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From: RonO <rokimoto@cox.net>
Newsgroups: talk.origins
Subject: Re: A cyanobacteria may be evolving organelle like characteristics
Date: Fri, 12 Apr 2024 19:03:11 -0500
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On 4/11/2024 11:10 PM, Pro Plyd wrote:
> 
> https://www.uri.edu/news/2024/04/evolution-in-action-new-study-finds-possibility-of-nitrogen-fixing-organelles/
> 
> Nitrogen is a nutrient essential for all life
> on Earth. Although nitrogen gas (N2) is
> plentiful, it is largely unavailable to most
> organisms without a process known as nitrogen
> fixation, which converts dinitrogen to
> ammonium — a major inorganic nitrogen source.
> 
> While there are bacteria that are able to
> reduce dinitrogen to ammonium, researchers at
> the University of Rhode Island, Institut de
> Ciències del Mar in Barcelona, University of
> California at Santa Cruz and the Massachusetts
> Institute of Technology have discovered
> nitrogen-fixing symbiotic organisms exhibiting
> behaviors similar to organelles. In fact,
> researchers posit these symbiotic organisms –
> UCYN-A, a species of cyanobacteria – may be
> evolving organelle-like characteristics.
> ...
> 
> paper here
> 
> https://www.sciencedirect.com/science/article/pii/S009286742400182X
> Metabolic trade-offs constrain the cell size
> ratio in a nitrogen-fixing symbiosis
> 

My take is that nitrogen fixers are usually anaerobic.  This one likely 
isn't very efficient at nitrogen fixing, and it gave up on 
photosynthesis and can't use light to fix carbon so it needs help in 
order to maintain it's nitrogen fixing ability.  Some how it needs to 
sequester itself away from oxygen inside an aerobic host.  Legumes help 
their nitrogen fixers by having leghemoglobin to sop up oxygen and keep 
the levels low enough in the root nodules so that their nitrogen fixing 
bacteria can fix nitrogen for them.  In return the plants give the 
bacteria a carbon source.

Sequestering from oxygen inside of an aerobic cell has to be difficult 
and is likely the reason that such an endosymbiosis has not occurred.

Ron Okimoto