Lack of Methylated Hopanoids Renders the Cyanobacterium Nostoc punctiforme Sensitive to Osmotic and pH Stress

Abstract

To investigate the function of 2-methylhopanoids in modern cyanobacteria, the gene coding for the radical-adenosyl methionine (SAM) methylase protein that acts on the C-2 position of hopanoids was deleted from the filamentous cyanobacterium ATCC 29133S. The resulting Δ mutant lacked all 2-methylhopanoids but was found to produce much higher levels of two bacteriohopanepentol isomers than the wild type. Growth rates of the Δ mutant cultures were not significantly different from those of the wild type under standard growth conditions. Akinete formation was also not impeded by the absence of 2-methylhopanoids. The relative abundances of the different hopanoid structures in akinete-dominated cultures of the wild-type and Δ mutant strains were similar to those of vegetative cell-dominated cultures. However, the Δ mutant was found to have decreased growth rates under both pH and osmotic stress, confirming a role for 2-methylhopanoids in stress tolerance. Evidence of elevated photosystem II yield and NAD(P)H-dependent oxidoreductase activity in the Δ mutant under stress conditions, compared to the wild type, suggested that the absence of 2-methylhopanoids increases cellular metabolic rates under stress conditions. As the first group of organisms to develop oxygenic photosynthesis, are central to the evolutionary history of life on Earth and the subsequent oxygenation of the atmosphere. To investigate the origin of cyanobacteria and the emergence of oxygenic photosynthesis, geobiologists use biomarkers, the remnants of lipids produced by different organisms that are found in geologic sediments. 2-Methylhopanes have been considered indicative of cyanobacteria in some environmental settings, with the parent lipids 2-methylhopanoids being present in many contemporary cyanobacteria. We have created a Δ mutant strain that does not produce 2-methylhopanoids to assess the influence of 2-methylhopanoids on stress tolerance. Increased metabolic activity in the mutant under stress indicates compensatory alterations in metabolism in the absence of 2-methylhopanoids.

Hopanoid lipids: from membranes to plant–bacteria interactions

“…2Me-hopanoids seem to be generally associated with pH stress in diverse bacteria but have negligible or very subtle effects in the presence of other stressors, depending upon the bacterial strain 37,86,89–91 . Extended hopanoids are more important in this context, as they improve growth during exposure to bile salts, high temperatures, low pH, detergents, osmotic stress and antimicrobial peptides 34,86 .…”

Bacteriohopanepolyols across environmental gradients in Lake Vanda, Antarctica

“…In fact, genomic data suggest that the capacity for C-2 methylation may have arisen in the alphaproteobacteria (Ricci et al, 2015). 2-MeBHP have more recently been investigated as markers of particular ecological niches (Ricci, Morton, Kulkarni, Summers, & Newman, 2017) and environmental conditions such as nutrient limitation (Doughty, Hunter, Summons, & Newman, 2009), pH and osmotic tension (Garby et al, 2017;Kulkarni, Wu, & Newman, 2013;Kulkarni et al, 2015;Poralla, Härtner, & Kannenberg, 1984;Welander et al, 2009), and desiccation (Poralla, Muth, & Härtner, 2000). However, few studies have examined the regulation of 2-MeBHP as a response to prevailing environmental conditions in natural systems.…”

Environmental controls on bacteriohopanepolyol profiles of benthic microbial mats from Lake Fryxell, Antarctica

“…Similar trends in penta‐functionalized BHP have been noted in marine oxygen minimum zones (Sáenz et al., ), where BHpentol is most abundant below the oxic–anoxic transition. It is possible that penta‐functionalized BHPs have a greater diversity of source organisms than previously reported (Sáenz et al., ), or are produced in response to environmental conditions (Garby et al., ), such as low light and oxygen. However, studies that further examine the physiological role of penta‐functionalized BHP must be performed in order to test this hypothesis.…”

“…It should be noted that the abundance of BHT II produced by any one organism may also vary with environmental conditions. BHPs may be regulated in response to particular environmental conditions, such as temperature (Kulkarni, Wu, & Newman, ; Osborne et al., ), pH (Garby et al., ; Kulkarni et al., ), and nutrient concentrations (Doughty, Hunter, Summons, & Newman, ). It is possible that environmental conditions present in marine OMZ systems may prompt the production of high abundances of BHT II while conditions in Lake Fryxell do not.…”

“…It should be noted that the abundance of BHT II produced by any one organism may also vary with environmental conditions. BHPs Lake Fryxell may be regulated in response to particular environmental conditions, such as temperature (Kulkarni, Wu, & Newman, 2013;Osborne et al, 2017), pH (Garby et al, 2017;Kulkarni et al, 2013), and nutrient concentrations (Doughty, Hunter, Summons, & Newman, 2009).…”