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      in Environment

      Predator interactions chiefly determine where Prochlorococcus thrive

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      Prochlorococcus are the smallest and most abundant photosynthesizing organisms on the planet. A single Prochlorococcus cell is dwarfed by a human red blood cell, yet globally the microbes number in the octillions and are responsible for a large fraction of the world’s oxygen production as they turn sunlight into energy.

      Prochlorococcus can be found in the ocean’s warm surface waters, and their population drops off dramatically in regions closer to the poles. Scientists have assumed that, as with many marine species, Prochlorococcus’ range is set by temperature: The colder the waters, the less likely the microbes are to live there.

      But MIT scientists have found that where the microbe lives is not determined primarily by temperature. While Prochlorococcus populations do drop off in colder waters, it’s a relationship with a shared predator, and not temperature, that sets the microbe’s range. These findings, published today in the Proceedings of the National Academy of Sciences, could help scientists predict how the microbes’ populations will shift with climate change.

      “People assume that if the ocean warms up, Prochlorococcus will move poleward. And that may be true, but not for the reason they’re predicting,” says study co-author Stephanie Dutkiewicz, senior research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS). “So, temperature is a bit of a red herring.”

      Dutkiewicz’s co-authors on the study are lead author and EAPS Research Scientist Christopher Follett, EAPS Professor Mick Follows, François Ribalet and Virginia Armbrust of the University of Washington, and Emily Zakem and David Caron of the University of Southern California at Los Angeles.

      Temperature’s collapse

      While temperature is thought to set the range of Prochloroccus and other phytoplankton in the ocean, Follett, Dutkiewicz, and their colleagues noticed a curious dissonance in data.

      The team examined observations from several research cruises that sailed through the northeast Pacific Ocean in 2003, 2016, and 2017. Each vessel traversed different latitudes, sampling waters continuously and measuring concentrations of various species of bacteria and phytoplankton, including Prochlorococcus. 

      The MIT team used the publicly archived cruise data to map out the locations where Prochlorococcus noticeably decreased or collapsed, along with each location’s ocean temperature. Surprisingly, they found that Prochlorococcus’ collapse occurred in regions of widely varying temperatures, ranging from around 13 to 18 degrees Celsius. Curiously, the upper end of this range has been shown in lab experiments to be suitable conditions for Prochlorococcus to grow and thrive.

      “Temperature itself was not able to explain where we saw these drop-offs,” Follett says.

      Follett was also working out an alternate idea related to Prochlorococcus and nutrient supply. As a byproduct of its photosynthesis, the microbe produces carbohydrate — an essential nutrient for heterotrophic bacteria, which are single-celled organisms that do not photosynthesize but live off the organic matter produced by phytoplankton.

      “Somewhere along the way, I wondered, what would happen if this food source Prochlorococcus was producing increased? What if we took that knob and spun it?” Follett says.

      In other words, how would the balance of Prochlorococcus and bacteria shift if the bacteria’s food increased as a result of, say, an increase in other carbohydrate-producing phytoplankton? The team also wondered: If the bacteria in question were about the same size as Prochlorococcus, the two would likely share a common grazer, or predator. How would the grazer’s population also shift with a change in carbohydrate supply?

      “Then we went to the whiteboard and started writing down equations and solving them for various cases, and realized that as soon as you reach an environment where other species add carbohydrates to the mix, bacteria and grazers grow up and annihilate Prochlorococcus,” Dutkiewicz says.

      Nutrient shift

      To test this idea, the researchers employed simulations of ocean circulation and marine ecosystem interactions. The team ran the MITgcm, a general circulation model that simulates, in this case, the ocean currents and regions of upwelling waters around the world. They overlaid a biogeochemistry model that simulates how nutrients are redistributed in the ocean. To all of this, they linked a complex ecosystem model that simulates the interactions between many different species of bacteria and phytoplankton, including Prochlorococcus.

      When they ran the simulations without incorporating a representation of bacteria, they found that Prochlorococcus persisted all the way to the poles, contrary to theory and observations. When they added in the equations outlining the relationship between the microbe, bacteria, and a shared predator, Prochlorococcus’ range shifted away from the poles, matching the observations of the original research cruises.

      In particular, the team observed that Prochlorococcus thrived in waters with very low nutrient levels, and where it is the dominant source of food for bacteria. These waters also happen to be warm, and Prochlorococcus and bacteria live in balance, along with their shared predator. But in more nutrient-rich enviroments, such as polar regions, where cold water and nutrients are upwelled from the deep ocean, many more species of phytoplankton can thrive. Bacteria can then feast and grow on more food sources, and in turn feed and grow more of its shared predator. Prochlorococcus, unable to keep up, is quickly decimated. 

      The results show that a relationship with a shared predator, and not temperature, sets Prochlorococcus’ range. Incorporating this mechanism into models will be crucial in predicting how the microbe — and possibly other marine species — will shift with climate change.

      “Prochlorococcus is a big harbinger of changes in the global ocean,” Dutkiewicz says. “If its range expands, that’s a canary — a sign that things have changed in the ocean by a great deal.”

      “There are reasons to believe its range will expand with a warming world,” Follett adds.” But we have to understand the physical mechanisms that set these ranges. And predictions just based on temperature will not be correct.” More

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      in Environment

      SMART researchers develop method for early detection of bacterial infection in crops

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      Researchers from the Disruptive and Sustainable Technologies for Agricultural Precision (DiSTAP) Interdisciplinary Research Group (IRG) ofSingapore-MIT Alliance for Research and Technology (SMART), MIT’s research enterprise in Singapore, and their local collaborators from Temasek Life Sciences Laboratory (TLL), have developed a rapid Raman spectroscopy-based method for detecting and quantifying early bacterial infection in crops. The Raman spectral biomarkers and diagnostic algorithm enable the noninvasive and early diagnosis of bacterial infections in crop plants, which can be critical for the progress of plant disease management and agricultural productivity.

      Due to the increasing demand for global food supply and security, there is a growing need to improve agricultural production systems and increase crop productivity. Globally, bacterial pathogen infection in crop plants is one of the major contributors to agricultural yield losses. Climate change also adds to the problem by accelerating the spread of plant diseases. Hence, developing methods for rapid and early detection of pathogen-infected crops is important to improve plant disease management and reduce crop loss.

      The breakthrough by SMART and TLL researchers offers a faster and more accurate method to detect bacterial infection in crop plants at an earlier stage, as compared to existing techniques. The new results appear in a paper titled “Rapid detection and quantification of plant innate immunity response using Raman spectroscopy” published in the journal Frontiers in Plant Science.

      “The early detection of pathogen-infected crop plants is a significant step to improve plant disease management,” says Chua Nam Hai, DiSTAP co-lead principal investigator, professor, TLL deputy chair, and co-corresponding author. “It will allow the fast and selective removal of pathogen load and curb the further spread of disease to other neighboring crops.”

      Traditionally, plant disease diagnosis involves a simple visual inspection of plants for disease symptoms and severity. “Visual inspection methods are often ineffective, as disease symptoms usually manifest only at relatively later stages of infection, when the pathogen load is already high and reparative measures are limited. Hence, new methods are required for rapid and early detection of bacterial infection. The idea would be akin to having medical tests to identify human diseases at an early stage, instead of waiting for visual symptoms to show, so that early intervention or treatment can be applied,” says MIT Professor Rajeev Ram, who is a DiSTAP principal investigator and co-corresponding author on the paper.

      While existing techniques, such as current molecular detection methods, can detect bacterial infection in plants, they are often limited in their use. Molecular detection methods largely depend on the availability of pathogen-specific gene sequences or antibodies to identify bacterial infection in crops; the implementation is also time-consuming and nonadaptable for on-site field application due to the high cost and bulky equipment required, making it impractical for use in agricultural farms.

      “At DiSTAP, we have developed a quantitative Raman spectroscopy-based algorithm that can help farmers to identify bacterial infection rapidly. The developed diagnostic algorithm makes use of Raman spectral biomarkers and can be easily implemented in cloud-based computing and prediction platforms. It is more effective than existing techniques as it enables accurate identification and early detection of bacterial infection, both of which are crucial to saving crop plants that would otherwise be destroyed,” explains Gajendra Pratap Singh, scientific director and principal investigator at DiSTAP and co-lead author.

      A portable Raman system can be used on farms and provides farmers with an accurate and simple yes-or-no response when used to test for the presence of bacterial infections in crops. The development of this rapid and noninvasive method could improve plant disease management and have a transformative impact on agricultural farms by efficiently reducing agricultural yield loss and increasing productivity.

      “Using the diagnostic algorithm method, we experimented on several edible plants such as choy sum,” says DiSTAP and TLL principal investigator and co-corresponding author Rajani Sarojam. “The results showed that the Raman spectroscopy-based method can swiftly detect and quantify innate immunity response in plants infected with bacterial pathogens. We believe that this technology will be beneficial for agricultural farms to increase their productivity by reducing their yield loss due to plant diseases.”

      The researchers are currently working on the development of high-throughput, custom-made portable or hand-held Raman spectrometers that will allow Raman spectral analysis to be quickly and easily performed on field-grown crops.

      SMART and TLL developed and discovered the diagnostic algorithm and Raman spectral biomarkers. TLL also confirmed and validated the detection method through mutant plants. The research is carried out by SMART and supported by the National Research Foundation of Singapore under its Campus for Research Excellence And Technological Enterprise (CREATE) program.

      SMART was established by MIT and the NRF in 2007. The first entity in CREATE developed by NRF, SMART serves as an intellectual and innovation hub for research interactions between MIT and Singapore, undertaking cutting-edge research projects in areas of interest to both Singapore and MIT. SMART currently comprises an Innovation Center and five IRGs: Antimicrobial Resistance, Critical Analytics for Manufacturing Personalized-Medicine, DiSTAP, Future Urban Mobility, and Low Energy Electronic Systems. SMART research is funded by the NRF under the CREATE program.

      Led by Professor Michael Strano of MIT and Professor Chua Nam Hai of Temasek Lifesciences Laboratory, the DiSTAP program addresses deep problems in food production in Singapore and the world by developing a suite of impactful and novel analytical, genetic, and biomaterial technologies. The goal is to fundamentally change how plant biosynthetic pathways are discovered, monitored, engineered, and ultimately translated to meet the global demand for food and nutrients. Scientists from MIT, TTL, Nanyang Technological University, and National University of Singapore are collaboratively developing new tools for the continuous measurement of important plant metabolites and hormones for novel discovery, deeper understanding and control of plant biosynthetic pathways in ways not yet possible, especially in the context of green leafy vegetables; leveraging these new techniques to engineer plants with highly desirable properties for global food security, including high-yield density production, and drought and pathogen resistance; and applying these technologies to improve urban farming. More

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      in Environment

      Zeroing in on the origins of Earth’s “single most important evolutionary innovation”

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      Some time in Earth’s early history, the planet took a turn toward habitability when a group of enterprising microbes known as cyanobacteria evolved oxygenic photosynthesis — the ability to turn light and water into energy, releasing oxygen in the process.

      This evolutionary moment made it possible for oxygen to eventually accumulate in the atmosphere and oceans, setting off a domino effect of diversification and shaping the uniquely habitable planet we know today.  

      Now, MIT scientists have a precise estimate for when cyanobacteria, and oxygenic photosynthesis, first originated. Their results appear today in the Proceedings of the Royal Society B.

      They developed a new gene-analyzing technique that shows that all the species of cyanobacteria living today can be traced back to a common ancestor that evolved around 2.9 billion years ago. They also found that the ancestors of cyanobacteria branched off from other bacteria around 3.4 billion years ago, with oxygenic photosynthesis likely evolving during the intervening half-billion years, during the Archean Eon.

      Interestingly, this estimate places the appearance of oxygenic photosynthesis at least 400 million years before the Great Oxidation Event, a period in which the Earth’s atmosphere and oceans first experienced a rise in oxygen. This suggests that cyanobacteria may have evolved the ability to produce oxygen early on, but that it took a while for this oxygen to really take hold in the environment.

      “In evolution, things always start small,” says lead author Greg Fournier, associate professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “Even though there’s evidence for early oxygenic photosynthesis — which is the single most important and really amazing evolutionary innovation on Earth — it still took hundreds of millions of years for it to take off.”

      Fournier’s MIT co-authors include Kelsey Moore, Luiz Thiberio Rangel, Jack Payette, Lily Momper, and Tanja Bosak.

      Slow fuse, or wildfire?

      Estimates for the origin of oxygenic photosynthesis vary widely, along with the methods to trace its evolution.

      For instance, scientists can use geochemical tools to look for traces of oxidized elements in ancient rocks. These methods have found hints that oxygen was present as early as 3.5 billion years ago — a sign that oxygenic photosynthesis may have been the source, although other sources are also possible.

      Researchers have also used molecular clock dating, which uses the genetic sequences of microbes today to trace back changes in genes through evolutionary history. Based on these sequences, researchers then use models to estimate the rate at which genetic changes occur, to trace when groups of organisms first evolved. But molecular clock dating is limited by the quality of ancient fossils, and the chosen rate model, which can produce different age estimates, depending on the rate that is assumed.

      Fournier says different age estimates can imply conflicting evolutionary narratives. For instance, some analyses suggest oxygenic photosynthesis evolved very early on and progressed “like a slow fuse,” while others indicate it appeared much later and then “took off like wildfire” to trigger the Great Oxidation Event and the accumulation of oxygen in the biosphere.

      “In order for us to understand the history of habitability on Earth, it’s important for us to distinguish between these hypotheses,” he says.

      Horizontal genes

      To precisely date the origin of cyanobacteria and oxygenic photosynthesis, Fournier and his colleagues paired molecular clock dating with horizontal gene transfer — an independent method that doesn’t rely entirely on fossils or rate assumptions.

      Normally, an organism inherits a gene “vertically,” when it is passed down from the organism’s parent. In rare instances, a gene can also jump from one species to another, distantly related species. For instance, one cell may eat another, and in the process incorporate some new genes into its genome.

      When such a horizontal gene transfer history is found, it’s clear that the group of organisms that acquired the gene is evolutionarily younger than the group from which the gene originated. Fournier reasoned that such instances could be used to determine the relative ages between certain bacterial groups. The ages for these groups could then be compared with the ages that various molecular clock models predict. The model that comes closest would likely be the most accurate, and could then be used to precisely estimate the age of other bacterial species — specifically, cyanobacteria.

      Following this reasoning, the team looked for instances of horizontal gene transfer across the genomes of thousands of bacterial species, including cyanobacteria. They also used new cultures of modern cyanobacteria taken by Bosak and Moore, to more precisely use fossil cyanobacteria as calibrations. In the end, they identified 34 clear instances of horizontal gene transfer. They then found that one out of six molecular clock models consistently matched the relative ages identified in the team’s horizontal gene transfer analysis.

      Fournier ran this model to estimate the age of the “crown” group of cyanobacteria, which encompasses all the species living today and known to exhibit oxygenic photosynthesis. They found that, during the Archean eon, the crown group originated around 2.9 billion years ago, while cyanobacteria as a whole branched off from other bacteria around 3.4 billion years ago. This strongly suggests that oxygenic photosynthesis was already happening 500 million years before the Great Oxidation Event (GOE), and that cyanobacteria were producing oxygen for quite a long time before it accumulated in the atmosphere.

      The analysis also revealed that, shortly before the GOE, around 2.4 billion years ago, cyanobacteria experienced a burst of diversification. This implies that a rapid expansion of cyanobacteria may have tipped the Earth into the GOE and launched oxygen into the atmosphere.

      Fournier plans to apply horizontal gene transfer beyond cyanobacteria to pin down the origins of other elusive species.

      “This work shows that molecular clocks incorporating horizontal gene transfers (HGTs) promise to reliably provide the ages of groups across the entire tree of life, even for ancient microbes that have left no fossil record … something that was previously impossible,” Fournier says. 

      This research was supported, in part, by the Simons Foundation and the National Science Foundation. More