The surprising ability of this organism to change shape without altering its DNA is helping scientists better understand the adaptation of ocean ecosystems
Essential to life on Earth, marine microalgae produce a significant portion of the oxygen we breathe, capture carbon, and form the base of ocean food chains. But how did these organisms evolve? A team from the Institute for Integrative and Systems Biology at Laval University has shed light on the unique life cycle of
Pseudoscourfieldia marina, an ancient lineage of green algae that occupies a central place in evolutionary history.
An assortment of green algae seen by optical microscopy
Images, NNehring
For decades, biologists observed two forms of
P. marina, long considered as two distinct species: a spherical form lacking flagella and a flagellated, elongated form covered with small scales. However, the study, published in the journal
Communications Biology, reveals that it is actually a single species with a surprisingly complex life cycle.
To reach this discovery, the scientists compared the DNA of these two forms. Although their DNA sequences are nearly identical, they do not activate the same genes. In other words, the microalga can radically change its appearance and function without modifying its DNA. In the flagellated form, for example, the team highlighted the set of genes necessary for producing the fine scales that cover the cell surface.
A scanning electron microscopy image of the spherical and flagellated forms of Pseudoscourfieldia marina — Courtesy The research team also showed that the spherical form is diploid, meaning it has two copies of each chromosome, while the flagellated form is haploid, so it has only one copy. This alternation suggests that
P. marina has a life cycle where haploid and diploid stages coexist in distinct forms. "This type of cycle is typically found in land plants and some multicellular algae, but rarely in an organism as simple as a unicellular microalga," reports Claude Lemieux, a professor at the Faculty of Science and Engineering who led the study.
The scientists also highlighted two so-called "outlier" chromosomes, very different from the rest of the genome, which could play a role in defense against viruses. These might therefore be involved in the ecological adaptation of the species.
According to Professor Lemieux, this study opens the door to new research on the evolution of microalgae, pillars of ocean ecosystems, and on the mechanisms of adaptation to the marine environment in response to environmental pressures.