4.4.6. CAB as a source of cyanocobalamin synthesising
prokaryotes
Organisms within all domains of life require the cofactor cobalamin
(vitamin B12), which is usually produced only by a subset of bacteria
and archaea [69]. Previous studies reported that
the cobalamin in ocean surface water is due to de nova synthesis by
Thaumarchaeota. Moreover, few members of Alphaproteobacteria,
Gammaproteobacteria and Bacteroidetes genomes were reported to have the
cobalamin synthesising gene [69]. In our analysis,
the CAB of Temora spp. was found to have a high proportion of
Thaumarchaeota, whereas Alpha-gammaproteobacteria were found to be high
in the CAB of Acartia spp., Calanus spp. andPleuromamma spp. In this regard, further studies on CAB diversity
from different ocean realms would shine a light on the actual potential
of CAB in the global biogeochemical cycles.
5. Conclusion
Herein, five copepod genera viz.Acartia spp., Calanus spp., Centropages sp.,Pleuromamma spp., and Temora spp., and their associated
bacteriobiome were investigated. The use of meta-analysis in the present
study reveals the difference in bacterial diversity indices within the
alpha and beta-diversity. To be more specific, the meta-analysis showed
significant variations in the alpha diversity between the copepod
genera. Moreover, it revealed that Calanus spp have high Shannon
index (H-index) and Pleuromamma spp. have high Faith’s
Phylogenetic Diversity. Furthermore, the meta-analysis revealed that the
CAB within the phylogenetically closer Pleuromamma spp. andCalanus spp. expressed a mere 7.604% (axis 1) dissimilarity
distance in PCoA analysis (Unweighted Unifrac distance matrix based on
the phylogenetic index). Likewise, from the meta-analysis, we were able
to identify the bacterial taxa which are significantly abundant in each
copepod genera in comparison with others.
In earlier studies, the core bacterial OTUs were identified based on
their presence/absence [1] as well as by using
distribution-based clustering (DBC) algorithms[2]. Herein, machine learning models were used to
predict the important copepod associated bacterial genera within the
five different copepod genera. In specific, we used supervised machine
learning models to predict the important bacterial s-OTUs. We predicted
28 bacterial taxa and one archeael taxon (SML-GBC) as important s-OTUs
in the five copepod genera. Among the predicted bacterial genera, in
common, Vibrio shilonii, Acinetobacter johnsonii,
Piscirickeesiaceae, and Phaeobacter were reported as important
s-OTUs in the Calanus spp. and Marinobacter, Limnobacter.
Methyloversatilis, Desulfovibrio, Enhydrobacter, Sphingobium,
Alteromonas andCoriobacteriaceaewere predicted as important s-OTUs in Pleuromamma spp. for the
first time. Additionally, the prediction accuracy (for Calanusspp. and Pleuromamma spp.) of the machine learning models used
here showed high accuracy, which indicates the reliability of the
predicted important s-OTUs in the copepod genera. Notably, from the
machine learning-based classification it was evident that specific
bacterial s-OTUs do exist for copepods.
Furthermore, our meta-analysis revealed that the five copepod genera
have bacterial communities that are capable of mediating methanogenesis
(with evidence of interlinking the methane production, DMSP degradation
and phosphate utilisation) and methane oxidation. We also found the five
copepod genera to have more potential Assimilatory Sulfur Reducing (ASR)
microbial communities than the Dissimilatory Sulfate Reducing (DSR)
communities within the CAB. Likewise, the bacterial community with
potential genes involved in nitrogen fixation, denitrification and DNRA
were also observed among the CAB of these five copepod genera. We also
found the potential genes that perform carbon fixation, iron
remineralisation and Cyanocobalamin (vitamin B12) synthesis in the CAB
of the five copepod genera.