title: 2-Methylhopanoids are maximally produced in akinetes of Nostoc punctiforme: geobiological implications.
abstract: 2-Methylhopanes, molecular fossils of 2-methylbacteriohopanepolyol (2-MeBHP) lipids, have been proposed as biomarkers for cyanobacteria, and by extension, oxygenic photosynthesis. However, the robustness of this interpretation is unclear, as 2-methylhopanoids occur in organisms besides cyanobacteria and their physiological functions are unknown. As a first step toward understanding the role of 2-MeBHP in cyanobacteria, we examined the expression and intercellular localization of hopanoids in the three cell types of Nostoc punctiforme: vegetative cells, akinetes, and heterocysts. Cultures in which N. punctiforme had differentiated into akinetes contained approximately 10-fold higher concentrations of 2-methylhopanoids than did cultures that contained only vegetative cells. In contrast, 2-methylhopanoids were only present at very low concentrations in heterocysts. Hopanoid production initially increased threefold in cells starved of nitrogen but returned to levels consistent with vegetative cells within 2 weeks. Vegetative and akinete cell types were separated into cytoplasmic, thylakoid, and outer membrane fractions; the increase in hopanoid expression observed in akinetes was due to a 34-fold enrichment of hopanoid content in their outer membrane relative to vegetative cells. Akinetes formed in response either to low light or phosphorus limitation, exhibited the same 2-methylhopanoid localization and concentration, demonstrating that 2-methylhopanoids are associated with the akinete cell type per se. Because akinetes are resting cells that are not photosynthetically active, 2-methylhopanoids cannot be functionally linked to oxygenic photosynthesis in N. punctiforme.
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge, MA, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: Doughty
authors.forename: R C
authors.initials: RC
authors.lastname: Hunter
authors.forename: R E
authors.initials: RE
authors.lastname: Summons
authors.forename: D K
authors.initials: DK
authors.lastname: Newman
mesh_terms: D002462:Cell Membrane / Q000737:chemistry; D015394:Molecular Structure; D046937:Nostoc / Q000737:chemistry / Q000254:growth & development* / Q000378:metabolism*; D014315:Triterpenes / Q000378:metabolism*
chemical_list: D014315:Triterpenes; C025206:hopane