New evolutionary study shows evidence for LUPA (Last Universal Photosynthetic Ancestor) 

Life on Earth as we know it essentially depends on photosynthesis. The process of photosynthesis is arguably the most important biochemical process on the planet because it captures the energy from the sun and produces biomass to support nearly all life on Earth. 

You may associate photosynthesis mainly with plants or algae, which are important contributors to this process, however there are far more photosynthetic bacteria than plants and algae combined in terms of sheer numbers and individual organism count on the planet. While plants and algae hold a larger total biomass, photosynthetic marine bacteria and phytoplankton are vastly more numerous, dominating photosynthesis in the oceans. 

Furthermore, just like multicellular life originated from ancestral single cell organisms, so does the origin of the photosynthetic machinery in all plants and algae. There is much similarity between the bacterial photosynthetic systems and the algal and plant systems, and it is generally accepted that oxygenic photosynthesis (that produces oxygen, like in plants and algae) originated from ancient bacterial photosynthetic organisms (that did not produce oxygen). 

However, there are still several uncertainties and controversies about the order of events on how photosynthesis has evolved. One fundamental question is whether photosynthesis was established only once in these ancient photosynthetic bacteria or whether there were multiple points of origin that led to the different types of photosynthesis that we observe today. 

The recent paper by Drs. Imhoff and Kyndt takes a multilayered phylogenetic approach to answer this fundamental question. Previous studies have focused on individual sets of evidence, however, unlike those previous studies, the recent paper used a comprehensive approach that studies three independent lines of evidence: phylogenetic analysis of the reaction centers, and the key enzymes of bacteriochlorophyll biosynthesis, BchN and AcsF, as well as gene synteny of the photosynthetic gene clusters. The convergence of the findings across these different sets provides a compelling and resilient argument for a single origin of photosynthesis, more robust than previously achieved.

The study was entitled “Photosynthesis Has Been Established Only Once—Evolution of Photosynthetic Reaction Center Proteins and Bacteriochlorophyll Biosynthesis” and published in the journal ‘Current Issues in Molecular Biology: https://www.mdpi.com/3783556

The paper describes that there was likely an ancient bacterial-like species that can be considered ‘the last universal photosynthesis ancestor (LUPA)’. This species had a homodimeric Type I photosynthetic reaction center, most similar to the systems we observe in Heliobacteria, Chlorobi, and Chloracidobacteria today. This system was likely present in very special ecological niches as long as 3.5 billion years ago. For comparison, algae have ‘only’ been around for an estimated 1-1.5 billion years and land plants only a mere 500 million years on the planet! So the bacterial origin of photosynthesis far outdates the presence of these organisms. 

The authors do caution that with studying evolution over such long time scales, one must consider that most of the intermediate forms of photosynthetic systems that existed are now extinct. Nevertheless, the occurrence of photosynthesis was a process of major significance for early life on Earth and very likely considerably promoted early bacterial life on the planet. 

Although this study shows the single establishment of photosynthesis in ancient photosynthetic anoxygenic bacteria, there are many undiscovered bacterial species on the planet and possibly yet-to-be-discovered bacterial species in special ecological niches that could be impacting this evolutionary puzzle in the future.