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The Geochemical Society presents the 2012 GAST Lecturer
Dr. Rosalyn Rickaby
Department of Earth Sciences, Oxford University (England)
I am fascinated by the jigsaw of complex interactions between the evolution of mineralising organisms, ocean chemistry, atmospheric composition and Earth’s climate. The extraction of chemical signatures from fossil shells of marine micro-organisms as a tool for constraining past ocean conditions and their influence on climate, on timescales ranging from hundreds to millions of years, is fundamental to my research. Seeking innovative alternative approaches to constraining past climates has increasingly directed my research towards an understanding of the physiological response of phytoplankton to the changing carbon cycle in the past and for the future.
The chemistry of the environment is intimately tied to the chemistry of life in a feedback system. The original reductive chemistry of life inexorably drove oxidation of Earth’s surface. The contrast in solubility products between hydroxides and sulfides lead to a predictable sequence of altered environmental concentrations of metals. Some metals increased in availability: notably the latecomers, Cu and Zn, but others decreased e.g. Fe. Such changes are charted in the chemistry of ancient sediments. Signals of the changing metal environment can also be found in the genomes of extant life. A rise in the number of Cu and Zn metal domains in proteins, fundamental to the advancement of life towards complexity and multicellularity, parallels the increasing Cu and Zn in the environment. This implies that the initial rise of oxidative products, and newly available metals, at first toxic to life, could be turned to advantage by leading to the emergence de novo of catalysts allowing increased complexity to evolve. By contrast, a decrease in environmental availability of metals already exploited by enzymes, can drive substitution by new metals to perform old functions, at least in labile enzymes. An example is the change in metallocentre found in “cytochrome-c oxidase”. The original protein was likely based on Fe only but later it became the copper-dependent, O2-reducing enzyme cytochrome oxidase, seen today.
The recent identification of an unusual Cd/Zn carbonic anhydrase from the marine diatom Thalassiosira weissflogii has revealed a biochemical function for Cd, substituting for Zn. This discovery holds the potential to explain the long-standing mystery as to why Cd, regarded as toxic to life, displays a nutrient-like profile in the modern ocean and isotopic (114Cd/110Cd) compositions which indicate removal into phytoplankton. We investigate whether this single known biological role for Cd can account for its apparent nutrient like behaviour by analysing the isotopic fractionation of Cd in the model bacteria, Escherichia coli, which have been transformed to express the CDCA1 gene. Our results demonstrate that whole cells accumulate and fractionate Cd regardless of CDCA1 expression. Our analysis of different cellular components suggests that, rather than direct use of Cd, it is detoxification in response to inadvertent Cd acquisition which is responsible for the fractionation of Cd isotopes. Such detoxification mechanisms are highly conserved across the diversity of life, and we conclude that detoxification acts as the driving force of the observed seawater Cd concentration and isotopic variations. In order to explain the negligible response of Cd uptake to expression of CDCA1, we shall explore if this unusual enzyme is a labile form of the conventional, non-exchanging carbonic anhydrases.