Photosynthesis Got an Early Start

Blue-green algae (cyanobacteria) have a bad reputation, but they played an essential role in the history of our planet. They produce toxins that can make people sick, but around 2.4 billion years ago (Giga annum or Ga) they played a key role in the oxygenation of the atmosphere and oceans, a period known as the Great Oxidation Event, and they are still important as primary producers of oxygen. Cyanobacteria are also the precursor of chloroplasts, organelles where oxygen-producing photosynthesis can take place within many living organisms, specifically those organisms that became plants. Although the production of oxygen is essential for most life as we know it, there has been no unambiguous evidence for thylakoids in the early fossil record until now. An elegant study by Catherine Demoulin, Yannick Lara, Alexandre Lambion, and Emmanuelle Javaux has provided such evidence and has moved the dawn of photosynthesis back 1.2 Ga! Light micrograph of Navifusa majensis, 1.75 billion years old, from the McDermott Formation, Australia. Length of organism = 110 µm. Photo copyright, E.J. Javaux. Demoulin et al. showed that analysis of the ultrastructure of fossils is an underappreciated tool for demonstrating chloroplast-like structures. They examined fossils from around the globe (Australia, Canada, and the Democratic Republic of the Congo) using transmission electron microscopy (TEM), specifically looking for thylakoids. Thylakoids are membrane-bound compartments where photosynthesis takes place in cyanobacteria and chloroplasts. Demoulin et al. searched for thylakoids in Navifusa majenis, microfossils that may be cyanobacteria. They isolated the carbonaceous microfossils from the mineralized matrix with a series of strong acid treatments and then embedded them in agarose. The specimens were dehydrated in a graded series of alcohol and embedded in resin. After sectioning, some sections were stained. Interestingly, Demoulin et al. observed no difference between stained and unstained specimens. Although this is difficult to explain, one factor is the microscope they used. The FEI Tecnai T12 provides suitable contrast in unstained sections. Another factor might be that during fossilization, the organic material was stained naturally by diverse elements from the sediment in which the cells were buried. The ultrastructure of unstained resin-embedded specimens showed a set of intracellular membranes with sharp, darker edges that appeared to be either parallel to the cell wall or locally contorted. Each membrane comprised one medium electron-dense layer surrounded by two electron-dense layers. These membranes are from 10–20 nm thick. Whereas these structures resemble basic units of chloroplasts, they are clearly not fully formed chloroplasts. They are probably thylakoids. Together with additional studies of the arrangement of intracellular membranes, this provides direct evidence for oxygen-producing photosynthesis. The value of using TEM to examine thin sections of microfossils was convincingly demonstrated by Demoulin et al. Their discovery of preserved thylakoids within N. majenis unambiguously establishes a minimum age of about 1.75 Ga for the divergence of cyanobacteria with and without thylakoids. Demoulin et al. predict that studies examining the ultrastructure of well-preserved fossils may more clearly define the organisms that created the Great Oxidation Event, leading to the metabolisms that have existed ever since then.

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