4.4.1 Methanogenesis
In the present analysis, the bacterial taxa involving in the methane production, viz . methanogenesis, methylphosphonate, DMSP and DMSO, were observed in all the copepod genera but the relative proportion varied between them. A similar observation in Acartiasp. and Temora sp. has been reported [38].
In the present analysis, we found that CAB has a complete set of aerobic methanogenesis genes (PhnL, M, J, H and G) which converts methylphosphonate (MPn) to methane (CH4)[53]. Some copepods, like Acartia sp. andTemora sp., were reported to associate with bacteria that involve in CH4 production from MPn [38]. De Corte et al. [9] suggested that different copepod species have different CAB, and only some copepods have specific CAB for methanogenesis and other biogeochemical cycles.
A previous study (with 14 C-labelled experiments) observed high methane production in Temora longicornis compared to Acartia spp.[54]. Also, the methanogenic archeae i.e.Methanobacterium bryantii-like sequences andMethanogenium organophilum, Methanolobus vulcani-like sequencesand Methanogenium organophilum were noted in Acartia clausi and Temora longicornis faecal pellets[55]. Meanwhile, in the present study, we observed that Pleuromamma spp. has a high proportion of mcrA gene (Figure. S2).
T. longicornis fed with a high content of TMA/DMA rich phytoplankton produced the maximum amount of CH4, which suggests that this production might be due to the micro-niches inside the copepods [56]. But in our present analysis, CAB of Pleuromamma spp. found to have a high proportion of dmd-tmd gene.
In our meta-analysis Acartia spp. found to have a high proportion of dmdA gene. The taxa detected in the present study, like Pelagibacteraceae, some Alpha and Gammaproteobacteria, are known to have dmdA genes [57].
Copepods feeding on phytoplankton liberate DMSP, which, in turn, is utilised by the DMSP-consuming bacteria in the gut (Acartia tonsa ), leading to methane production [58]. Moreover, the methane enrichment in the Central Baltic Sea was due to the dominant zooplankton Temora longicornis feeding on the DMSP/DMSO-rich Dinophyceae resulting in methane release[54].
Instead of analysing faecal pellets [58] and anaerobic incubation experiments [59], further research should also consider CAB-mediated aerobic methanogenesis as one of the factors to solve the ‘Ocean methane paradox’.