Geobiology: September 2011

The Institute for Astronomy (IfA) invites applications for a Postdoctoral Fellowship with interests in the origin of Earth's water to work with the University of Hawai'i's NASA Astrobiology Institute lead team (see The UH lead team maintains an innovative and multi-disciplinary research environment linking astronomical, biological, microbiological, chemical, and geological sciences to investigate the origin, history, distribution and role of water as it relates to life in the universe. The program centers around interactions with an interdisciplinary group of postdoctoral fellows. We have a particular need for an individual interested in the origin of Earth's water, and, by analogy, terrestrial planetary volatiles. The work involves geological field work to sample primitive, deep-mantle-plume materials, preparation of samples of melt inclusions in olivines from Hawaiian and Icelandic basalts for isotopic measurements using the petrographic microscope, scanning electron microscope, and electron microprobe, and measurements of D/H ratios and hydrogen abundances in the melt inclusions using the UH Cameca ims 1280 ion microprobe. The Fellowship is for one year and may be renewable up to a total of 3 years assuming satisfactory progress and continued availability of funds. The fellow will receive a stipend of approximately $5,000 per month, a small relocation allowance and basic research costs.

Fossils are essential to our understanding of the history and origins of complex life. New work from NAI's MIT and Penn State teams describes exquisitely preserved microfossils from mid-Neoproterozoic (811-717 million years old) rocks of the Fifteenmile Group, Yukon. These fossils are interpreted as biomineralized plates that covered the surface of a single-celled alga.

Their findings suggest that the minerals used by the ancient marine organisms have changed through time, perhaps linked to changing ocean chemistry. While the relationship of these fossils to modern organisms is difficult to determine, the researchers argue that it's likely that these unique fossils are the plates of an organism most closely related to green algae. Their paper appears online in Geology.

A New Look at the Sulfur Cycle

The record of Earth's sulfur cycle preserved in sedimentary rocks is commonly used to track the evolution of microbial sulfur metabolisms and levels of atmospheric oxygen throughout geologic history. Sulfur isotope evidence suggests the Earth's atmospheric oxygen appeared about 2.4 billion years ago, but its level remained rather low until about 650 million years ago.

New studies by NASA Astrobiology Institute scientists have questioned the extent to which the record of the sulfur cycle reflects the oxygenation. The team has demonstrated that a laboratory culture of a marine sulfate-reducing bacterium can produce sulfur isotope signatures beyond the threshold previously used to define the boundaries for different sulfur metabolisms. This finding suggests that oxygenation is not the only mechanism that can explain similar signatures in modern and ancient sediments. The team's paper was published in the July 1 issue of Science.

Cycling Nitrogen in Ancient Oceans

In recent years, scientists have found evidence that a 'near complete' biological nitrogen cycle existed in the oceans during the late Archean to early Proterozoic (from 2.5 to 2 billion years ago). Modern bacteria use an enzyme called nitrogenase to cycle nitrogen from one form to another. This enzyme is dependent on the presence of metallic elements like iron (Fe), vanadium (V) and, most often, molybdenum (Mo). However, ancient oceans didn't contain much molybdenum. Could Fe-nitrogenase or V-nitrogenase have played a larger role in the archaean oceans than they do today?

To answer this question, a team of researchers at NAI's Montana State University and Arizona State University teams studied the phylogenetic relationships between the proteins that allow nitrogenase to interact with each of the three elements. Their results suggest that the protein (known as Nif protein) actually developed in methanogenic microorganisms, and was then incorporated into bacteria by lateral gene transfer around 1.5-2.2 billion years ago.

Ultimately, if Mo-nitrogenase originated under anoxic conditions in the Archaean, it would have likely happened in an environment where both methanogens and bacteria coexisted, and where molybdenum was present for at least part of the time.

The emergence of enzymes like Mo-nitrogenase was a significant step in the evolution of life, and had powerful repercussions for planet Earth and its biosphere as a whole. This research can help answer important questions about the environmental conditions that were present on the early Earth, and the interactions that occurred between life and the ancient planet.

The results were published in the May edition of the journal Geobiology