Gut microbial communities are known to influence the physiology of the host fish, including the general health, immune responses, feeding, digestive system and metabolism (Liu et al. 2016; Butt and Volkoff 2019; Yukgehnaish et al. 2020). Their association with the fitness of host fish has been previously reported in various fish species, including damselfish and cardinalfish (Parris et al. 2016). Besides that, fish are free-living aquatic organisms, where the microbial composition is believed to shift and evolve along with the host in order to adapt to the dynamically changing water conditions (Sullam et al. 2012).
In the present study, we explored the microbial diversity in different gut sections, as each section plays specialised functional roles, which contention made upon the unique cell structures (Salleh et al. 2019) and gene expression profiles (Martin et al. 2016) in these gut sections. Foregut and midgut are where enteric digestions occur while the hindgut, with its larger lumen is important for enteric immune functions (Buddington et al. 1997; Egerton et al. 2018; Salleh et al. 2019). Thus, the microbial compositional differences among these gut locations revealed in the present study were presumably due to gut functional division, as described in the rainbow trout (Betiku et al. 2023). However, irrespective of the health state, the most abundant phyla in all gut sections were Proteobacteria, Actinobacteria, Firmicutes, Fusobacteria and Tenericutes, known to be the persistent groups in fish gut (Wang et al. 2018).
This study found that Halomonas colonization was higher in healthy foreguts than in moribund groupers. Protease-producing strains, such as Halomonas, are particularly important in supporting high trypsin activity in carnivorous fish (Liu et al. 2016). Higher abundance of Halomonas residing in healthy foreguts, implies better protein-digesting processes in healthy orange-spotted groupers. The findings also revealed multiple aromatic compound degradation pathways and higher abundance of Curvibacter in healthy foreguts. Curvibacter genus is composed of anaerobic bacteria that are involved in the degradation of aromatic compounds and was widely studied for its application in bioremediation (Muratçobanoğlu et al. 2020). In this case, Curvibacter is potentially beneficial in enhancing host tolerance by removing the aromatic pollutants. This finding may push the boundaries of possibility to the use of Curvibacter in supporting the aquaculture activities, especially in waters of developing countries that are heavily polluted by aromatic compounds, such as Yellow River (Li et al. 2006), Tianjin rivers (Shi et al. 2005), Jiulong River Estuary and Western Xiamen Sea in China (Maskaoui et al. 2002), as well as Soltan Abad River, Iran (Kafilzadeh 2015) and Meliane river, Tunisia (Khabouchi et al. 2020).
Hindgut is considered a vital structure in fish immunity (Salleh et al. 2019). Healthy hindguts with a significantly higher abundance of Bacillus strains, may indicate its involvement in regulating host gut health, as in the Nile tilapia (Kuebutornye et al. 2020). Bacillus was previously found to only inhabit in the gut of fast-growing groupers (Sun et al. 2009) and its various enzyme and antimicrobial activities successfully ameliorated intestinal damage in trout (Yamamoto et al. 2010; Abriouel et al. 2011). Furthermore, inclusion of host-associated Bacillus spp. in the diet greatly enhanced mucosal immunity and tolerance of Nile Tilapia towards pathogen infection (Kuebutornye et al. 2020).
There was higher abundance of Fusobacteria residing in moribund hindguts. This gut microbiota is deemed invasive and is often associated with a number of diseases in other animal models (Swidsinski et al. 2011; Chun et al. 2020). As Fusobacteria appeared as a common persistent microbiota in fish guts of both groups, as well as in other fish species (Yukgehnaish et al. 2020), it is unlikely linked to the poor health state of moribund groupers in this study. Meanwhile, the higher abundance of Vibrio and Photobacterium in moribunds could be correlated with significantly increased degradation of chitin derivatives in moribund hindguts. Both are pathogenic chitinolytic bacteria (Hunt et al. 2008) in aquaculture and past evidences showed that increased Vibrio and Photobacterium may disrupt gut ecological balance, reduced the dominance of beneficial microflora and alter gut-microbiota-mediated functions in groupers (Sun et al. 2009; Deng et al. 2020; Liu et al. 2020). This is reflected in our study, as the moribund hindguts were found with enhanced lipid metabolisms, coupled with reduced metabolisms of energy, carbohydrate and protein (Fig. 5c).
In conclusion, our study reveals that corresponding with other investigations in other grouper species (Sun et al. 2009; Deng et al. 2020; Liu et al. 2020), gut microbiota changed dynamically and reflected the health state of farmed orange-spotted grouper, E. coioides. A number of microbial genera, including Halomonas, Curvibacter and Bacillus are potentially beneficial gut microbiome that could be developed into a probiotic in the aquaculture industry to enhance the general health and pollution tolerance of host fish, in particular, the future prospects in using Curvibacter to support the aquaculture water bodies with aromatic pollutants. The abundance of Vibrio and Photobacterium may impact the gut ecological balance and hence the metabolic stability. However, the functions of unique although minor microbiota in these gut regions could not be elucidated due to limited available data, but we believe they may play critical roles in maintaining gut health. Despite the low number of moribund fishes sampled in this study, the findings parallel previous studies and could serve as a good metric of data validity. Even so, we strongly recommend the inclusion of higher number of biological replicates in future studies.