Paulinella chromatophora, described in 1895 by the German zoologist Robert Lauterborn, is considered to be one of the first documented cases of endosymbiosis, that is, a “plant-like” cell living inside an “animal-like” cell. Therefore, Paulinella occupies a pivotal position in the endosymbiosis field. However, because this is a rare and difficult to culture single-celled organism, very few studies have been done, until distinguished professor Debashish Bhattacharya (Department of Biochemistry and Microbiology) and his colleagues were able to culture enough cells to sequence the genome as well as do many other experiments.
The origin of photosynthesis in algae and plants occurred over a billion years ago, whereas the Paulinella event occurred about 100 million years ago. The Paulinella is a “work in progress” model of endosymbiosis that helps explain how the ancient endosymbiosis, that shaped our planet, succeeded. Therefore, Paulinella is a critical tool for understanding the origin of primary production—the conversion of the sun’s energy into organic material through photosynthesis—important because it is the process that forms the foundation of food webs in most ecosystems.
The team determined the 707 million base pair genome sequence of Paulinella and studied how photosynthesis works in this species. To do this, they identified genes that are active over the daytime-nighttime cycle and determined how Paulinella responds to high light, a common stress factor that algae and plants need to deal with. The team’s findings suggest that massive gene evolution was needed to allow photosynthesis in Paulinella.
Why does it matter? Photosynthesis is critical for our planet because algae and plants use this process to “fix” inorganic carbon into sugars that support many ecosystems, including crops that feed humans. How this complex process evolved is shrouded in mystery because it occurred over 1 billion years ago in algae and plants and the early steps have been lost to time. Paulinella offers the opportunity to access this information and learn about the major early innovations that allowed photosynthesis to take hold. We can also apply this information to potentially improve primary production in crop plants, algae, and edible seaweeds.
What are the next steps?
Ongoing work funded by NASA in the Bhattacharya lab is aimed at better understanding how the photosynthetic machinery works in Paulinella and what sorts of genomic changes have occurred in this species. Working with their partner on the NASA grant, Arthur Grossman at the Carnegie Institution in Stanford, they are identifying newly arisen genes in Paulinella that may help improve photosynthesis in other algae and plants. The Grossman lab will express these novel Paulinella genes in photosynthetic cyanobacteria to test their impacts on reducing stress caused by excess high light that disrupts the photosynthetic machinery. The Lab is also developing single cell methods to study gene expression to understand how individual Paulinella cells are regulating photosynthesis and coordinating their circadian clock throughout the light-dark cycle.
The published preprint in Oxford Academic is at https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msaa206/5892262. Pubmed.gov National Library of Medicine also published at https://pubmed.ncbi.nlm.nih.gov/32790833/. The final paper will appear in the November issue.