By Laura Frederiksen
Photosynthesis is perhaps the most important biological process on earth. It sustains almost all life, from tiny photoautotrophic bacteria to complex heterotrophs such as ourselves. It is widely accepted that photosynthesis first occurred as a tiny genetic variation in prokaryotes (Cribb, 2012). Over time, this variation evolved into a widespread process that ultimately changed the world. So by what mechanism did photosynthesis leap from prokaryotic to eukaryotic cells? The answer, it would seem, lies with the Endosymbiont Hypothesis (Reece et al. 2012).
According to this hypothesis, photosynthesising prokaryotes known as cyanobacteria began to live inside larger cells approximately 1.5 billion years ago (Carnegie Institution, 2012). Originally engulfed as prey or parasites, the cyanobacteria began to live in a mutual, symbiotic relationship with their host cells (Reece et al. 2012). Eventually evolution progressed so far that the two cells became inseparable. The cyanobacteria transported most of their DNA to the host cell’s nucleus, so that both cells became reliant on each other to survive (Carnegie Institution, 2012). In this way, the endosymbiont became a chloroplast; a vital organelle found in plant cells today.
These events happened such a long time ago that researching the Endosymbiont Hypothesis is a challenging process. However, two researchers at the Carnegie Institution for Science in Stanford recently published a paper in the Proceedings of the Natural Academy of Sciences detailing their findings on early photosynthesis (Carnegie Institution, 2012). Arthur Grossman and Eva Nowack investigated the amoeba Paulinella chromatophora. These organisms contain photosynthetic compartments known as chromatophores (Carnegie Institution, 2012). While still originating from cyanobacteria, these endosymbionts are younger and show an earlier phase in the development of true organelles (Carnegie Institution, 2012).
Grossman and Nowack directed their research to the 30 cyanobacterium genes which were transferred to the host cell nucleus after endosymbiosis (Grossman & Nowack, 2012). This is approximately 70% of the chromatophore genome (Grossman & Nowack, 2012). The Carnegie team focused on three genes in particular. They found that these genes were manufactured in the cytoplasm of the cell before being transported to the chromatophore (Grossman & Nowack, 2012). They also learnt that proteins en-route to the chromatophore were first directed through the Golgi apparatus (Grossman & Nowack, 2012), an organelle commonly thought of as the cell “traffic controller”. This route shows an early process for protein transport into chromatophores and gives insight into how the more complex transport system works in chloroplasts (Carnegie Institution, 2012).
When interviewed about the implications of this research, Nowack said,
“Obtaining a comprehensive list of proteins imported into chromatophores, as well as understanding the pathway by which these proteins are imported, could provide insight into the mechanism that eukaryotic cells use to ‘enslave’ bacteria and turn them into organelles such as chloroplasts and mitochondria”
(Carnegie Institution, 2012).
P. chromatophora is proving to be a great tool for studying organellogenesis. Understanding the way that photosynthesis was adopted into eukaryotic cells has major ramifications for life as we know it. Photosynthesising eukaryotes were an incomparable evolutionary leap and as a result, we are completely dependent on them in everyday life.
Carnegie Institution 2012, Amoeba may offer key clue to photosynthetic evolution, Carnegie Institution for Science, Washington, viewed 14 March 2012, <>.
Cribb, T 2012, Lecture 6 – Prokaryotes, getting energy and their role on earth, Powerpoint slides, University of Queensland, Brisbane.
Grossman, AR & Nowack, ECM 2012, ‘Trafficking of protein into the recently established photosynthetic organelles of Paulinella chromatophora’, Proceedings of the National Academy of Sciences, viewed 16 March 2012, <http://www.pnas.org/content/early/2012/02/22/1118800109.full.pdf+html>.
Reece, J, Meyers, N, Urry, L, Cain, M, Wasserman, S, Minorsky, P, Jackson, R, Cooke, B 2012, Campbell Biology, Ninth Edition, Pearson Education Inc., Australia.