The main finding of a bibenzyl molecule detected in 28 mangrove species through this extensive GC-MS and GC-FID study initially seemed to be very intriguing. To date, among plants, bibenzyl derivatives have mainly been reported from some bryophytes, orchids, and Cannabis sativa, and some of these derivatives are well established for several clinical parameters, mainly based on their antimicrobial and cytotoxic potential (Nandy and Dey, 2020; Ahmad et al., 2022; Boddington et al., 2022; Chen et al., 2022). Several mangrove and mangrove associate species are often reported to possess bioactive secondary metabolite molecules of high curative, preventive, and pharmaceutical importance in medicinal practices (Bandaranayake, 1998; Jiang et al., 2018; Zhang et al., 2018; Bibi et al., 2019; Roy and Dutta, 2021; Sudhir et al., 2022; Wu et al., 2022). Nevertheless, to our best knowledge, the presence of bibenzyls has not yet been referred to in this context. Under this backdrop, it seemed challenging to interpret and rationalize the presence of bibenzyl molecules in 28 mangrove and mangrove associate species collected from the Indian Sundarbans without downstream purification of the actual bibenzyl derivatives detected. However, the repeated identification of the bibenzyl cores in such a high number of mangrove and mangrove associate species (28 species) with almost 95–100% reproducibility in the results irrevocably establishes the bibenzyl core molecule as a product of some yet-to-be-identified secondary metabolic pathway existing in these eco-physiologically enigmatic mangrove and mangrove associate species. As these findings are preliminary and we are yet to purify the “bibenzyl” derivative compounds from these mangrove and associate species, the ubiquitous presence of bibenzyl scaffold in all these 28 species, strengthens our hypothesis to justify that all these studied mangrove and associate species invest their energy in synthesizing this common secondary metabolite “bibenzyl”, because the presence of this metabolite could be related to the ROS-scavenging potential of the bibenzyl molecule, which is desperately needed by the mangrove and mangrove associate species to mitigate the salinity induced oxidative stress to survive in hypersaline mangrove niche. To validate our hypothesis to establish “bibenzyl” as a novel eco-physiological trait of mangrove species, hitherto unknown in existing literature, we carried out this extensive study of detection of the presence of bibenzyl core scaffold for 28 different mangroves and related plant species.

Several researchers have demonstrated bibenzyl biogenesis in plant systems as a by-product of branches of the core phenylpropanoid biosynthetic pathway. A well-known bibenzyl derivative (3,3’,5-trihydroxybibenzyl) identified from the bulb tissues of the orchids Barlia longibracteata and Orchis spp. was first validated to be a physiological intermediate of an offshoot of the phenylpropanoid pathway, with l-phenylalanine as a precursor and various hydroxycinnamic acids as intermediary substrates. This study conclusively established the origin of at least one of the aromatic rings in the trihydroxybibenzyl molecule derived via phenylpropanoid biosynthesis based on feeding experiment results (Fritzemeier and Kindl, 1983). A correlation between upregulated expression of the several intermediate enzymes of the phenylpropanoid pathway, including the starting enzyme PAL, in the roots, leaves, and several other plant parts with concurrent abundance of bibenzyls in the orchid Arundina graminifolia indirectly indicated the interrelationship between these molecules (Ahmad et al., 2022). The biosynthetic route of cyclic bis(bibenzyl) marchantin A, a derivative of the core bibenzyl from the bryophyte Marchantia polymorpha, was also demonstrated to originate from the benzene ring of l-phenylalanine via trans-cinnamic acid and p-coumaric acid. The researchers validated the biosynthesis of the bibenzyl monomers to constitute the marchantin A molecule via a unique branch of the core phenylpropanoid biosynthesis pathway (Friederich et al., 1999). Another bryophyte, Plagiochasma appendiculatum, was also successfully demonstrated to synthesize bisbibenzyls via a special branch of the core phenylpropanoid synthesis pathway, originating from l-phenylalanine and taking a diversion at p-coumaric acid to dihydro-p-coumaric acid, leading to the formation of dihydro-p-coumaryl coenzyme A (CoA) and other intermediates (Gao et al., 2015). A recent comprehensive study from Cannabis sativa explicitly proved the origin of the two aromatic rings of the bibenzyl scaffold derived from the core phenylpropanoid pathway via hydroxycinnamic acids becoming esterified to acyl-CoA to form dihydro-CoA derivatives and consequent condensation with malonyl-CoA to yield bibenzyl scaffolds (Boddington et al., 2022).

Thus, the detection of a bibenzyl scaffold from 28 mangrove and mangrove associate species of the Indian Sundarbans may be rationalized in light of the physiological adaptive convergence of mangrove species, where the primary acclimations are centered on osmotic adjustments and the secondary acclimative process controls oxidative stress-induced damage from the high-salinity environment (Nijam et al., 2022). The phenylpropanoid biosynthetic pathway that provides substrates for many secondary metabolites, including bibenzyls, is an integral component of this oxidative stress mitigation process. This explains the common presence of the bibenzyl scaffold in all 28 mangrove species studied. This interpretation finds stronger support from a study showing significant upregulation of enzymes of the phenylpropanoid pathway that were strongly induced under hypersalinity (500 mM NaCl) in Kandelia candel, a woody true mangrove species, demonstrated via transcriptomic analyses, enzymatic activity analyses, and succeeding “phenylpropanoid” accumulation with potential to scavenge ROS (Wang et al., 2016). Several reports corroborate the in vitro ROS detoxification attribute of purified biogenic bibenzyl derivatives, including from the orchid Dendrobium parishii (Kongkatitham et al., 2018); the aerial parts of Notholaena nivea Desv. (Pteridaceae) (Cioffi et al., 2011); purified moscatilin, gigantol, crepidatin, and chrysotoxin bibenzyl derivatives from Dendrobium spp. (Choonong et al., 2019); and purified bibenzyls, nobilin D and nobilin E, from the orchid Dendrobium nobile (Zhang et al., 2007). This ROS detoxification potential of ‘bibenzyl’ scaffold becomes more important for mangrove associate species, that grow luxuriously in the same mangrove habitats of Indian Sundarbans along with conventionally known true mangrove species irrespective of their habits like herb, shrub or tree. True mangrove species possess typical adaptive features like pneumatophores, vivipary, cryptovivipary etc., which are lacking in these semi-mangrove plants (mangrove associate species). Despite the absence of true mangrove adaptive features, these semi-mangrove plants (mangrove associate species) have the potential to survive the hypersalinity of the mangrove niches because of their acclimative osmotic adjustment potentials by synthesizing several compatible solutes (Begam et al., 2020). Almost all the mangrove associate species we have referred in this study ( Table 1 ) are known to synthesize one or more number of compatible solutes, helping them to acclimate with the hypersalinity of mangrove niches by osmotic adjustment, notwithstanding the possession of adaptive features of true mangroves (Begam et al., 2020). This study establishes the presence of another bioactive secondary molecular scaffold of “bibenzyl” being common to all these mangrove associate species, which might confer to these species additional salinity induced ROS scavenging potential via antioxidant activities described for this molecule by earlier researchers (Zhang et al., 2007; Cioffi et al., 2011; Kongkatitham et al., 2018; Choonong et al., 2019). Thus this study signifies “bibenzyl” scaffold, like other secondary metabolites, might be a potential acclimative response that is especially beneficial for mangrove associate species.

In summary, this study confirmed detection of a bibenzyl scaffold from 28 mangrove and mangrove associate species of the Indian Sundarbans growing in mangrove niches where the prevailing conductivity of the sediment and riverine water is ~5–12 dS/m and ~32–40 dS/m ( Table 1 ) respectively, during major part of the year (Chowdhury et al., 2019; Begam et al., 2020). We propose bibenzyl as another signature molecule of salinity acclimation in mangroves, with potential for detoxification of salinity-induced ROS, thereby shielding the photosynthetic apparatus and other cell organelles and protecting function of the downstream metabolic processes, similar to the effects of compatible solutes/osmolytes. To our knowledge, this is the first report to reveal the presence of the secondary metabolite bibenzyl in mangroves thought to have an eco-physiological functional role towards salinity acclimation. However, purification of the bibenzyl derivatives from mangrove species is yet to be carried out, along with validation of its biosynthetic steps. This preliminary study should attract further research interest on mangrove eco-physiological functional traits, especially with regard to the mechanisms underlying salinity stress acclimation, amid a unique ecological environment. Bibenzyl seems a potential signature molecule for stress alleviation of mangrove species adapted to hypersaline ecology.