For my dissertation, I evaluated the molecular fossil and microfossil record of early eukaryotes using modern organisms as a platform for interpreting the past. I believe that new tools applied to historical geobiology, such as the use of molecular fossils or microchemical techniques, must be understood from an phylogenetic and biological perspective to be useful to geologists. Dissertation research
I characterized biomarkers commonly used as molecular fossils within the phylogenetic context of their source organisms. I focused on the commonly used biomarkers from two distinct biochemical classes, steroid lipids and algaenan biopolymer. I characterized both classes in depth for the green algae. Green algae are a diverse and well constrained monophyletic group with a deep fossil record, making them ideal for this type of study. I cultured all the organisms used in my study, and extracted and characterized their sterols with GC-MS, algaenan with pyrolysis GC - MS, and sequenced their 18s ribosomal RNA genes for phylogenetic analysis. The algaenan work has allowed me to help redefine algaenan as an operational category of lipid-based biopolymer, likely formed at least in part during diagenesis. My work with green algal sterols has helped support green algae as the primary source for C29 sterols and thus the C29 steranes that are abundant in the rock record. In addition, I have profiled the sterol composition of choanoflagellates, a close unicellular sister group of animals. To complement the sterol profile of these protists, I have helped to characterize the key genes in their sterol biosynthetic pathway. Biosynthetic genes reveal the biosynthetic capacity of the organisms that contain them, over and above the expressed products profiled in natural products surveys. Phylogenetic and genomic comparisons of these genes provide an avenue to investigate the evolutionary history of a biosynthetic pathway.
I also have used microchemical techniques (pyrolysis GC-MS and micro-FTIR) to characterize microfossils and their putative modern analogs. Some spheroidal microfossils bear a striking morphological and ultrastructual similarity to the phycomate prasinophytes, the earliest diverging group of green algae. Many have assumed, based on this relationship, that all spheroidal microfossils represent green algae. Phycomate prasinophytes cannot be cultured and field collections are difficult because their geographic distribution is unknown. I was fortunate to find a population in Puget Sound, from the areas surrounding the Friday Harbor Marine Labs, and was able to characterize their cell wall ulturastructure and chemistry. The SEM, TEM, and chemical analysis have supported earlier work from our lab suggesting a greater diversity of organisms producing spheroidal microfossils than previously thought. It has also called into question long standing ideas about microfossil presesrvation. I have proposed a new model for microfossil preservation based on the chemical properties of modern phycomate prasionphytes and microfossils that challenges the existing selective preservation hypothesis.
In the future, I intend to build my research program around the same fundamental principals I used in my dissertation: using the biology of modern organisms to inform our interpretations of the evidence of past life.
Many of the questions I am interested in surround the origin of eukaryotes and understanding modern eukaryotic microbial diversity. The timing and nature of the origin of eukaryotic cells remains elusive. Likewise the full nature of the eukaryotic tree of life remains controversial because many kingdom level clades are poorly known and new kingdom level groups are being discovered with molecular survey techniques. Many of these new clades occur in anoxic environments and may shed light on the presumptive oxygen requirements for eukaryotes and, more broadly, help to provide environmental constraints on eukaryotic rigins and early evolution. I would like to use comparative cell biology, biochemistry, and genomic approaches to study these organisms. Specifically, I will focus on sterol biosynthesis and physiology in these organisms because sterols appear to be one of a few characters found in all eukaryotes. [collaborator: Dr. Scott Dawson, University of California-Davis Department of Microbiology]
I am also interested in functional aspects of sterols in organisms. Sterols are well known as membrane components in eukaryotes and function in membrane stabilization. It is unknown if these molecules have other functions. My dissertation work, characterizing sterol profiles and sterol biosynthesis genes in many microorganisms, shows that some organisms synthesize a great diversity of sterols. Sterol diversity and abundance in particular organisms may be related to aspects of physiology and behavior. Studies of sterol function have been largely limited to animals, which only make a single sterol product and to yeasts which are secondarily unicellular. Building on my dissertation work, I would like to investigate the function of sterols in microorganisms, specifically in the choanoflagellates. Choanoflagellates represent the ideal organisms for this study because the group has unicellular and colonial members, they have a rich sterol profile, they are the unicellular relatives of animals and therefore are evolutionarily interesting, and they have a sequenced genome. [collaborator: Dr. Nicole King, University of California-Berkeley, Department of Molecular and Cellular Biology]
A continuing research interest of mine is the biology and diversity of the phycomate prasinophyte green algae, their evolutionary history, and their relationship to organic walled microfossils. I have developed a research base on these unique phytoplankton at the Friday Harbor Labs over the last two years of my dissertation work. I have recognized and described the one species found in Puget Sound as a new species, Halosphaera dubei, and have begun to characterize its life history, development, and cell biology of this organism. Halosphaera dubei is still unculturable and I intend to continue to experiment with culturing strategies using the population from Puget Sound. This population is currently the only predictable population in which to sample in the world. When in culture, this organism will be a good candidate for genomic research because of its unusual life cycle, cell biology and basal position within green algae. In addition, I am very interested in the geographic distribution of Halosphaera dubei in the Pacific Ocean, as well as the distributions of all phycomate prasinophytes. This group of organisms has received relatively little attention from biologists, considering its phylogenetic and paleontological status. If able, I will collect other genera and species of phycomate prasinophytes for microchemical and ultrasturctural analysis and characterize their global distribution. Another interesting geobiological aspect of prasinophyte green algae is the are the only major group of green algae in the phytoplankton today, though they are believed to be the most abundant phytoplankton in pre-Mesozoic oceans. I am interested in modern interactions between prasinophytes and diatoms and dinoflagellates, the most abundant marine phytoplankton in modern oceans. This modern relationship and its interaction with ocean chemistry or ecosystem dynamics may shed light on the conditions in past oceans that led to the Mesozoic transition in phytoplankton. anywhere…Updated April 2007