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2-Methylhopanoids are maximally produced in akinetes of Nostoc punctiforme: geobiological implications

Authors

  • D. M. DOUGHTY,

    1. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA, USA
    2. Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, MA, USA
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  • R. C. HUNTER,

    1. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA, USA
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  • R. E. SUMMONS,

    1. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
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  • D. K. NEWMAN

    1. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA, USA
    2. Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, MA, USA
    3. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
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Corresponding author: D. K. Newman. Tel.: (617) 324 2770; fax: (617) 324 3972; e-mail: dkn@mit.edu

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.

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