A star in the pond
In the freshwater ponds of Massachusetts and other aquatic habitats worldwide, a microscopic organism defies the typical bacterial shapes of rods, spheres, and spirals. Known to science as Stella, this bacterium has a distinctly star-like, six-pronged body. This radial symmetry is exceptionally rare among prokaryotes. The cells are flat, nonmotile, and typically measure between 0.7 and 3.0 micrometers in diameter.
These unusual bacteria are prosthecates, meaning their bodies have appendages that are direct extensions of the cell wall and membrane, filled with cytoplasm. These are not limbs for movement, like flagella, but semi-rigid stalks called prosthecae. Stella belongs to the Alphaproteobacteria class and is a chemoorganotroph, obtaining energy by oxidizing organic compounds like amino acids. They thrive in oligotrophic, or nutrient-poor, water, a detail that is central to understanding their strange form.
The complete genomes of three Stella strains have been sequenced, showing the genetics behind its unique morphology. Analysis shows that Stella possesses many genes for determining a typical rod shape, suggesting it evolved from a more conventional ancestor. The star shape likely arises from polar growth, an elongation process at the tips of the cell, possibly controlled by specific pole-organizing proteins.
The function of form
The purpose of Stella's star shape is being studied, with two primary hypotheses. The first relates to nutrient absorption in its low-nutrient habitat. The prosthecae dramatically increase the bacterium's surface-area-to-volume ratio. This larger surface area helps the bacterium take up of the scarce organic molecules it needs to survive. The appendages are like microscopic antennae, maximizing contact with the surrounding water to scavenge for food.
The second hypothesis proposes that the star shape is a defense mechanism. Microscopic predators, such as protozoa and other bacteria, hunt by engulfing their prey, a process called phagocytosis. A simple spherical or rod-shaped bacterium is relatively easy to consume. Stella's rigid and flat structure makes it an awkward meal. The size and shape may be just large enough and complex enough to deter predation, increasing its chances of survival. This form of morphological defense is a known strategy in the microbial world.
Reproduction in Stella is also atypical. Instead of simple binary fission where one cell splits into two identical halves, Stella divides symmetrically to form two three-pronged daughter cells. This process ensures the continuation of its unusual and advantageous shape.