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’.