Bacterial and viral infections are among the major challenges faced by the global shrimp farming industry. White spot disease can lead to mortality rates of up to 100% within a span of 3 to 5 days.
It is known that the shrimp Penaeus vannamei is highly susceptible to the white spot syndrome virus (WSSV) throughout its life cycle, with the postlarval developmental stage being considered the most vulnerable to viral attacks.
One potential strategy for disease control in shrimp involves interfering with viral gene expression using double-stranded RNA (dsRNA) administered orally.
Scientists from the Center of Excellence for Shrimp Molecular Biology and Biotechnology (Centex Shrimp) at Mahidol University, the National Center for Genetic Engineering and Biotechnology (BIOTEC), University College London, and the University of Kent have reported the production of antiviral dsRNA in the microalga Chlamydomonas reinhardtii and its use in protecting shrimp against white spot syndrome virus (WSSV) infections.
Currently, DNA transformation of the nuclear genome is possible for numerous species of microalgae. One application of these technologies is the recombinant production of functional biomolecules for oral administration to aquatic animals.
Previous studies have demonstrated that recombinant dsRNA can be produced in the chloroplast of the edible microalga Chlamydomonas reinhardtii and used to control diseases in shrimp.
Chlamydomonas reinhardtii, a freshwater microalga, is possibly the most studied microalga species and has an advanced set of molecular tools for nuclear and chloroplast genetic engineering.
Another advantage of C. reinhardtii is its ability to grow heterotrophically when provided with a fixed carbon source like acetate.
To evaluate the efficacy of the TN72:VP28 strain of C. reinhardtii as an additive in shrimp feed to protect against white spot syndrome virus (WSSV) infections, scientists conducted a feeding experiment followed by a challenge test.
In the feeding trial, groups of shrimp postlarvae were fed for four days with either a standard feed or feed mixed 1:1 with dried algae. On the fourth day, the shrimp groups were challenged by feeding them with tissue infected with the white spot virus.
“The survival of shrimp that received feed supplemented with dried algae containing dsRNA significantly improved (~69% survival) compared to a negative control (<10% survival),” they reported.
TN72:VP28 Strain of C. reinhardtii
According to the study’s results, the TN72:VP28 strain of C. reinhardtii specifically accumulates a fraction of RNA that can be amplified by RT-PCR followed by treatment with RNase A.
“These results provide strong evidence for the production of VP28 dsRNA in the chloroplast of algae. Although this represents a 10-fold increase over what was previously reported in our system, the amount is much lower than in a bacterial system,” the scientists emphasized.
However, they highlight that the value of the C. reinhardtii system is that this microalga is considered safe as food and feed ingredient, and consequently, the entire cell can be used as feed.
Furthermore, the scientists recommend further research to confirm the trend of dsRNA production in the C. reinhardtii system.
On the other hand, the study highlights that there is no risk in using the transgenic microalga C. reinhardtii in shrimp feed because the dried biomass used as an ingredient is an effective method of killing the microalga, thereby reducing the possibility of environmental contamination by genetically modified organisms.
“Drying produces a powder suitable for formulation in shrimp feed where dsRNA is potentially stable at room temperature for extended periods. Further analysis is required to establish the exact half-life of dsRNA under these conditions, along with the required dosage level for effective protection, so that a comprehensive technical-economic assessment and cost calculation per dose can be performed,” they concluded.
Due to the susceptibility of vannamei shrimp to WSSV, it is essential for shrimp producers to ensure the supply of disease-free postlarvae before transferring them to shrimp farms.
In this regard, “feeding shrimp postlarvae with transgenic microalgae expressing dsRNA can reduce or delay massive mortality after viral infection,” the scientists stated.
The findings suggest that this new dsRNA production platform could be employed as a low-cost, low-tech control method for aquaculture.
The study was funded by the Royal Society of the United Kingdom and by Thailand Science Research and Innovation (TSRI) through a Newton Advanced Fellowship.
Reference (open access)
Charoonnart, P.; Taunt, H.N.; Yang, L.; Webb, C.; Robinson, C.; Saksmerprome, V.; Purton, S. Transgenic Microalgae Expressing Double-Stranded RNA as Potential Feed Supplements for Controlling White Spot Syndrome in Shrimp Aquaculture. Microorganisms 2023, 11, 1893. https://doi.org/10.3390/microorganisms11081893