Geobiology Research at RSES

Planet Earth and the biosphere have evolved together for more than 3.5 billion years. Geobiology is the science that studies this co-evolution of life and environment in Earth's distant past and in present ecosystems. Local geobiological processes at present determine e.g. Whether estuaries and water reservoirs remain stable or whether they will drift into oxygen starvation. On a global scale, the interplay between life and the environment drives our planet's climate e.g. through generation and removal of greenhouse gases. A changing atmosphere and climate, in turn, may lead to the loss of biodiversity and a shift in ecosystems. Humans cause major perturbations in these subtle ecological balances, which impacts the environment in significant negative ways. If we want to learn how the Earth-Life system reacts to human perturbation we have to understand how geobiological networks responded to disturbances in the past; we have to learn how to read Earth's laboratory notebook. A chronicle of Earth's past ecosystems and their geochemical cycles is recorded in fresh sediments and sedimentary rocks millions to billions of years old. At RSES, we read these subtle microbial and environmental signatures using molecular fossils (biomarkers), elemental abundances and isotopic ratios in sedimentary rocks, corals and shells of planktonic organisms.

The following staff members are involved in Geobiology research at RSES.				Other Links:
	Dr. Jochen Brocks	Web Page	Email	Student Research Projects in Geobiology
	Mr Richard Schinteie		Email
	Ms Janet hope		Email

Current Research projects

Lake Tyrrell

The biogeochemistry of a Salt Lake

Lake Tyrrell is a large salt lake in outback Australia . We aim to make the lake one of the most completely understood hypersaline ecosystems in the world. To achieve this, we are combining environmental genomics where the genome of all predominant microorganism in the lake water are sequenced (>1 billion base pairs) with biomarker geochemistry where we will quantify the entirety of all lipids and pigments. Once the biological origins of lipids and pigments are known by assigning them to genes of individual organism, we will trace them back in time more than 100,000 years by studying the fossil remains of the pigments in the 6 meters of mud that have accumulated in the lake bed. The 1 billion base pairs of environmental genome will give a very detailed view of how the microbial community in Lake Tyrrell is constructed and how the ecosystem works; from the molecular fossil record we will learn how microbial communities evolved through time through changes of climate and salinity.

Lake Tyrrell is a large salt lake in outback Victoria . It was chosen as our study site as it is the hydrologically best understood salt lake in Australia and contains a well preserved sedimentary record of the past ~120,000 years. In winter, the lake contains ~50 cm water, but in summer, the water evaporates, leaving a salt crust up to 7 cm thick. However, around 120,000 years ago, Tyrrell was a ~13 meter deep freshwater (?) lake. During subsequent climate changes, the water level dropped dramatically and the lake went through cycles of drying and partial refilling. The history of Lake Tyrrell is representative of the region. It may hold valuable information about climatic changes in the southern hemisphere and the desiccation of the Australian continent.

Were oceans in Earth's middle age anoxic, sulfidic and toxic?

Artist impresion of the Earth' early oceans

Today, Earth's oceans are teaming with life, and even deep marine trenches contain enough oxygen to support complex organisms. However, oceans in Earth's distant past were fundamentally different. In the first half of Earth history, ~4.5 to 2.3 billion years (Ga) ago, the world's oceans and atmosphere were almost entirely devoid of oxygen. Surprisingly, for the interval between 1.8 to 0.8 Ga, the state of the oceans remains mysterious. According to one view, the oceans were essentially oxygenated. However, a recent hypothesis suggests that this scenario might not be correct and that the oceans remained widely oxygen-deficient and partly anoxic and sulphidic. If the oceans really were anoxic and sulphidic throughout Earth's middle age (~1.8 – 0.8 Ga), then 20% of the planet's history would have to be rewritten.

In the Biogeochemistry Laboratory at RSES, we test the existence of a mid-Proterozoic sulphidic ocean using hydrocarbon biomarkers. Biomarkers are the molecular fossils of biological lipids. They can be preserved in sedimentary rocks for billions of years and often contain information about ancient microbial ecosystems and environmental conditions. In particular, we are looking for the biomarkers of green and purple sulphur bacteria. Green and purple sulphur bacteria are anoxygenic phototrophs. They require light, oxygen-free conditions and reduced sulphur species such as sulphide and sulphur. Therefore, in planktonic environments, they may only exist where anoxic and sulphidic waters rise into the photic zone of the water column (photic zone euxinia). Sulphidic oceans would have provided perfect growing conditions for these organisms. The mid-Proterozoic may have been “The Age of Phototrophic Sulfur Bacteria”. A search for these biomarkers in 1.6 Ga old rocks from the McArthur Basin in northern Australia yielded some first surprising results (Brocks et al., Nature 2005).

Molecular fossils and early life on Earth

In lakes and oceans, organic matter from dead organisms is usually recycled back into CO 2 and water. However, a small fraction of this dead biomass, particularly pigments and lipids, escapes the remineralization process and accumulates in the bottom sediment. Over millions of years, the lipids will turn into hydrocarbon fossils, or biomarkers. Many biomarkers are extremely stable. Preserved in sedimentary rocks over billions of years, they are witnesses to past ecosystem and environmental conditions.

At RSES, we search for the oldest biomarkers on Earth in Precambrian rocks from Australia and other places around the world. The molecules help us reconstruct ancient microbial ecosystems. For instance, we found that the seas around the north of Australia 1.6 billion years ago were anoxic and sulfidic. The waters were inhabited by green and purple sulfur bacteria. Algae and other forms of complex life did not exist or were rare in this toxic broth.

Biomarkers and Genomics

Biomarkers: Life in the Precambrian was dominated by bacteria and archaea, organisms that rarely leave diagnostic cellular remains in the fossil record. However, hydrocarbon biomarkers, the molecular fossils of natural products such as lipids and pigments, can yield a wealth of information about Precambrian ecosystems. Biomarkers often retain the diagnostic carbon skeleton of their biological precursors and may survive in sedimentary rocks for hundreds of millions of years. Many biomarkers are diagnostic for specific microbial groups such as methanogens, methanotrophs or phototrophic bacteria and, thus, may give information about ancient biodiversity. Therefore, biomarkers can answer outstanding questions about Precambrian ecology and evolution, such as how ecosystems responded to the oxygenation of the atmosphere ~2.4 billion years ago, whether Earth oceans were anoxic and sulfidic during the mid-Proterozoic, or how life responded to massive glaciations in the Neoproterozoic.

The Problem: However, there is a major obstacle that hampers the application of biomarkers as paleoenvironmental proxies: the incomplete knowledge of the lipid biosynthetic capacity of living organisms. According to some estimates, less than 1% of microorganisms can be isolated from the environment and grown in pure culture, and the biomarker content of these uncultivated microbes remains almost always unknown. Detecting and describing the lipids and pigments produced by those 99% of microorganisms that can not yet be cultured would boost the value of biomarkers extracted from ancient rocks.

The Solution? The might be able to assign many biomarkers to organisms in the coming decade by combining lipid research with environmental genomics and microbial community proteomics. Of particular interest will be studies of communities where it is possible to reconstruct nearly complete genomes, proteomes and lipid profiles of dominant microorganisms taken directly from modern environments without cultivation. These genomes will be screened for genes involved in lipid and pigment biosynthesis and matched with corresponding enzymes and lipids detected in the same sample. In addition to defining the diversity of as yet unknown branches of the tree of life, this will contribute to new understanding of the phylogenetic distribution of potential biomarkers produced by microorganisms, even if they evade isolation. The intricate knowledge of the lipid biosynthetic machinery of present ecosystems will then serve to elucidate new biomarkers and biomarker patterns in ancient sedimentary rocks.