Why I'm Going...
Peter Rejcek, the Antarctic Sun editor, sent me an e-mail today asking about our research project, my role in it, implications for understanding early life, and relevance to possible life on other planets. I thought you might like to hear my answers as well. Here's an explanation of what I'm up to:
My main research questions focus on interpreting the early evolution of life, mostly by characterizing the morphology of fossil microbial communities. I am particularly interested in intricate microbial structures that are preserved in some rocks that are 2.5 to 3 billion years old (fenestrate microbialites); they were abundant until just before O2 accumulated in the atmosphere, so these structures might be able to answer some important questions about the evolution of cyanobacteria. There are examples of similar structures in younger rocks as well as in some modern environments. In the modern environments, these structures (reticulates) are formed by motile filamentous cyanobacteria. Thus, I am working with various other scientists to see how variations in the modern microbial structures reflect biological and environmental conditions with the goal of developing models for growth of the ancient structures. Lake Joyce plays a very important role in these studies. Most other examples of these structures are found in very shallow water hot springs and in temperate lakes. We think similar structures are also growing in the very different environmental conditions under ice in a number of Dry Valley lakes, and we want to use this very different environment to help sort out biological influences on morphology from environmental influences. We chose Lake Joyce over other Dry Valley lakes because some of the Lake Joyce microbial structures might be calcifying, which will lead to additional insights into how the fossilization process influences morphology.
There are two very important aspects of using Lake Joyce microbialites and other similar modern examples to interpret ancient structures. First, the cautionary note. The modern examples we've identified so far are all dominated by motile, filamentous cyanobacteria. Our results so far suggest, however, that it is the motility and filamentous shape of the bacteria and not the fact that they are photosynthetic that makes these structures. Specifically, we have yet to make any observations that require the photosynthetic activity of the bacteria to make the structures of interest. Thus, although I am very interested in constraining the origin of cyanobacteria, we need to be very careful to identify the actual mechanisms that create the mat morphology. It is possible that light-limited environments will provide new insights into this distinction, but we have to understand the Lake Joyce microbial behaviors very well before we can extrapolate too much to the rock record.
Second, the good news. The structures we are looking at have very distinctive morphological properties. If they do form only when elongate microbes move parallel to their elongation (our current hypothesis), these structures might form when any elongate cells or groups of cells behave this way. Thus, one might expect these structures from any microbial communities on any planet if cells live as filaments. (A filamentous cell shape has evolutionary advantages in water because filaments are less subject to brownian motion and are more stable in some turbulent flows than spherical cells or aggregates. Thus, one might expect evolution to produce this simple shape even if the biochemistry of the organisms was entirely different.) The morphological properties we're looking at are centimeter-scale and easily preserved in rocks; if present, they could be easily identified in images from a lander or rover. Thus, these are easily observable structures that might indicate the presence of a filamentous microbial community on another planet.
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My specific role in the Lake Joyce project is to help analyze images to constrain the morphology of microbial structures to help direct sampling and in situ measurements of geochemical gradients. Also, I'll perform experiments to characterize how the bacteria move, looking for correlations between biological behaviors and morphology. I have a lot of expertise in microbial-mineral interactions and looking for signatures of these interactions in rocks, and I'll be looking at the locations of calcite precipitation relative to microbial activity and prepare samples for isotopic analyses.
My main research questions focus on interpreting the early evolution of life, mostly by characterizing the morphology of fossil microbial communities. I am particularly interested in intricate microbial structures that are preserved in some rocks that are 2.5 to 3 billion years old (fenestrate microbialites); they were abundant until just before O2 accumulated in the atmosphere, so these structures might be able to answer some important questions about the evolution of cyanobacteria. There are examples of similar structures in younger rocks as well as in some modern environments. In the modern environments, these structures (reticulates) are formed by motile filamentous cyanobacteria. Thus, I am working with various other scientists to see how variations in the modern microbial structures reflect biological and environmental conditions with the goal of developing models for growth of the ancient structures. Lake Joyce plays a very important role in these studies. Most other examples of these structures are found in very shallow water hot springs and in temperate lakes. We think similar structures are also growing in the very different environmental conditions under ice in a number of Dry Valley lakes, and we want to use this very different environment to help sort out biological influences on morphology from environmental influences. We chose Lake Joyce over other Dry Valley lakes because some of the Lake Joyce microbial structures might be calcifying, which will lead to additional insights into how the fossilization process influences morphology.
There are two very important aspects of using Lake Joyce microbialites and other similar modern examples to interpret ancient structures. First, the cautionary note. The modern examples we've identified so far are all dominated by motile, filamentous cyanobacteria. Our results so far suggest, however, that it is the motility and filamentous shape of the bacteria and not the fact that they are photosynthetic that makes these structures. Specifically, we have yet to make any observations that require the photosynthetic activity of the bacteria to make the structures of interest. Thus, although I am very interested in constraining the origin of cyanobacteria, we need to be very careful to identify the actual mechanisms that create the mat morphology. It is possible that light-limited environments will provide new insights into this distinction, but we have to understand the Lake Joyce microbial behaviors very well before we can extrapolate too much to the rock record.
Second, the good news. The structures we are looking at have very distinctive morphological properties. If they do form only when elongate microbes move parallel to their elongation (our current hypothesis), these structures might form when any elongate cells or groups of cells behave this way. Thus, one might expect these structures from any microbial communities on any planet if cells live as filaments. (A filamentous cell shape has evolutionary advantages in water because filaments are less subject to brownian motion and are more stable in some turbulent flows than spherical cells or aggregates. Thus, one might expect evolution to produce this simple shape even if the biochemistry of the organisms was entirely different.) The morphological properties we're looking at are centimeter-scale and easily preserved in rocks; if present, they could be easily identified in images from a lander or rover. Thus, these are easily observable structures that might indicate the presence of a filamentous microbial community on another planet.
The top left and bottom two images are rocks that are 2.5 billion years old with the darker areas representing fossil microbial communities. The top left and bottom images show the bottom of the microbial layers and the middle image shows the rock from bottom to top. The image on the upper right shows modern cyanobacteria that migrated into these reticulate shapes in about 13 hours in the lab. The patterns in the experiments and rocks are very similar. We'll be doing similar experiments with bacteria from Lake Joyce to see if they make these types of patterns.
This image shows reticulate mats from Pavilion Lake, which shows a larger view of what we think the ancient structures looked like while they were growing. It was taken by Bekah Shepard.
*****
My specific role in the Lake Joyce project is to help analyze images to constrain the morphology of microbial structures to help direct sampling and in situ measurements of geochemical gradients. Also, I'll perform experiments to characterize how the bacteria move, looking for correlations between biological behaviors and morphology. I have a lot of expertise in microbial-mineral interactions and looking for signatures of these interactions in rocks, and I'll be looking at the locations of calcite precipitation relative to microbial activity and prepare samples for isotopic analyses.
