Introduction
Biofloc technology (BFT)is presented as a sustainable aquaculture method due to its low water exchange and increased stocking density compared to conventional methods. The bacteria population present at the biofloc is responsible of nitrogen recycling, maintaining the total nitrogen at safe levels for the cultured species. Furthermore, bacteria form protein-rich flocs, resulting in an improvement of growth rate and associated feed costs. On the other hand, microalgae are a source of beneficial fatty acids that improve the health and growth of cultivated species. Studies have demonstrated that continuous supplementation with different types of microalgae has enhanced BFT in different ways, such as increased survival rates and growth in P. vannamei (Dong et al., 2022). The main objective of this study is to find the ecological balance between the bacterial population and the inoculated microalgae. It is a known fact that microalgae require a light/dark regime for productive photosynthesis (specific photoperiod). Therefore, this experiment was conducted at different photoperiods to determine the most suitable for microalgal growth and P. vannamei in BFT.
Material and Methods
Two microalgae, Chlorella sp. (CH) and Phaeodactylum tricornutum (PH), with a final absorbance of 0.15 AU (λ= 680 nm) were added into a biofloc system (total suspended solids (TSS): 150 mg/L) under three different photoperiods (PP) (16:8, 12:12, and 8:16 light:dark). Additionally, a control group was included that consisted in a biofloc without microalgae inclusion and under darkness conditions (0:24). All groups had 3 replicates, with a total of 21 experimental units of 90 L. The experiment was carried out with a density of 123 animals/m2, and an initial weight of 3.6 g and 5.1 g for CH and for PH, respectively. P. vannamei were fed twice a day with commercial feed (Le Gouguessant Aquaculture, France). The experiment had a duration of 14 days. Water parameters were monitored daily by the multiparametric HANNA equipment HI19829 Model, with salinity values of 19±4 g/L, temperature of 28 ºC, pH of 8-8.8, dissolved oxygen >5 mg/L, and alkalinity of 207 ± 40 mg/L. Ammonium, nitrite, nitrate, TSS < 500 mg/L values were measured twice a week. Microalgae population was quantified with total chlorophyll extraction and spectrophotometry (DLAB SP-UV1100). Finally, the animals were weighed at the end of the experiment. The results of the study were analyzed using multiparametric ANOVA and One-way ANOVA (p-value <0.05) with the Statgraphics 19 software.
Results and discussion
The concentration of chlorophyll showed different dynamics based on the PP, as can be seen in Figure 1. Using CH as microalgae, mostly, the control group showed lower levels of chlorophyll until 7th day. PH group showed significantly higher levels after microalgae inoculation respect to control, but only four days later decreased dramatically, remaining low the rest of the experiment. Except 16:8 group, which increased on day 14, showing differences with the rest of the groups. (Figure 1).
Regarding water quality, meanwhile CH group was able to maintain the ammonia, nitrite and nitrate concentration to non-toxic levels, in PH group, ammonium and nitrate concentration increased until the 11th day, to decrease later on, but never achieving toxic levels.
In relation to growth performance, no significant differences in weight gain were observed between PP within each experimental group (Table 1).
Nevertheless, when we compared PP and microalgae, generally, higher growth was obtained using Chorella spp than P. tricornutum, with significant differences in the PP 12:12 and 8:16 PP. Thus, it seems that medium bacterial and microalgae growth are the best conditions for shrimp production using Chorella spp. On the other hand, likely, due to the bad adaptation of PH to the BFT, the highest growth was obtained in the control group, without microalgae inoculation. The poorer adaptation of P. tricornutum could be due to being a diatom, it may face issues of insufficient silica for its growth.
Therefore, we can conclude that based on the major stability of water parameters, minor decline in chlorophyll concentration and the greatest weight gain in animals indicates that Chlorella sp. may be the most optimal candidate to use under BFT. Similar effects were also found in the study by Ge et al. (2016), with better shrimp growths using microalgae. Longer experimental trials are necessary to confirm these results, but it is the first study under our knowledge that is able to maintain a stable microalgae population to produce P. vannamei under BFT.
Bibliography
Dong, S., Li, Y., Huang, F., Lin, L., Li, Z., Li, J. Zhang, Y. & Zheng, Y. (2022). Enhancing effect of Platymonas addition on water quality, microbial community diversity and shrimp performance in biofloc-based tanks for Penaeus vannamei nursery. Aquaculture, 554, 738057.
Ge, H., Li, J., Chang, Z., Chen, P., Shen, M., & Zhao, F. (2016). Effect of microalgae with semicontinuous harvesting on water quality and zootechnical performance of white shrimp reared in the zero water exchange system. Aquacultural Engineering, 72, 70-76.
Acknowledgments
This work and the research contract of T. Cascales was supported by European Union Next Generation-Plan of Recuperación-Ministerio de Ciencia e Innovación-Gobierno de España (TED2021-129272B-C21) and Conselleria d’innovació, Universitats, Ciència i Societat Digital of Generalitat Valenciana (INVEST/2022/434), respectively.