Biologists have isolated a new type of multicellular bacterium, named Jeongeupia sacculi HS-3, from an underground stream in northern Kyushu Island, Japan. Jeongeupia sacculi HS-3 represents an unusual form of bacterial multicellularity — an organism that can exist in dense, filamentous multicellular structures and clusters of coccobacillus (short rod-shaped) daughter cells. The experiments that mimic the periodic immersion that the new species experiences in its natural cave environment suggest that water immersion plays a role in these life-cycle dynamics.
“The emergence of multicellularity is one of the greatest mysteries of life on Earth,” said Professor Kouhei Mizuno, a researcher at the National Institute of Technology (KOSEN).
“The point is that we already know the superior function and adaptability of multicellularity, but we know almost nothing about its origins.”
“Established function and adaptability are not necessarily their own formative driving force.”
“A curiosity of multicellularity is the conflict between the ‘benefits of individuals’ versus the ‘benefit of the group’ that must have existed in the early stage of the evolutionary transition.”
“We don’t have a good existing model to study multicellularity except theoretical models.”
“One such model is ‘ecological scaffolding’, which proposes that environmental factors provide a selection pressure to an evolving population, suggesting that Darwinian natural selection is applicable even to unicellular organisms.”
Professor Mizuno and colleagues isolated Jeongeupia sacculi HS-3 from a limestone cave wall that is intermittently submerged by an underground river in northern Kyushu Island, Japan.
Most bacteria on agar show an opaque texture due to their disordered structure, but a colony of the new species appeared transparent, with an iridescent hue.
The authors found that Jeongeupia sacculi HS-3 has two distinct life phases:
(i) on a solid surface it self-organizes into a layer-structured colony with the properties of a liquid crystal;
(ii) after maturation, the colony forms a semi-closed sphere housing clusters of ‘daughter’ coccobacillus cells, which are released upon contact with water.
“We needed 10 years to be sure that this was not a contamination of two different species and that it was not just a mutation,” Professor Mizuno said.
“First, we used a series of microscopic observations to film the entire process from a single cell to a colony, for which we developed our own methods.”
“Then, we found that the morphological changes in cells and colonies were both controlled and reversible.”
“Those data led us to believe that it is a ‘multicellularity’ of HS-3.”
“The first stage of HS-3’s life cycle suggests that the liquid crystal-like organization is involved in the emergence of multicellularity, which has not been reported before,” said Professor Kazuya Morikawa, a researcher at the University of Tsukuba.
“The existence of the second life stage implicates the involvement of dynamic water environment in the emergence of HS-3’s multicellularity.”
“We have been surprised by the various curious properties that HS-3 encompasses, one of which is that the multicellular behavior of this new species fits well with the recently proposed ‘ecological scaffolding’ hypothesis.”
“We now think that the leap towards multicellularity would be more elaborate and beautiful process than the one we have imagined so far.”
The discovery is described in a paper published online in the journal eLife.
Kouhei Mizuno et al. Novel multicellular prokaryote discovered next to an underground stream. eLife, published online October 11, 2022; doi: 10.7554/eLife.71920
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