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Bacterial communities associated with the rotated positioning syndrome in gilthead sea bream larvae

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By Milthon Lujan

Gilthead sea bream (Sparus aurata). Source: CSIC.
Gilthead sea bream (Sparus aurata). Source: CSIC.

In the Mediterranean, Greece stands out as one of the main producers and has a large aquaculture sector that relies heavily on the farming of Gilthead Seabream (Sparus aurata). At the heart of this success is larviculture, the delicate process of raising healthy larvae until they are ready to be raised in tanks or cages in the sea.

However, larviculture is a critical stage that presents a significant obstacle: outbreaks of diseases that devastate Gilthead Seabream larvae production.

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Recently, a new concerning disorder affecting S. aurata larvae has emerged. These small fish exhibit unusual behaviors such as rolling movements, surface distribution, and lethargy, ultimately leading to death. Identifying the cause of this ‘rotated positioning’ syndrome is crucial to safeguard future production.

In this regard, scientists from the University of Patras, University of Crete, Avramar Aquaculture SA, Avramar Aquaculture SA, and the Athens Veterinary Center combined their knowledge to study the microbiome of Gilthead Seabream larvae (Sparus aurata) to uncover the secrets of the ‘rotated positioning’ syndrome.

Rotated Positioning Syndrome of Gilthead Seabream larvae

According to various studies, the agent responsible for the ‘rotated positioning’ syndrome of Gilthead Seabream larvae does not appear to be a specific known pathogen. Scientists believe that this syndrome is associated with variations in diversity and interactions within bacterial communities.

The microbiome: a possible culprit?

Recent research suggests that the larvae’s microbiome, the community of bacteria living inside them and their environment, plays a crucial role in their health. Changes in this bacterial composition, known as dysbiosis, could be driving the observed symptoms in the ‘rotated positioning’ syndrome.

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Studies point to opportunistic pathogens or imbalances within the existing beneficial, neutral, and harmful bacteria inhabiting the larviculture ecosystem as possible culprits.

Unraveling the microbiome of healthy and symptomatic larvae

Researchers used an advanced technique called amplicon sequencing to analyze the bacterial fingerprints of both healthy and ‘rotated’ Sparus aurata larvae. They monitored these bacterial communities during the initial development of the larvae (18 days) and even analyzed the bacteria present in the surrounding ecosystem, including breeding water, food sources such as Chlorella, Rotifer, and Artemia, and even organic matter.

This study delves into the bacterial communities within healthy and ‘rotated’ Sparus aurata larvae, along with the bacteria surrounding breeding water, organic matter, and food. Using high-tech sequencing techniques, researchers aim to map these bacterial landscapes and compare them at different development stages (3 to 18 days after hatching).

A tale of two microbiomes

The results revealed fascinating differences in bacterial profiles between healthy and symptomatic larvae. Healthy larvae harbored a dynamic microbiome, dominated by different bacterial groups at various stages: Vibrio and Bacillus early on, Alphaproteobacteria in the middle, and Marinifilum towards the end. In contrast, ‘rotated’ larvae showed a striking prevalence of a specific bacterial genus: Psychrobacter, which increased significantly as they developed.

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Furthermore, the study also identified the role of diet in shaping the larval microbiome. Chlorella algae favored Firmicutes bacteria, while Rotifer and Artemia hosted Proteobacteria, including several species of Psychrobacter. Particularly in later stages, live Artemia appeared to specifically increase the presence of Psychrobacter in symptomatic larvae.

By analyzing the complex network of interactions between bacterial species, researchers discovered that Psychrobacter played a central role in the microbiome network of ‘rotated’ larvae. In contrast, healthy larvae relied on a more diverse network that included bacteria from the phyla Proteobacteria, Bacteroidota, and Firmicutes.

Application in Gilthead Seabream aquaculture

This study sheds light on the crucial role of the microbiome in the health of Sparus aurata larvae. The distinct bacterial fingerprints associated with healthy and symptomatic larvae, particularly the dominance of Psychrobacter in ‘rotated’ individuals, offer valuable clues for early detection and potential intervention strategies. By monitoring the ratio of gammaproteobacteria/alphaproteobacteria, a promising indicator of larval health, fish farmers can take proactive measures to prevent outbreaks and safeguard their precious young populations.

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Understanding the dynamics of the microbiome and its link to larval health, this research paves the way for:

  • Early monitoring and control: Identifying changes in the microbiome associated with the rotated positioning syndrome, allowing early intervention to prevent outbreaks.
  • Improved production performance: Optimization of larviculture conditions and feeding to promote a healthy microbiome, leading to stronger and more resistant larvae.

Conclusion

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This research marks a significant step in unraveling the mysteries of the rotated positioning syndrome and unlocking the full potential of larviculture in the Mediterranean and beyond, as it provides valuable information that can be used as indicators to monitor the health status of Sparus aurata larvae.

Harnessing the power of the microbiome, we can ensure a bright future for sustainable marine aquaculture and secure the supply of healthy larvae to ensure the success of Gilthead Seabream farmers.

The study was funded by the Hellenic Ministry of Rural Development and Food under the project ‘Study of the genome and microbial communities in the development and production of aquacultured sea bream and sea bass (FishμBiome).’

Reference (open access)
BEL MOKHTAR, N., Apostolopoulou, G., Koumoundouros, G., Tzokas, K., Toskas, K., Gourzioti, E., … & Tsiamis, G. Bacterial community structures and dynamics associated with rotated positioning syndrome in Gilthead Sea Bream (Sparus aurata) larviculture. Frontiers in Aquaculture, 2, 1270932.

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