Dissertation Enhancement: Investigating Chloroflexi mats at a Japanese Hot Spring as a Model for Anoxygenic Stromatolites

California Institute Of Technology, Pasadena CA

Investigators

Abstract

Although we usually think of life on Earth in terms of complex multicellular organisms like plants and animals, these have evolved only relatively recently, in the last 600 million years of Earth?s 4.5 billion year history. Earlier, all life on Earth was microbial, made up of organisms like bacteria and algae. Although these organisms look simple they are responsible for some of the most important inventions in biology, including oxygenic photosynthesis?the ability to take water, sunlight, and CO2 and to produce sugar and oxygen. While this process occurs in plants today it was first invented by a group of organisms called Cyanobacteria. The production of oxygen by photosynthetic Cyanobacteria completely changed how the Earth?s surface worked, by producing the ozone layer, providing oxygen for aerobic respiration, and allowing oxygen to play a role in chemical weathering of rocks. This change occurred about 2.3 billion years ago. It isn?t well understood, however, whether Cyanobacteria evolved and invented oxygenic photosynthesis right around this time or much earlier in Earth history. Because individual bacteria don't leave good fossils, one of the best ways of looking for when they evolved is by observing preserved microbial mats in ancient rocks. This award supports doctoral dissertation research by U.S. graduate student Lewis Ward to study microbial mats forming in hot springs in Japan, where similar microbial mats are being formed today by Cyanobacteria and Chloroflexi, a group of bacteria which performs photosynthesis but without releasing oxygen. By studying these two types of mats and seeing how they differ, it is possible to determine which features of microbial mats are preserved in the rock record and can be used to distinguish which group of organisms produced them. By comparing these features to preserved microbial mats throughout the rock record, we can better understand the history of Cyanobacteria, photosynthesis, and oxygen and, by extension, the origins of life on Earth. This research will be conducted in collaboration with Professor Shawn McGlynn, an expert on microbial communities in the environment, at Tokyo Metropolitan University in Japan. Stromatolites are layered, accretionary sedimentary structures often formed by the interaction
of microbes with sediments. With a record dating back 3. 5 billion years, they are among the
 earliest evidence for life on Earth. While similar structures may form in the absence of biology, extensive investigation of modern analogs provides robust criteria for identifying the biogenicity
 of many ancient stromatolites. Known modern stromatolite analogs are formed by oxygenic Cyanobacteria, which leads to interpretation of ancient stromatolites as indicators of the presence of Cyanobacteria performing oxygenic photosynthesis. However, independent geological proxies suggest that Cyanobacteria and oxygen appear relatively late, closer to the rise of oxygen ca. 2. 35 Ga. Because Cyanobacteria evolved from anoxygenic ancestors, it is conceivable that early stromatolites were formed by anoxygenic phototrophs, but we have a poor understanding of whether and how this would 
occur. In order to resolve this conundrum we propose to develop and study a model stromatolite-forming system composed of Chloroflexi--a phylum of facultatively aerobic filamentous bacteria capable of anoxygenic photoheterotrophy. To catalyze this work, this award supports a collaboration with researchers at Tokyo Metropolitan University who are among the foremost experts in the
world at the isolation, cultivation, and physiological study of Chloroflexi. This project will be enhanced by access to natural Chloroflexi mats at a hot spring at the Nakabusa Onsen in Japan. This unique opportunity will allow effective development of a model system for
anoxygenic stromatolites using Chloroflexi in both pure culture and using natural hot spring mats. This system will directly test whether or not stromatolites can form in the absence of photosynthesis (by growing them as aerobic heterotrophs), and if organisms performing anoxygenic phototrophy can form stromatolites.