Scientists in Japan have found a type of bacteria in a cave that exhibits multicellular behavior and a unique two-phase life cycle.
The bacterium, HS-3, was isolated from the wall of a limestone cave that is periodically submerged by an underground river. HS-3 has two different life phases; on a solid surface, it self-organizes into a layered, structured colony with qualities similar to that of a liquid crystal. The HS-3 colony matures into a semi-enclosed sphere containing clusters of “daughter” coccobacillus cells, or short rod-shaped cells, which are released when in contact with water.
“The emergence of multicellularity is one of the greatest mysteries of life on Earth,” says corresponding author Kouhei Mizuno, a professor at the National Institute of Technology (KOSEN), Tokyo, Japan. “The point is that we already know the higher 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 ‘individual benefits’ versus ‘group benefit’ that must have existed in the early stage of evolutionary transition. We don’t have a good existing model to study multicellularity except for theoretical models.”
One such model, called “ecological scaffolding,” holds that the environment exerts selection pressure on a developing population, arguing that Darwinian natural selection still applies to single-celled organisms.
Mizuno and her lab student, Ohta, identified HS-3 from water dripping down the wall of a limestone cave on the northern Japanese island of Kyushu in 2008. They initially looked for lipid-accumulating bacteria, but Ohta discovered a small colony with an extraordinarily beautiful color and texture when inspecting old bacterial agar plates before discarding them. Due to their disorganized structure, most bacteria on agar have an opaque texture, however, this colony was transparent and had an iridescent hue. Phenotypic comparisons with closely related species verified this colony as a new species, HS-3, which the scientists named jeongeupia bag (meaning “cradle”).
The team used microscopies to analyze the growth of the colony. The cells began to reproduce simply as coccobacilli, but cell elongation caused the colony to form a single-layered structure, oriented like a liquid crystal. Lumps form particularly at the edge of the colony, relieving internal pressure and giving HS-3 the unique ability to maintain this two-dimensional liquid arrangement for an extended period, which may be a prerequisite for HS-3 to establish a multicellular behaviour.
The colony then expanded to form additional layers. The inner filamentous cells folded, generating vortex-structured domains. These domains and the liquid crystal arrangement explain the transparency observed in HS-3 colonies on agar. After two days, rapid cell reproduction occurred internally and the colony began to swell three-dimensionally, forming a semi-closed sphere that housed the coccobacillus cells. After the fifth day, the inner cells were expelled from the colony, causing a chain reaction of this event in adjacent colonies and thus indicating some multicellular control.
As the HS-3 cave wall sampling site was regularly subjected to flowing water in the cave, the team submerged colonies of mature hemispheres in water. Internal coccobacilli were released into the water, leaving behind the filamentous cell architecture. By seeding these daughter cells on fresh agar, they found that the cells were able to reproduce the original filamentous structure, demonstrating that the two distinct phases of the HS-3 life cycle are reversible and may have arisen due to changing conditions within the cell. cave. .
“We needed 10 years to be sure that this wasn’t contamination from two different species and that it wasn’t just one mutation,” says Mizuno. “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 controlled and reversible. These data led us to believe that it is a ‘multicellularity’ of HS-3”.
“The first stage of the HS-3 life cycle suggests that liquid crystal-like organization is involved in the emergence of multicellularity, something that has not been reported before. The existence of the second life stage implies the involvement of the dynamic aquatic environment in the emergence of HS-3 multicellularity,” says co-author Kazuya Morikawa, a professor in the Division of Biomedical Sciences. Tsukuba UniversityJapan
“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. Now we think that the leap towards multicellularity would be a more elaborate and beautiful process than the one we have imagined up to now”. commented Mizuno and Morikawa.
Reference: “New Multicellular Prokaryote Discovered Alongside an Underwater Stream” by Kouhei Mizuno, Mais Maree, Toshihiko Nagamura, Akihiro Koga, Satoru Hirayama, Soichi Furukawa, Kenji Tanaka, and Kazuya Morikawa, October 11, 2022, eLife.