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Investigation of the Roles of Hopanoids in the Lifecycle of Bradyrhizobium diazoefficiens in the Context of Climate Change


Tookmanian, Elise M. (2022) Investigation of the Roles of Hopanoids in the Lifecycle of Bradyrhizobium diazoefficiens in the Context of Climate Change. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/h0xe-jb65.


Rhizobia are a group of bacteria that participate in plant-growth promoting symbioses with legumes, where the bacteria supply the plant with a source of useable nitrogen. In agriculture, crop rotation capitalizes on this symbiosis by planting legumes to restore the nitrogen content of depleted soils. The effects of climate change, such as increased temperature and changing precipitation patterns, threaten the future viability of agriculture. Rhizobia exemplify the role bacteria can play to improve agriculture’s resilience to climate change and prevent land degradation and food insecurity. However, in order for bacteria to realize this potential, they need to survive the challenges of climate change. In my thesis, I detail the environments that rhizobia experience throughout their lifecycle and how the soil environment will likely change as the climate changes. Then, I connect these environmental parameters, especially hypo and hyperosmolarity, to the outer membrane. The outer membrane is the first line of defense for bacteria against external assaults. Rhizobia make many changes to their outer membrane compared to commonly studied enteric bacteria. For example, the ability to synthesize hopanoids, steroid-like lipids, is overrepresented in rhizobia.

Hopanoids are known to help protect bacteria against a wide range of stresses – but, surprisingly, we found that the extended hopanoid class is not required for a moderately successful symbiosis between rhizobia strain Bradyrhizobium diazoefficiens and the tropical legume Aeschynomene afraspera. The main defect was in the initiation of the symbiosis, perhaps due to motility defects in the extended hopanoid—deficient mutant. As we investigated this paradox, we discovered that hopanoids are conditionally essential in B. diazoefficiens depending on the medium in which the organism is grown. Specifically, we investigated the role of hypoosmolarity and divalent cation concentration, discovering that extended hopanoids confer robustness to the physicochemical environment. This property indicates that extended hopanoids may be important in the soil environment, which is prone to osmotic variability, especially as the climate changes. This work increases our understanding of the role of the outer membrane and hopanoids in bacterial resilience which may help with engineering or selection of better crop additives in the future.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:hopanoid; climate change; bacteria; osmolarity; soil; symbiosis; environmental stress
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Newman, Dianne K.
Thesis Committee:
  • Ondrus, Alison E. (chair)
  • Sessions, Alex L.
  • Rees, Douglas C.
  • Newman, Dianne K.
Defense Date:21 July 2021
Non-Caltech Author Email:etookmanian (AT)
Record Number:CaltechTHESIS:09072021-015258902
Persistent URL:
Related URLs:
URLURL TypeDescription Video S1 Video S2 Video S3 Video S4 Table S1 adapted for Chapter 2 adapted for Chapter 3 adapted from Chapter 4
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14354
Deposited By: Elise Tookmanian
Deposited On:09 Sep 2021 21:38
Last Modified:16 Sep 2021 18:22

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