus orientalis.

[0050] Example 5: The promoting effect of Alteromonas macleodii A1 on the growth of Platyhelminthes under low temperature (10℃) conditions. 1. Co-culture experiment with Platycodon grandiflorus (1) Experimental groups: In two 250 mL sterile Erlenmeyer flasks, add 100 mL of sterile F / 2 culture medium and 50 mL of *Platycodon grandiflorus* in the logarithmic growth phase to each flask. Experimental group: On day 0 and day 14 of the experiment, add an additional 1 mL of the washed bacterial culture prepared in step 2 to each Erlenmeyer flask. Control group: On day 0 and day 14 of the experiment, add an additional 1 mL of sterile F / 2 culture medium to each Erlenmeyer flask as a blank control.

[0051] (2) Cultivation conditions: Both the experimental and control groups were placed in a light incubator, and the culture conditions were controlled as follows: temperature 10℃, light intensity 4000 Lux, and light-dark cycle 12 hours:12 hours. During the culture period, the incubator was shaken several times a day to prevent sedimentation.

[0052] (3) Initial concentration: At the start of cultivation (day 0), the initial cell density of *Alteromonas macleodii* in the control group was 46.77 ± 14.73 cells / μL, and the chlorophyll a concentration was 756.00 ± 78.49 μg / L. The initial cell density of the experimental group was 65.08 ± 15.00 cells / μL, and the chlorophyll a concentration was 796.27 ± 57.79 μg / L. In the experimental group, after the first addition of bacteria, the initial concentration of strain *Alteromonas macleodii* A1 in the co-culture system was 9016.69 ± 556.62 cells / μL. At this point, there was no significant difference in cell density and chlorophyll concentration between the two groups.

[0053] (4) Secondary addition of bacteria: On day 14 of cultivation, the initial cell density of *Alteromonas macleodii* in the control group was 165.17 ± 16.73 cells / μL, and the chlorophyll a concentration was 1307.03 ± 54.13 μg / L. The initial cell density of the experimental group was 182.25 ± 16.03 cells / μL, and the chlorophyll a concentration was 1455.50 ± 108.74 μg / L. In the experimental group, bacteria were added for the second time; at this point, the concentration of strain *Alteromonas macleodii* A1 in the co-culture system was 22797.60 ± 895.53 cells / μL. At this stage, there was still no significant difference in cell density and chlorophyll concentration between the two groups.

[0054] 2. Indicator Testing and Results Starting from day 0 of cultivation, samples were taken from each group every 24 hours for 18 consecutive days. Chlorophyll a concentration and algal cell density were measured as described in Example 2 above.

[0055] (1) Changes in chlorophyll a concentration: such as Figure 10 As shown, after approximately 7 days of adaptation, the chlorophyll a concentration in both groups of *Alteromonas macleodii* began to increase slowly, but until day 14 of culture, there was no significant difference in chlorophyll a concentration between the experimental and control groups. Two days after the second addition of the strain (day 16), the chlorophyll a concentration in the experimental group began to be significantly higher than that in the control group (P<0.05), reaching 1821.50±128.77 μg / L, and this statistically significant difference remained between the two groups during subsequent culture (P<0.05). This indicates that under low-temperature conditions of 10℃, when *Alteromonas macleodii* is growing normally, the strain *Alteromonas macleodii* A1 can still effectively promote photosynthetic pigment synthesis in *Alteromonas macleodii*.

[0056] (2) Changes in algal cell density: such as Figure 11 As shown, the cell density growth trend of both groups of *Platycodon grandiflorus* was consistent with the changes in chlorophyll a concentration. The cell density of both groups began to increase slowly after 7 days, and by day 14, there was no significant difference between the experimental and control groups. Two days after the second addition of the strain (day 16), the cell density of the experimental group significantly (P<0.05) surpassed that of the control group, reaching 220.38±18.37 μg / L, and this statistically significant difference remained between the two groups during subsequent culture (P<0.05). This indicates that under low-temperature conditions of 10℃, when *Platycodon grandiflorus* is growing normally, strain A1 still has a significant promoting effect on cell proliferation and can still significantly promote chlorophyll a accumulation and increase cell density when co-cultured with *Platycodon grandiflorus*.

Claims

1. A type of *Metaseridae*, characterized in that, The preservation number of the *Metaseridae* strain is CGMCC No. 35130.

2. The *Methionella micranthae* as described in claim 1, characterized in that, The 16S rRNA gene sequence of *Metasequoia glyptospira* is SEQ ID NO:

1.

3. The application of the *Metaseridae* strain according to claim 1 in promoting microalgae growth.

4. The application as described in claim 3, characterized in that, The microalgae mentioned are feed microalgae used for aquaculture breeding and seedling cultivation.

5. The application as described in claim 3, characterized in that, The bait microalgae mentioned is Chaetoceros muelleri.

6. The application as described in claim 3, characterized in that, The bait microalgae are golden algae or flat algae.

7. A method for promoting the growth of Chaetoceros muelleri, characterized in that, The method involves co-culturing Chaetoceros muelleri with the *Metasemonas muelleri* strain described in claim 1.

8. The method as described in claim 7, characterized in that, The co-culture was carried out at a temperature of 25±2℃, a light intensity of 3000–5000 Lux, and a light-dark cycle of 12 h:12 h.

9. A biological agent for promoting the growth of Chaetoceros muelleri, characterized in that, The biological agent comprises live Mycobacterium micranthum as described in claim 1.

10. The biological agent as described in claim 9, characterized in that, The biological agent is a bacterial suspension, lyophilized powder, or immobilized particles.

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