A surprising discovery by scientists from the Carnegie Institution has brought a new advancement to the study of photosynthesis, which is considered one of the most crucial biological processes on Earth.
Through photosynthesis, plants, algae, and some types of bacteria provide energy for most living organisms by creating food from sunlight. In this process, these organisms release oxygen and absorb carbon dioxide.
However, two studies conducted by Arthur Grossman and his colleagues have shown that some microorganisms living in the ocean have evolved a method of photosynthesis that does not conform to the aforementioned rules. These organisms generate a significant portion of energy without the need to absorb carbon dioxide or release oxygen. These two studies were published in Biochimica et Biophysica Acta and Limnology and Oceanography. Arthur Grossman’s discovery not only challenges scientists’ fundamental understanding of photosynthesis but may also help explain why marine microorganisms contribute to the increased levels of carbon dioxide in the atmosphere.
Grossman and his team studied the photosynthesis process in the marine bacterium Synechococcus – a type of bacterium that can perform photosynthesis, commonly known as cyanobacteria (previously referred to as green algae). These unicellular organisms dominate the entire phytoplankton in the world’s oceans and are significant contributors to global primary productivity.
Grossman and his colleagues aimed to understand how Synechococcus thrives in iron-poor waters that cover most of the oceans, while normal photosynthesis requires high levels of iron. Some researchers believe that oxygen plays a potential role in receiving electrons from photosynthetic machinery as a substitute for carbon dioxide. However, Grossman’s research team demonstrated that this process is meaningful when conducted in nutrient-poor marine regions that account for nearly half of the ocean’s surface.
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The sausage-shaped cells are unicellular Synechococcus bacteria, while the filaments are actually sulfur-free green bacteria. (Photo: Richard W. Castenholz, University of Oregon) |
Grossman stated: “To some extent, it appears that Synechococcus bacteria in nutrient-poor oceans have addressed the iron issue by bypassing the typical photosynthesis process. They have skipped steps involving iron in photosynthesis, which are also the steps where carbon dioxide is absorbed from the atmosphere.”
Shaun Bailey, a postdoctoral researcher collaborating on the project, mentioned: “We quickly recognized the differences in Synechococcus bacteria. The absorption of carbon dioxide and photosynthesis in this bacterium do not align. Therefore, we knew there was something other than carbon dioxide being used in the photosynthesis process. And indeed, it was oxygen.” The researchers identified an enzyme involved in this process called PTOX (plastoquinol terminal oxidase). They emphasized that this new photosynthesis process needs to be considered in understanding the primary productivity of marine ecosystems.
In standard photosynthesis, light energy breaks down water molecules, releasing oxygen and providing electrons used to fix carbon dioxide taken from the atmosphere, producing energy-rich molecules such as sugars. In the newly discovered process, a large portion of these electrons is not used to fix carbon dioxide; instead, they bind water molecules together, resulting in less oxygen produced during photosynthesis.
Bailey stated: “It seems that these organisms are conducting an inefficient cycle of turning water into water under the influence of light. However, that is not entirely accurate because this unusual cycle is also a way to use light to generate energy while protecting the photosynthetic machinery from damage that can be caused by light absorption.”
Generating energy from this light-driven cycle of turning water into water plays a crucial role as cyanobacteria use this energy to obtain scarce nutrients in their environment. This novel phenomenon has been demonstrated by graduate student Kate Mackey, who conducted direct photosynthesis studies on samples taken from the Pacific and Atlantic Oceans.
Mackey stated: “Nutrient-poor, iron-poor environments cover about half of the Earth’s oceans. This indicates that a large portion of the Earth’s surface is utilized for photosynthesis. Our discovery shows that this unusual cycle occurs in both major ocean basins; it also indicates that essential energy derived from sunlight does not proceed simultaneously with the fixation of carbon dioxide in the photosynthesis process. This means that photosynthetic organisms in the ocean absorb less carbon dioxide from the atmosphere than we previously thought.”
Joe Berry from the Global Ecology Department at the Carnegie Institution remarked: “This discovery changes our understanding of the photosynthesis process in organisms living in vast but nutrient-poor marine environments. We have always assumed that, like higher plants, the goal of photosynthesis is to produce carbohydrates from carbon dioxide and store them for later use as an energy source for various cellular functions or for growth. But now we know that some organisms have bypassed this complex process. They use light minimally to power cellular cycles through a simpler, more economical photosynthetic machinery in nutrient-deficient environments like those lacking iron. We do not yet fully understand the implications of this phenomenon, but it will definitely change how we comprehend the optical framework of photosynthetic pigments in the ocean and how we model ocean productivity.”
Wolf Frommer, director of the Plant Biology Department at the Carnegie Institution, endorsed the transformative significance of the discovery. “If we think we have understood the process of photosynthesis, this research shows that there is still much we need to learn about these fundamental physiological cycles. The research conducted by Grossman’s lab, along with earlier evidence provided by Greg Vanlerberghe from the University of Toronto, has demonstrated that the gene encoding the PTOX enzyme appears to be widely distributed in marine cyanobacteria. It will provide a solid foundation for building models of primary productivity in the ocean.”
The authors of the study include: Shaun Bailey, Anastasios Melis, Katherine RM Mackey, Pierre Cardol, Giovanni Finazzi, Gert van Dijken, Gry M Berg, Kevin R Arrigo, Jeff Shrager, Arthur R Grossman.
