Research Article, J Mar Biol Oceanogr Vol: 11 Issue: 5

Proposal of a Culture Method for Maintaining High Growth of Sterile Ulva spp./species (Chlorophyta)

Shin Hirayama^1*, Shuichi Tashiro², Kohei Inoue², Kazuya Urata², Masafumi Iima³ and Yasuyuki Ikegami²

¹Regional Innovation Center, Saga University, 1 Honjo, Saga, Japan

²Institute of Ocean Energy, Saga University, 1 Honjo, Saga, Japan

³Faculty of Environmental Science, Nagasaki University, Bunkyo-machi, Nagasaki, Japan

*Corresponding Author:Shin Hirayama
Regional Innovation Center, Saga University, 1 Honjo, Saga, Japan
E-mail:st9828@cc.saga-u.ac.jp

Received date: March 1, 2022, Manuscript No. JMBO-22-59398;
Editor Assigned date: March 3, 2022, PreQC No. JMBO-22-59398(PQ);
Reviewed date: March 17, 2022, QC No. JMBO-22-59398;
Revised date: March 23, 2022, Manuscript No. JMBO-22-59398(R);
Published date: March 30, 2022, DOI: 10.4172/2324-8661.1000230

Citation: Hirayama S, Tashiro S, Inoue K, Urata K, Iima M, et al. (2022) Proposal of a Culture Method for Maintaining High Growth of Sterile Ulva spp./species (Chlorophyta). J Mar Biol Oceanogr 11:3.

Abstract

Keywords: Ulva spp.

Keywords

Ulva lactuca; Sterile Ulva; Growth rate, Specific growth rate, Photosynthetic photon flux density

Introduction

The eutrophication of warm inner bays progresses along with social activities such as the excessive fertilization of fields [1-3] and the accumulation of food residues on the seabed by sea surface aquaculture [4-6]. This is especially the case in the coastal bay area where the replacement of seawater is small, and there are concerns about the impact on aquatic fisheries such as the occurrence of red tide [7,8].

However, in such areas, the overgrowth of sterile Ulva spp. and their seawater remediation function have been reported, especially in shallow water. Furthermore, in Southern Kyushu in Japan, attempts have been made to suppress the accumulation of N and P in the aquaculture area by adjoining a cage for the growth of sterile Ulva spp. As a result, the improvement of marine environmental technology is desirable. As part of this technological development, sterile Ulva spp. are being cultured and produced as a functional food and feed for fish and shellfish. This has the potential to reduce N and P levels in the inner bays [9-11]. Furthermore, while biomass-based global warming response technology is drawing attention, sterile Ulva have the property of easily maintaining grown organisms, so they not only absorb CO2 but also fix CO2. It can also be expected as a biological resource for so-called blue carbon [12,13]. Thus far, we have evaluated the growth rate of sterile Ulva spp. for 37 days. This was done using a culture tank on land that simulates the sea; as a result, the production of edible parts of rice is five times that of grains. It has been confirmed that it exhibits an above high growth rate [14]. Furthermore, in order to maintain a high growth rate, it has been suggested that culture control using the Leaf Area Index (LAI) as an index is an important factor for the total leaf/culture area.

Nevertheless, in order to measure LAI, it is necessary to calculate the total area of sterile Ulva spp. over time during culture. For this measurement, the following procedure is followed: 1) Collect sterile Ulva spp. from the culture tank, 2) Measure and confirm the number of individual sterile Ulva spp. 3) Measure the area of each sterile Ulva spp. (numerous steps involving measurement), and 4) Calculate the total area measured. Therefore, LAI is an index that is difficult to use easily for efficiently producing sterile Ulva spp. In particular, the index was found to be difficult to apply when assuming the production of large-scale sterile Ulva spp. Consequently, in this paper, we focus on the Photosynthetic Photon Flux Density (PPFD) on the bottom of the culture tank as an alternative index to LAI. In addition, we clarify the relationship between the photosynthetic photon bundle density on the bottom of the culture tank and the proliferation of sterile Ulva lactuca. Furthermore, the efficient culture production mode of sterile U. lactuca is described, using the photosynthetic photon bundle density at the bottom of the culture tank as an index.

Materials and Methods

Selection of Ulva species

In this study, we chose to use U. lactuca, obtained from the marine park at Yokohama in Tokyo Bay, as this species is known to stably increase in culture.

Measurement of photosynthetic photon flux density on the bottom of the culture tank

The PPFD (μmol m-2 s-1) at the bottom of the culture tank of U. lactuca was measured three times a day, and the average PPFD was calculated every week.

Cultivation equipment

The main cultivation equipment consisted of a seawater pump, seawater tank, water pump, aeration device, and culture tank. Seawater was pumped by the seawater pump and collected in the tank. From here it was supplied to the culture tank using the water pump. The 42-L culture tank had light-shielding material covering its sides so that sunlight could only enter from the top surface (0.1 m2), so as to simulate the light reception situation in the sea. The culture tank was installed in the daylight solar irradiation area on the north side of Imari satellite, Institute of ocean energy, Saga university, Japan.

Culture conditions

The locations of the U. lactuca sampling sites in Japan are shown in Figure 1. U. lactuca was isolated from Tokyo Bay [8] and the growth rates (g-dry.m−2.d−1) evaluated using the culture tank in Imari (Figure 1).

A total of 254 sheets of φ 48.5 mm U. lactuca was added to the culture tank to give an LAI of five. A culture was performed at a seawater flow rate of 2 L/h and an aeration rate of 24 L/h. Because the growth rate of U. lactuca has been shown to depend more on the concentration of NO3-N than PO4-P [8], PO4-P was set to a constant value of 0.04 ppm in the culture. However, concentrations of NO3-N were maintained at 0.8 ppm.

Measurement of growth rate

During the culture period, the wet weight of U. lactuca was measured once a day as an index of growth status. The dry growth rate (g-dry.m-2.d-1), meanwhile, was measured after the culture was completed.

Results and Discussion

Culture process for Ulva lactuca

The transition of wet weight was measured as an evaluation index of the growth degree of U. lactuca in the installed culture tank, with the results shown in Figure 2. The wet weight increased with time, and it was confirmed that it grows smoothly in 0.8 ppm-NO3-N seawater by stirring. Furthermore, in order to grasp the transition of the growth rate more accurately, the specific growth rate was measured, with the results shown in Figure 3. From this measurement, it was found that the specific growth rate became maximum on the seventh day of culture, and then decreased. In this culture environment, the supply of CO2 by NO3-N and aeration is sufficient, suggesting it may be light limitation that is a growth factor after seven days (Figures 2 and 3).

As a result, as an index for grasping the degree of light used in the culture tank, we focused on the PPFD at the bottom of the tank. We then measured the transition to clarify the relationship with the growth rate. In other words, if the light condition that maintains a high specific growth rate can be found, it is thought to be possible to obtain more real-time information on the growth of U. lactuca. The PPFD at the bottom of the culture tank was therefore measured, and the results aggregated every week (which is the growth evaluation period for U. lactuca). Although it was measured three times a day, the average PPFD reached a maximum of 20.6 μmol m-2 s-1 after seven days (one week). It then decreased significantly after seven days, and seven after 28 days. In other words, it was found that the PPFD decreases to 4.0 μmol m-2 s-1 after 14 days, to 3.7 μmol m-2 s-1 after 21 days, to 1.2 μmol m-2 s-1 after 28 days, and to 1/17 after seven days. In this study, the PPFD was measured three times a day, and the relationship with the growth rate verified every seven days. In order to improve the degree of accuracy, it was necessary to increase the measurement frequency of the PPFD and the specific growth rate of U. lactuca. Figure 4 shows the relationship between the LAI and the average value of the PPFD on the bottom of the culture tank. With the increase in LAI, the following became clear: The PPFD at the bottom of the culture tank decreased, and after seven days of culture, the PPFD at the bottom of the culture tank became 20.6 μmol m-2 s-1 under the condition of LAI 13.3. Up until this point, we have determined the specific growth rate using LAI as an index, and have shown that the specific growth rate peaks on the seventh day of culture in some Ulva spp [14]. The LAI can be measured by 1) Harvesting U. lactuca from the culture tank, 2) Measuring and confirming the number of U. lactuca, 3) Measuring the area of U. lactuca, and 4) Calculating the total area of the measured U. lactuca. This involves a significant number of steps consisting of calculating the sum. Furthermore, in order to suppress the effects on U. lactuca, it was necessary to complete these operations in a short time and return the U. lactuca to the culture tank. Accordingly, LAI is an index that is difficult to use as an index for the efficient production of sterile U. lactuca. It was also found to be difficult to apply this index when the large-scale production of U. lactuca is assumed.

In this respect, the measurement of the PPFD at the bottom of the culture tank is simple. This method of finding the conditions for maintaining a high specific growth rate, based on the PPFD, is easy to apply to large-scale culture tanks. It is therefore highly likely that the technology will be directly linked to industrial applications. Nevertheless, the water temperature of the culture tank during the daytime when it is exposed to sunlight. After 20 days of culturing, the days when the temperature fell below 15°C became prominent even during the day. It was thought that the effect of the decrease in specific growth rate due to temperature might be added.

Evaluation of the growth rate of Ulva lactuca

Based on the above culture results, 28 days after culturing, U. lactuca was washed with water, the dry weight measured and the growth rate (g-dry.m-2.d-1) determined. As a result, it was as high as 12.6 g-dry.m-2.d-1. The water in the culture tank contained 0.8 mg/L (ppm) NO3-N and 0.04 mg/L (ppm) PO4-P, which are the normal concentrations recorded in Tokyo Bay. Therefore, it was shown that efficient biomass production is possible even under relatively low temperature conditions with this level of NO3-N concentration, temperature, and air (CO2) supply.

Potential for culture production of Ulva lactuca

During the 28 days of U. lactuca culture, no contamination of other seaweeds or adhesion of diatoms in the culture vessel were observed, and the biomass yield of the edible portion was 8.4 times that of rice cultivation. From this, it is considered that the Ulva spp. culture type shows a high productivity of edible parts.

Accordingly, an evolved version of the continuous production system of U. lactuca, has been proposed, using LAI as an index. Figure 5 shows the U. lactuca culture production system, using the PPFD at the bottom of the U. lactuca culture tank as an index. The production system for U. lactuca includes the following six steps: 1) Select a sterile Ulva sp., 2) Cultivate in a culture tank equipped with an aeration device under NO3-N concentration control, using the PPFD as a growth index, 3) Collect and effectively use the plants approximately every seven days, 4) Adjust the collected part of U. lactuca that is cut out, 5) Transfer the cut out U. lactuca to the culture tank as a seed, and 6) Effectively use U. lactuca residue after drilling. This system enables the continuous production of U. lactuca under culture conditions while maintaining a high growth rate (Figure 5).

Sterile U. lactuca can be used as a feed and functional food for farmed fish and poultry [15,16]. Therefore, this system has the potential to contribute to food and feed production. Furthermore, the installation of this production system in countries with warm seas would allow food self-sufficiency to be achieved, which is currently an issue for Japan. In particular, this system, which does not use LAI as an index, has the potential to become a labor-saving production system with less complicated operations for production control.

The development of a continuous culture production method for U. lactuca, which allows production and harvesting to occur in eutrophic areas, is becoming increasingly feasible. Furthermore, U. lactuca collected from the coast contain a useful sulfur-containing amino acid (D-cysteinolic acid) and have also been reported to have singlet oxygen and neutral fat suppression effects [16]. In addition, anti-Methicillin Resistant Staphylococcus Aureus (MRSA) suppression effect has also been reported [17-19]. The next step will therefore be to determine the content of useful substances in U. lactuca grown under culture conditions, with the ultimate goal of accumulating these substances in cultured plants, adding further to their value.

Conclusion

In this study, it was found that sterile U. lactuca can be effectively produced under enriched culture conditions, controlled by an index as the PPFD at the bottom of the culture tank. The measurement of the PPFD at the bottom of the tank is simple, and this method of finding the conditions for maintaining a high specific growth rate based on the PPFD is easy to apply to large-scale culture tanks. As a result, it is highly likely that the technology will be directly linked to industrial applications. In other words, this Japanese production system could be applied to eutrophic inner bays in warmer regions of the world. Furthermore, since this production system is capable of high biomass production, it has potential as a food production system using enriched seawater.

Acknowledgment

 We gratefully acknowledge the work of past and present members of Institute of ocean energy, Saga University.

Conflicts of Interest

No conflict of interests declared.

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