Management of Freshwater Microalgae in Aquaculture

Abstract

Algae refers to organisms across different kingdoms, including Protista, Plantae, Chromista, and Bacteria, which explains their diverse roles in the biosphere. In the ecosystem, algae stand as the foundational pillar of the food web; as photoautotrophs, they utilize solar energy and convert it into chemical energy, thereby initiating energy transfer. In fish farming, algae provide shading and nutrients for microorganisms that fish larvae can consume for their growth. When those aquatic animals respire and excrete, the carbon dioxide (CO₂) and excretions become the essential ingredients to promote algae growth. In some aquaculture scenarios, the ecosystem between algae and other organisms can be thrown off balance, leading to an overabundance or insufficient algae in the system. Certain algae species are favored when conditions such as specific temperature, sufficient sunlight, and an overload of nutrients are introduced to the environment. It leads to domination by a single or a couple of algae species in the ecosystem, also known as the algal blooms. Algal blooms can lead to fluctuations in water parameters, typically shifting pH values and dissolved oxygen during photosynthetic activity differences between day and night; these fluctuations cause stress to the aquatic animals and affect the production yield. For the blooming of toxic algae species, the accumulation of toxins in the food web can further lead to fish health issues, fish mortality, and even public health concerns.

This thesis seeks to manage and balance aquaculture algal populations, highlighting mitigation strategies such as hydrogen peroxide (H₂O₂) and ozone nanobubbles to reduce harmful algal blooms in fish ponds, demonstrating feasibility in real farming situations. Microcystis sp., a harmful cyanobacterium commonly seen in earthen ponds, was one of our targeted toxin-releasing algal bloom species. In the first study, low-dose hydrogen peroxide (7mg/L) was tested in both the laboratory and ponds; the treatment successfully reduced this toxin-releasing cyanobacterium (Microcystis sp.) by 64.7% in laboratory setting, and in ponds it was reduced on average by 44% without harming other algae (Scenedesmus sp.), resulting a shift of the algae community towards the beneficial algae after the application of the H₂O₂ treatment. During the pond trials, the gill status of the Jade perch and Giant tiger prawns were examined, and the histology results suggested that the H₂O₂ treatment did not harm the animals' gills. Although the results were promising, we were concerned that the death of Microcystis sp. might result in the release of additional toxins into the aquatic environment, which brought us to the second study. Two trials were conducted to investigate whether the low-dose hydrogen peroxide treatment (7mg/L) resulted in microcystin-LR release under laboratory conditions. Our laboratory experiment shows that the hydrogen peroxide did not lead to further microcystin-LR release. During the first trial, the release of microcystin-LR was not detected in the H₂O₂-treated group, which was similar to that of the control group (no treatment). In contrast, 0.933 ± 0.381 ppb of microcystin-LR was extracted from 2,344,230 Microcystis sp. cells in the positive control group (Where all the cells were destroyed during a boiling step in methanol). In the second trial, the total Microcystis sp. cells in each sample were increased to 64,125,000, 16.933 ± 0.303 ppb and 16.933 ± 0.109 ppb of microcystin-LR were detected from the control group (no treatment), and the H₂O₂ treatment group respectively. In contrast, on average, 19.333 ± 0.742 ppb of microcystin-LR were detected in the positive control, suggesting that extra microcystin-LR existed within the Microcystis sp. cells but were not released after applying the H₂O₂ treatment.

Another technique using low-dose (0.025ppm) ozone nanobubbles was assessed for its effect on reducing non-cyanobacteria algae species in pond water. The technique successfully reduced the diatom species Nitzschia sp. by 66.4% within 5 minutes, and an even higher killing effect (68.2%) was observed after 9 minutes of the treatment. Besides, a 24-hour delayed effect was also detected after the treatment, with a further reduction of approximately 10% of the algae count. The use of ozone at a low level (0.025ppm) was also suggested to be beneficial for increasing the dissolved oxygen levels in pond systems when the ozone breaks down into oxygen.

After evaluating strategies to reduce harmful algae, we investigated using nanobubbles to promote the growth of beneficial algae for algae production. We demonstrated that nanobubble technology could be used to inject CO₂ gas to promote the growth of a typical green algae species, Chlorella vulgaris. The technology addressed one of the bottlenecks of algae production, inefficient CO₂ injection. Carbon dioxide nanobubbles enhanced the CO₂ diffusion rate by over 113% compared to the traditional method of delivering this gas (i.e., injection through air stone). For instance, the CO₂ nanobubble group had a carbonate ion concentration of 815.536 ± 14.893 mg/L, while only 381.857 ± 16.294 mg/L carbonate ion was detected in the group that injected compressed CO₂ via air stone for 30 minutes. After two 30 minute-treatments within a five-day culturing period, the CO₂ nanobubble group had a significantly higher yield (18%) than the CO₂ group compared to when the CO₂ was injected via air stone, and almost 65% better algal yield than the group where only air was injected to the culture system through an air stone. This study highlights the potential of CO₂ nanobubble technology in industrial applications, providing a cost-effective and an efficient method for boosting yield in algae production.

In conclusion, this thesis successfully demonstrated how low-dose hydrogen peroxide (7mg/L) and low-dose ozone nanobubble (0.025 ppm) can be used to mitigate algal blooms in pond water. Further, CO₂ injected with nanobubble technology was used to tackle the inefficient CO₂ injection challenge in algae production and promote beneficial algal growth. The results provided a range of viable strategies to mitigate the imbalance of the algal dynamics in freshwater aquaculture.

Date of Award	25 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sophie Natasha ST-HILAIRE (Supervisor)