Goal 5: Understand the evolutionary mechanisms and environmental limits of life
Determine the molecular, genetic, and biochemical mechanisms that control and limit evolution, metabolic diversity, and acclimatization of life.
The diversity of life on Earth today is a result of the dynamic interplay between genetic opportunity, metabolic capability and environmental challenges. For most of its existence, our habitable environment has been dominated by microorganisms and subjected to their metabolism and evolution. As a consequence of such microbial activities on a geological time scale, the physical-chemical environment on Earth has been changing, thereby determining the path of evolution of subsequent life. For example, the release of molecular oxygen by cyanobacteria as a by-product of photosynthesis as well as the colonization of the Earth's surface by metazoan life induced fundamental, global changes in the Earth's environment. The altered environment, in turn, posed novel evolutionary challenges to the organisms present, which ultimately resulted in the formation of our planet's major animal and plant species. Therefore this ‘co-evolution' between organisms and their environment is apparently an inherent feature of living systems.
Life survives and sometimes thrives under what seem to be harsh conditions on Earth. For example, some microbes thrive at temperatures of 113°C. Others exist only in highly acidic environments or survive exposures to intense radiation. While all organisms are composed of nearly identical molecules, evolution has enabled such microbes to cope with this wide range of physical and chemical conditions. What are the features that enable one microbe to thrive under extreme conditions that are lethal to many others? An understanding of the tenacity and versatility of life on Earth, as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, will provide a critical foundation for the search for life beyond Earth. These insights will help us to understand the molecular adaptations that define the physical and chemical limits for life on Earth. They will provide a baseline for developing predictions and hypotheses about life on other worlds.
Background
The evolution of biogeochemical processes, genomes and microbial communities has created the complexity and robustness of the modern biosphere. However, we lack a fundamental understanding of how evolutionary forces, such as mutation, selection, and genetic drift, operate in microorganisms that act on and respond to changing microenvironments. We can examine the reciprocal interactions between biosphere and geosphere that can shape genes, genomes, organisms, and species interactions. Accordingly, we will begin to develop an understanding of the evolution of biochemical and metabolic machinery that drives the global cycles of the elements, as well as the potential and limits of such evolution. Furthermore, we must observe their coordination into genetic circuitries, and their integration into more complex biological entities, such as whole cells and microbial communities.
While co-evolution of the Earth's physical-chemical environment and its living world is dynamic and proceeds at all organismic levels, prokaryotic microorganisms have played a critical role in shaping our planet. Microbes can serve as highly advanced experimental systems for biochemical, genetic, and genomic studies. To date, over 100 microbial genomes have been sequenced. This unprecedented wealth of information, together with the experimental tools now available, provides a tremendous opportunity for experimental studies to be conducted on the evolution of microbial genes, genomes, and microbial communities. Such studies will uncover fundamental principles of molecular, cellular and community level evolution with relevance to Earth and other planets. Of specific interest is observing or simulating the evolution of those molecular properties that facilitate the metabolic coupling of the oxidation/reduction cycles of elements and the adaptation to novel environments, especially extreme environments, created by simulated perturbations. Hypothesis-driven experimentation on microbial ecosystems using single species with known genome sequences can be employed to predict environmental changes and evolutionary solutions. Such studies can be extended to defined mixed communities to study the plasticity and adaptation of the "metagenome," comprising the genomes of all members of a microbial community, when subjected to environmental changes and genetic flux. The evolved genotypes and phenotypes should be correlated to the specific changes they induce in the physical-chemical environment.
Our ongoing exploration of the Earth has led to continued discoveries of life in environments that have been previously considered uninhabitable. For example, we find thriving communities in the boiling hot springs of Yellowstone, the frozen deserts of Antarctica, the concentrated sulfuric acid in acid-mine drainages, and the ionizing radiation fields in nuclear reactors. We find some microbes that grow only in brine and require saturated salts to live and we find others that grow in the deepest parts of the oceans and require 500 to 1000 bars of hydrostatic pressure. Life has evolved strategies that allow it to survive even beyond the daunting physical and chemical limits to which it has adapted to grow. To survive, organisms can assume forms that enable them to withstand freezing, complete desiccation, starvation, high-levels of radiation exposure, and other physical or chemical challenges. Furthermore, they can survive exposure to such conditions for weeks, months, years, or even centuries. We need to identify the limits for growth and survival, and to understand the molecular mechanisms that define these limits. Biochemical studies will also reveal inherent features of biomolecules and biopolymers that define the physical-chemical limits of life under extreme conditions. Broadening our knowledge both of the range of environments on Earth that are inhabitable by microbes and of their adaptation to these habitats will be critical for understanding how life might have established itself and survived in habitats beyond Earth.
Objective 5.1
Environment-dependent, molecular evolution
in microorganisms
Experimentally investigate and observe the evolution of genes, metabolic pathways, genomes, and microbial species. Experimentally investigate the forces and mechanisms that shape the structure, organization, and plasticity of microbial genomes. Examine how these forces control the genotype-to-phenotype relationship. Conduct environmental perturbation experiments on single microbial species to observe and quantify adaptive evolution to astrobiologically relevant environments.
Example investigations
- Experimentally observe the assembly of genes
into novel metabolic pathways as an adaptive response to environmental
changes.
- Examine microbial genome rearrangements,
including gene deletion and acquisition processes, in response to
nutrient change and physical-chemical stress.
- Investigate the diversity of genome stability in physiologically and genomically different microbes.
Objective
5.2
Co-evolution of microbial communities
Experimentally examine the metabolic and genetic interactions in microbial communities that have determined major geochemical processes and changes on Earth. Investigate how these interactions shape the evolution and maintenance of metabolic diversity in microbial communities. Investigate how novel microbial species establish and adapt into existing communities.
Example investigations
- Investigate small molecule interactions and
their role in coordinating metabolic activities in mixed phototrophic/chemotrophic
microbial communities.
- Examine adaptive mutations
in individual microbial species of mixed communities in response to
environmental perturbations.
- Examine the susceptibility
of established microbial communities to invasion by foreign microbes.
Objective
5.3
Biochemical adaptation to extreme environments
Document life that survives or thrives under the most extreme conditions on Earth. Characterize and elucidate the biochemical capabilities that define the limits for cellular life. Explore the biochemical and evolutionary strategies that push the physical-chemical limits of life by reinforcing, replacing, or repairing critical biomolecules (e.g., spore formation, resting stages, protein replacement rates, or DNA repair). Characterize the structure and metabolic diversity of microbial communities in such extreme environments.
Example investigations
- Investigate the intrinsic properties and stability
of critical biomolecules that allow microorganisms to survive severe
freezing and thawing cycles.
- Biochemically characterize DNA-repair mechanisms
that allow microorganisms to recover from radiation damage.
- Study survival strategies that might allow microbes to maintain their viability for very long periods of time (thousands to millions of years).