Polycyclic triterpenoids (sterols and hopanoids) are ringed lipids and are highly stable to adverse conditions like acidic/alkali and elevated temperature as well as withstand the conditions which can destroy the most of the organic compounds present in the sediments (McGarvey and Croteau, 1995; Shunthirasingham and Simpson, 2006). Biohopanoids are the pentacyclic triterpenoid lipids of the hopane series which can be synthesized by bacteria and few eukaryotes (Rohmer et al., 1984). Biohopanoids get converted to some other products during the diagenesis process, but it still retains the information of the number and position of the original functional group. For example, C32 and C34hopanediols and novel compounds C32hopanediol (XI) and C33hopanediol (XIII) can be related to a restricted number of BHP precursors (Rodier et al., 1999). However, these functionalisedgeohopanoids will not survive deep burial in the geosphere, and for the longer-term preservation of information the hopanoids in macromolecular sedimentary organic matter must be looked which are bound through the sites of original functionality, and may thus preserve source-specific information (Rodier et al., 1999). Hopanoids are used as molecular fossils by geologists and paleobiologists to study the history of Earth surface environments (Pearson et al., 2007). Hopanoids were also reported from the ancient samples which are >2500 billion years old (Brocks et al., 2003; Brocks et al., 1999).
A survey of hopanoid distribution reveals that specific hopanoids may be linked to specific bacterial taxa (Belin, 2009; Blumenberg et al., 2012; Cvejic et al., 2000; Silipo et al., 2014; Summons et al., 1999). It was established that Cyanobacteria are one of the most important producers of BHPs in the environmental samples (Blumenberg et al., 2010). Intact BHPs are reported from the ancient sample as old as 50 million year ago (van Dongen et al., 2006). Methylated C35 hopanoid is one the signature hopanoids for Cyanobacteria in ancient sedimentary environment, therefore, considered as the biomarker for oxygenic photosynthesis (Summons et al., 1999). 2-Methylhopanoids were reported from few methylotrophs, symbiotic nitrogen-fixing bacteria and Rhodopseudomonas palustris TIE-1, hence 2-methylhopanoids cannot be used as an indicator of cyanobacterial oxygenic photosynthesis (Blumenberg et al., 2006; Cvejic et al., 2000; Silipo et al., 2014; Welander et al., 2009). Type I methanotrophs can produces hexafunctionalized BHPs (XXIX) as major hopanoids and similarly type II methanotrophs produces tetra (XXIII) and pentafunctionalized BHPs (XXVIII) as major hopanoids (Cvejic et al., 2000; Silipo et al., 2014). It was observed that freshwater sediments are having an abundant quantity of penta- (XXVIII) and hexafunctionalized (XXIX) hopanoids, which can be linked to enriched methanotrophs in these environment sites (Blumenberg et al., 2006; Silipo et al., 2014).
1.4Role of hopanoids
1.4.1 Membrane integrity and rigidity
In the eukaryotic cell, sterols play a key role in membrane fluidity, stability and help in tolerating ethanol stress and temperature stress (Swan and Watson, 1998). Therefore, it is always a topic of interest that whether the sterol surrogate, bacterial hopanoids also performed the same role in a bacterial cell membrane. It was observed that hopanoids have condensing effect on the artificial membrane and in liposome it results in increased viscosity (Benz et al., 1983). Hopanoid deficient mutant of Rps.palustris TIE-1 became sensitive to bile salt which is the indicator of outer membrane damage and antibiotic like rifampicin and erythromycin which suggest that hopanoids similar to sterol play important role in membrane rigidity and decreases membrane permeability (Welander et al., 2009). Hopanoid intercalate in phospholipid bilayer and pack the hydrophobic centres of bilayers. This architecture helps in inhibiting the loss of protons and important in protecting the cell in extreme pH conditions (Welander et al., 2009).
1.4.2 Lipid Ordering
The ordered plasma membrane is an important property shared by all the three domains of life (Saenz et al., 2015). Lipid ordering gives the property of freedom of motion and lateral segregation of lipids without losing the integrity of a membrane which contributes to membrane fluidity, membrane permeability and signal transduction (Saenz et al., 2015). The property of planner geometry and interaction with sphingomyelin endows natural tendency to sterols to form the ordered lipid phase (Saenz et al., 2015). Lipid A in bacteria is analogous to sphingomyelin and it is reported that hopanoid can covalently linked to lipid A molecule which will enhance the outer membrane resistance (Silipo et al., 2014).
In a model membrane, it was observed that diplopterol (VI) can interact with saturated lipids and form ordered lipid phase (Saenz et al., 2012a). Reports showed that hopanoid might play a crucial role in lipid ordering but the mechanism and significance of lipid ordering in bacteria is poorly understood. Still, there are various bacteria which do not produce hopanoids, so how lipid ordering will be maintained in the absence of hopanoids remains an open question.
1.4.3 Plant microbe interaction
Symbiosis is the mutual interaction between two organisms from which both the organisms get benefited. Bradyrhizobium diazoefficiens is nitrogen fixing bacterium shows symbiotic association with legume Aeschynomeneafra spera and soybean (Kulkarni et al., 2015). ?hpnP and ?hpnH mutants of B. diazoefficiens were unable to synthesize methylated hopanoids and C35 hopanoids respectively. In B. diazoefficiens methylated hopanoids and C35 hopanoids are important in microaerobic growth. It was observed that C35 hopanoids are important in outer membrane rigidity, aerobic growth and stress tolerance (Kulkarni et al., 2015). ?hpnP and ?hpnH mutants of B. diazoefficiens induce fewer nodules on soybean and nitrogenase activity was also reduced. But on the other hand, when A. afraspera was infected with ?hpnH mutants of B. diazoefficiens, it was able to induce nodule organogenesis but plant shows typical nitrogen starvation symptoms like reduced plant growth, a decrease in acetylene reduction activity and foliage chlorosis(Kulkarni et al., 2015). In conclusion, it was reported by Kulkarniet al., that C35 hopanoids are essential for growth in symbiotic microenvironment and symbiosis between B. diazoefficiens and A. afraspera(Kulkarni et al., 2015).