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

On 12/04/2024 06:10, 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
>

Very interesting !

I don't think they're super-clear about the line between endosymbiont
and organelle. At the end they discuss "protein trafficking and/or gene
migration" between the symbiotic partners as something that would
demonstrate it is indeed an organelle, and the potential instability of
the symbiosis.

I also thought this bit was very interesting:

"One might question why an N2-fixing organelle-like entity has not yet
evolved or is evolving so slowly compared with mitochondria and
plastids. Although we cannot answer this question, we could speculate
that at least the evolution of the B. bigelowii/UCYN-A symbiosis is
based on recent events in a geological timescale. For example, ocean
conditions on Earth during the mid to late Cretaceous, such as a warm
tropical surface ocean and global anoxia,40 together with the dominance
of diazotrophic cyanobacteria41 and B. bigelowii species turning into a
more phagotrophic strategy to survive and recover from the
end-Cretaceous darkness period caused after the bolide impact on
Earth,32 might have favored the encounter of N2-fixers and eukaryotes.
Accordingly, not only did the B. bigelowii/UCYN-A symbiosis originate
ca. 91 mya,42 i.e., in the late Cretaceous, but also the origin of other
marine (e.g., marine planktonic diatom diazotroph associations43) and
non-marine (e.g., plants with specialized root organs [nodules] where
N2-fixing bacteria are hosted44) N2-fixing symbioses have been dated to
the Cretaceous period."

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