Abstract The history of research on microbial rhodopsins offers a novel perspective on the history of the molecular life sciences. Events in this history play important roles in the development of fields such as general microbiology, membrane research, bioenergetics, metagenomics and, very recently, neurobiology. New concepts, techniques, methods and fields have arisen as a result of microbial rhodopsin investigations.
In addition, the history of microbial rhodopsins sheds light on the dynamic connections between basic and applied science, and hypothesis-driven and data-driven approaches. The story begins with the late nineteenth century discovery of microorganisms on salted fish and leads into ecological and taxonomical studies of halobacteria in hypersaline environments. These programmes were built on by the discovery of bacteriorhodopsin in organisms that are part of what is now known as the archaeal genus Halobacterium. The transfer of techniques from bacteriorhodopsin studies to the metagenomic discovery of proteorhodopsin in 2000 further extended the field. Microbial rhodopsins have also been used as model systems to understand membrane protein structure and function, and they have become the target of technological applications such as optogenetics and nanotechnology. Analysing the connections between these historical episodes provides a rich example of how science works over longer time periods, especially with regard to the transfer of materials, methods and concepts between different research fields.
Microbial rhodopsins: exemplary membrane proteins For more than a century, research on microbial rhodopsins and the organisms that contain them has shed light on numerous facets of biology. Researchers have made important discoveries of diverse unanticipated organisms and physiological capacities in saline environments.
In 1970, the first microbial rhodopsin, bacteriorhodopsin, was isolated from the cell membrane of Halobacterium. This finding had a major impact on protein structure and membrane research. Recently, an even broader distribution of previously unknown microbial rhodopsin genes, proteorhodopsins, has added another dimension to the ecological, genetic and physiological understanding of these exemplars of membrane proteins. Although microbial or type I rhodopsins were first found in the membranes of organisms that are now known as Archaea, these proteins were also subsequently detected in eukaryotic microorganisms and bacteria. They function as light-dependent transporters or photoreceptors, either through ion transport or through signal transduction. The protein is formed by a single amino-acid chain with a characteristic fold that encompasses seven transmembrane -helices.
A light-sensitive retinal cofactor is bound via a Schiff base to a conserved lysine residue of the protein. Upon absorption of a single photon, the retinal isomerizes from the all- trans to the 13- cis form, causing a cyclic sequence of spectroscopically detectable intermediates. The production of these intermediates is accompanied by conformational changes in the protein. At the end of this photocycle, the protein returns to its initial state. The mechanism of microbial rhodopsin function has many similarities to type II rhodopsins, which include the photosensitive receptor proteins of animal retinas. However, these visual pigments function as G-protein-coupled receptors (GPCRs) and the cofactor is released from the opsin protein during the photocycle, whereas the retinal in microbial rhodopsins remains permanently bound for historical accounts of the history of research on visual rhodopsins and other seven transmembrane receptors, see and. Although type I and II rhodopsins share the retinal cofactor and an architecture consisting of seven transmembrane helices, it is unclear whether the two protein families have resulted from a single progenitor or are an example of convergent evolution.
John Spudich and colleagues prefer the latter explanation on the basis of current genome sequence data as well as protein structures. Yet, they also point out that all known examples of type I and II rhodopsins are from evolutionarily distant organisms, and that further genome projects might reveal a missing link between the two families.
Because a broad, but patchy distribution of type 1 rhodopsins has been found in Archaea, Bacteria and Fungi, suggest that lateral gene transfer has been the principal force for the distribution of these proteins. In addition to questions about the evolutionary relationships between microbial and visual rhodopsins, research on these two fields has intersected at other points in the last decades, most notably in the field of structural biology, but also in studies of protein dynamics. In the history of microbial rhodopsin research, bacteriorhodopsin plays a central role. The protein has a molecular mass of 26.8 kDa and forms two-dimensional crystalline patches in the cellular membrane of Halobacterium salinarum. Because of the conspicuous colour of the protein, these patches are called ‘purple membrane’ whether in cells or as biochemical preparations. Bacteriorhodopsin is known as a ‘light-driven proton pump’ because a proton is transported out of the cell during the protein's photocycle.
The purple membrane thereby provides the organism with the means for a phototrophic lifestyle. Halorhodopsin, also discovered in H.
Salinarum, is a light-dependent anion importer involved in osmotic homeostasis. Microbial rhodopsins can also have sensory functions. Through protein–protein interactions, these sensory rhodopsins mediate phototactic responses in their host organisms.
Channelrhodopsins, such as those found in the green alga Chlamydomonas, are involved in the perception of light, but function by triggering passive ion fluxes across the membrane. All the evidence on the recently discovered proteorhodopsins, which were found through metagenomic analyses of bacterial DNA from microbial communities in marine environments, indicates a bioenergetic rather than a signalling function. Today, microbial rhodopsin research is a flourishing research field in which new understandings of rhodopsin diversity, function and evolution are contributing to broader microbiological and molecular knowledge.
In addition, a variety of inventions have been devised on the basis of microbial rhodopsins – from photosensitive security cards to devices that can control specific neuronal activities. As well as being a dynamic field now, microbial ‘rhodopsinology’ has been a source of major insight in earlier membrane research, structural biology, photobiology, microbial ecology and physiology. Here, we link these many dimensions of microbial rhodopsin research together and show that the long history of the field enriches understanding of not only the development of a particular research programme but also the general history of the life sciences.
Aspects of this story, especially in regard to the early research on halophilic microorganisms, are presented in Aharon Oren's detailed and multidimensional discussion of the biology of these organisms. An extensive collection of sources on early halophile research was also compiled by Helge Larsen (1922–2005), a professor of biochemistry at the university of Trondheim. Additional insight into early research on the purple membrane and bacteriorhodopsin can be gleaned from Walther Stoeckenius's reflections on that period.
Discoveries of halobacteria in applied microbiology and microbial ecology A crucial factor in the discovery story of halophilic microorganisms was the striking red colour that many of these organisms display in mass growth. Red waters were already reported in the Bible and some Chinese manuscripts, and the Historia Naturalis by the Roman naturalist discusses in 77–79 AD the regionally differing colours of sea salt (Pliny XXXIII: xxxi, ). cites biblical episodes where water was turned into blood: e.g.
The Nile during the first Plague of Egypt (Exodus 7: 17–25; see also 2 Kings 3: 22). Saltern workers in San Francisco Bay saw the red tinge as an indication of increasing salt concentration in the brine, which was then transferred to other ponds for crystallization.
The discovery of microorganisms as the causal agents of redness in brines was catalysed by the use of sea salt in the preservation of food such as fish. In the late nineteenth century, a time of industrialization and increasing global trade, as well as the development of bacteriology and infection biology, microbial analyses were established as part of food hygiene practices. Because salted and dried codfish sometimes turned red in damp summers, the US Fish Commission commissioned an investigation from William G. Farlow (1844–1919), a professor of cryptogamic botany at Harvard. Farlow attributed the red discoloration of the fish to the growth of a ‘very minute plant’ named Clathrocystis roseo-persicina. He also stated that the reddish cells originated from the Mediterranean salt used for preservation.
To prevent contamination, which not only lowered the price of the product, but was also suspected to cause food poisoning, Farlow recommended the use of salt from less contaminated sources. Farlow's analysis and subsequent publications had legal and economic impacts. In France, a sale ban on reddened codfish was announced in 1885. In the same year, a Spanish newspaper reported Farlow's findings and warned that Spanish salterns could be affected by dwindling sales to the fish-producing regions of the north Atlantic. A scientific problem that continued to plague microbiological studies was the taxonomy of the halophile isolates, which were continually reclassified for many decades after their initial discovery for a detailed account of early isolates of halophilic microorganisms and their various reclassifications, see. Researchers working with fish or salt samples often found several types of cells, and it remained difficult to establish the identity of different isolates purely on the basis of traits such as cell shape or colour. An important step was to match organisms from fish samples with those found in salterns.
A study of San Francisco Bay saltwater ponds identified bacteria that were believed to be degrading organic material at extremely high salt concentrations. One of these bacteria, the ‘red chromogenic bacillus’, was found to have a colour similar to that of saltern brines and to smell like those brines when cultured. These cultures were also shown to thrive on salt cod. In the early 1920s, two extensive studies reviewed the current knowledge of halophiles on salt-fish and attempted to improve their classification through more specific culturing and staining methods. Using fish from Norway that had been shipped to North German ports, the German mycologist and phytopathologist, Heinrich Klebahn (1859–1942), managed to isolate from fish a gram-negative, rod-shaped microorganism that formed reddish colonies. He named it Bacillus halobius ruber (; for an English translation, see ).
He noted two phenomena that would continue to be of importance throughout the history of halobacterial research. Klebahn observed that the cells swell and lyse upon transfer into water. Reflecting on the relationship between the organism and its environment, he stated that ‘the process reminds one of the behavior of certain deep sea creatures that burst due to their internal pressure, when they are taken out of the deep’ (translation by, p.12).
Klebahn also managed to extract the pigments responsible for the red colour and to characterize them spectroscopically. Just two years later, another comprehensive paper on halophiles was published in Canada, which was a second hub of salt-fish production and trade. Bacteriologists Francis C.
Harrison and Margaret E. Kennedy examined various fish and salt samples and isolated an organism that they named Pseudomonas salinaria (; ). They described the lifecycle of this organism as ‘pleomorphic’, meaning that it possessed various cell shapes. The nomenclature was probably bestowed without knowledge of Klebahn's work, which was published in German during the period of economic and political instability after World War I, when the dissemination of science carried out in central Europe was limited. Salted, dried codfish showing discolouration caused by halobacteria (dark areas), and a Petri dish containing an agar medium prepared from codfish and containing 16% salt.
On the medium are colonies of the ‘red organism’ (bright dots) and salt crystals. Reproduced from, Plate 1. During the interwar period, halophile research was increasingly conducted by environmentally inclined microbiologists, some of whom had strong connections to the Delft school of microbiology.
Baas Becking, who moved from the Netherlands to the United States and became director of the Jacques Loeb Laboratory at Pacific Grove in California, studied organisms in Californian saltwater lakes. For him, the presence of life in seemingly hostile environments had broader implications than it did for the microbiologists examining fish and salt samples. He called these niches a ‘borderland of physiological possibilities’ and thought they suggested ‘that physiology heretofore has been founded on the properties of liquids such as freshwater, seawater and blood-serum’ (, p. In the Netherlands, halophiles were studied by microbiologist and biochemist Albert Jan Kluyver, who was involved in the isolation of Pseudomonas beijerincki and its identification as the causative agent of spoilage in salted beans. With help from both Kluyver and Baas Becking, Helena Petter, a PhD student in the Botanical Laboratory at the University of Utrecht, isolated bacteria from salted cod and herring as well as from sea salt samples. She named these organisms Bacterium halobium rather than Bacillus halobius because she did not observe any spore formation (as Klebahn had not earlier). In her paper, she also introduced the term ‘bacterioruberin’ for the coloured pigments, which she purified in crystalline form.
These substances appeared to be similar to carotenoids, but did not match any known forms. Trijntje Hof, working in Kluyver's laboratory at the Technical University of Delft, noticed the lack of attention that had been paid to bacteria in earlier studies of saline lake ‘biocoenoses’ (ecological communities) and observed: ‘Nevertheless, there can be scarcely any doubt that bacteria will form an essential link in the cycle of life occurring in these lakes’ (, p. Although the Netherlands had no saltwater lakes suitable for her research, she examined materials such as salt samples, brine mud and salted fermentation products in order to compare her findings with those from lakes. These studies successfully combined microbiological methods with ecological approaches and biochemical analyses, but the taxonomic problems persisted. The first edition of Bergey's Manual of Determinative Bacteriology (1923) and the fourth (1934) name the organism that was first described by Harrison and Kennedy as Serratia salinaria.
It was assigned, presumably, for the red hue of the cultures. When chromogenic, rod-shaped halophiles were also isolated from leather, these were consequently designated as Serratia cutirubrum, meaning ‘red hide’. However, using a trait such as colour as the key characteristic of a genus was subsequently questioned. It was found that not only the pigments of S.
Marcescens and those of other halophilic microorganisms were different but also that the physiology and the flagellar organization of these organisms were distinct. Because of their similarity to pseudomonads, the two halophile species reappeared in Bergey's 1948 edition as P. Salinaria or cutirubrum, which was the nomenclature proposed originally by Harrison and Kennedy. A solution to this taxonomic muddle was provided by the Israeli microbiologist, Benjamin Elazari-Volcani (1915–1999), who had also been a visitor to Kluyver's laboratory at Delft. Elazari-Volcani's first paper on halophiles dates back to 1936, when he published a short note, ‘Life in the Dead Sea’, in Nature, authored by his birth name Benjamin Wilkansky. He isolated several types of aquatic microorganisms, some of which were orange-red when cultured. Reflexive big kahuna reef 2 keygen.
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In a 1944 article, he describes several obligately halophile bacteria and suggests Halobacterium as a subgenus of Flavobacterium on the basis of the similarity of his isolates to those of Peirce, Klebahn, Harrison and Kennedy, Petter and others. Contributed Halobacterium as the genus name to Bergey's Manual the name Halobacterium had already been introduced in the 1930s for halophilic rods, but this original classification did not catch on. The reformed genus encompassed microorganisms from natural brines as well as those from salt-fish and other salted goods. It worked as a unifying category for these apparently related isolates. The type species H. Salinarium (later salinarum) was characterized as a gram-negative, rod-shaped bacterium. Today, former species such as Halobacterium cutirubrum or Halobacterium trapanicum, originally isolated from the salterns of Trapani in Sicily, are all considered to be different strains of H.
Salinarum. For Elazari-Volcani, as for Klebahn and Petter, and indeed, for the existing history of halophile research, the unusual pigmentation of the microorganisms was crucial to the observation, identification and biochemical analyses of the organisms. However, using the colour as a characteristic caused taxonomic problems when this highly visible trait had to be evaluated against microscopical or biochemical distinctions. The physiological function of the pigments remained unclear. The first microbial rhodopsin: bacteriorhodopsin, membranes and bioenergetics In the late 1960s, biological membrane research was an experimentally and conceptually heterogeneous territory.
Electron micrographs showed cell membranes as dark shaded lines that could be distinctively stained by heavy metal salts. It could not, however, be determined unambiguously how the lipids and proteins found in membrane material from cell fractionations were spatially organized. Moreover, the phenomenon of the active transport of, for example, ions or sugars across lipid barriers, was difficult to address with techniques such as radioactive transport assays for intact cells. Little was known about the material structures responsible for the transport process. In addition, the membranes of chloroplasts and mitochondria formed the battleground for the controversy over oxidative phosphorylation and Peter Mitchell's chemiosmotic theory.
According to the latter, membranes maintained a proton gradient that was required for ATP synthesis, whereas competing ‘chemical theories’ argued for the existence of a phosphorylated high-energy intermediate. Differing models of membrane organization in biochemistry textbooks of this era clearly illustrate the lack of consensus among scientists on these topics. Different models of membrane organization from the first edition (1970) of Albert L. Lehninger's Biochemistry textbook. The two figures at the top represent versions of the Davson–Danielli model, which is based on a lipid bilayer with proteins attached to both sides of the membrane. The figure at the bottom represents a subunit model of membrane structure that was proposed for Halobacterium. The ‘fluid mosaic model’ that became the basis for today's concept of biological membranes is not depicted (see ).
Reproduced with permission from. Adapted by G.
Sporn, FEMS Central Office. Biochemical studies of membranes were complicated by the difficulties in obtaining membrane material in sufficient amounts and by the problems of separating protein and lipid components. These factors may explain why Walther Stoeckenius, an electron microscopist at Rockefeller University who was studying myelin organization as well as erythrocytes and mitochondrial membranes, decided to work on halobacterial membranes in the mid-1960s. The publication that initially stimulated his interest reported a disintegration of the halobacterial cell envelope fractions into distinct molecular components when they were transferred into media of low osmotic strength. These data suggested a subunit model of the membrane as a planar array of lipoprotein particles. This was in contrast to the then prevalent Davson–Danielli model, which implied a lipid bilayer with protein adjacent to both sides.
However, the ‘auto-fractionation’ of halobacterial membranes in distilled water also provided a technical advantage as it considerably facilitated analyses of the different fractionation products. In addition, the fractionation products obtained from halobacterial membranes could easily be distinguished by their characteristic colours. Stoeckenius started to work on halobacterial membranes while still at Rockefeller. His project continued at the University of California Medical School at San Francisco, where he moved in 1969 with two new co-workers, Allen Blaurock and Dieter Oesterhelt. Oesterhelt, a trained chemist, was on sabbatical leave from Munich, where he had earned a doctorate in 1967 under the supervision of Nobel laureate biochemist, Feodor Lynen. Blaurock, a physicist, had worked with X-ray techniques in Maurice Wilkin's lab at King's College London. When Blaurock and Oesterhelt examined a curiously purple-coloured membrane fraction by X-ray diffraction, they found a sharp diffraction pattern that indicated a high degree of molecular order.
In addition, Oesterhelt observed that the purple material turned yellow upon the addition of organic solvents such as ether (; ). When Blaurock told him about a similar colour change in the frog retina he was working with at King's, Oesterhelt was inspired to look for the chemical substance that formed the visual pigment in the eyes of higher animals: vitamin A aldehyde or retinal (D. Oesterhelt, pers. (a) The rise of bacteriorhodopsin research from 1966 to 1990, based on the number of publication titles per year with the keywords ‘bacteriorhodopsin’ (BR) or ‘purple membrane’ (PM) in the title, abstract or keywords of papers in the ISI Web of Knowledge SM database (black bars). For comparison, the increase of total publications is shown in grey bars, which include all English language publications during the same years in the subject areas ‘Biochemistry & Molecular Biology’ and ‘Biophysics’ as indexed in Science Citation Index Expanded.
Searches were carried out using a new version of Web of Knowledge SM. The number of total publications is normalized to the number on BR/PM in 1990. The rapid growth of bacteriorhodopsin publication activity is clearly visible between 1972 and 1980. These data are indicative of a trend and not precise relationships. (b) Schematic timeline displaying the interrelations of selected areas in halobacterial/microbial rhodopsin research and their connections to other fields in the life sciences. Full arrows represent transfer primarily of research materials such as cells or proteins; dotted arrows depict transfers mostly of technologies and concepts, which often cross-fertilized fields unrelated before.
Developments in molecular genetics are indicated on the left and biochemical and structural biological research on the right. Dates are indicative. The purple membrane was soon revealed to function as a light-dependent proton exporter.
A structural model of the protein was established by Richard Henderson and Nigel Unwin from the Medical Research Council Laboratory of Molecular Biology (LMB) in Cambridge. Henderson's and Unwin's paper depicted the -helices of bacteriorhodopsin as parallel graded columns perpendicular to the membrane plane. This publication is the most extensively cited of all papers on bacteriorhodopsin or the purple membrane from 1970 to 1980. This high rate of citation might indicate the more general relevance of this particular publication to membrane and structural research The Science Citation Index shows 1591 citations for this paper as of February 2011. The next closest paper is paper with 1277 citations.
Henderson and Unwin qualify the purple membrane as ‘a simple example of an ‘intrinsic’ membrane protein, a class of structures to which many molecular pumps and channels must belong’ (, p.31). Transport was thought to result from quasi-mechanical conformational changes of the polypeptide chain. Bacteriorhodopsin also played a major role in the resolution of the controversy over oxidative phosphorylation, when it was shown that proteoliposomes containing it (in combination with the mitochondrial FOF1-ATPase) synthesized ATP upon illumination. Because a transmembrane proton gradient alone sufficed for ATP synthesis, this experiment was interpreted in favour of Peter Mitchell's chemiosmotic mechanism of energy generation. Promotion material ( c. 2006) concerning the projected use of recombinant bacteriorhodopsin as copy-protection pigment in ID cards. Photocopying the cards results in the reversible bleaching of purple membrane patches engineered into the document.
Picture courtesy of Nobert Hampp, Marburg. An important characteristic for all of these applications is the robustness of purple membrane, which is unusual for biological substances. Most applications require large amounts (several grams) of purple membrane, which makes techniques to scale up culturing and biochemical preparation prerequisites of profitable production.
In Germany, a biotechnological company has been created to establish the commercially viable production of purple membrane variants. Many of the technologies mentioned above require genetically engineered variants of bacteriorhodopsin with modified photocycles, and this explains why many publications and patents date from the early 1990s, when genetic tools such as shuttle vectors made it possible to synthesize recombinant bacteriorhodopsin in large amounts. Speaks of a ‘second evolution’ of purple membrane through genetic engineering, which enhances desirable properties without losing the naturally evolved rapid light response or the robustness of the protein. However, despite ongoing interest, there is no publicly accessible information showing that any of these technologies has been successfully commercialized yet. Another strategy, using recombinant DNA technology to make microbial rhodopsins function in other organisms, has recently become the focus of a dynamic field of application. In technologies now called ‘optogenetics’, neurons of various animals are transfected with microbial rhodopsin genes by viral vectors. The proteins expressed and transferred into the membranes of these cells can then be used as light-activated ‘switches’ to trigger or suppress action potentials.
Optogenetic technologies mostly use the chloride-importer halorhodopsin of Natronomonas pharaonis as an inhibitor of action potentials , or channelrhodopsin-2 of the alga, Chlamydomonas reinhardtii, to trigger neuronal responses. Channelrhodopsins, as mentioned earlier, are a member of a family of rhodopsins found in eukaryotic microorganisms. They have been studied in Chlamydomonas since the 1980s and shown to be inward-directed proton and cation channels rather than transporters.
Optogenetic tools are used to stimulate cell populations or regions of organs such as the brain, in the same way that was achieved earlier by electrophysiology. Microbial rhodopsin tools are non-invasive, however, and do not require any pharmacological agents or chemicals. Moreover, their photostimulation of neurons has a high spatial and temporal resolution. By implanting optical devices into the brain of laboratory animals, rhodopsin responses can be used to influence behavioural traits such as locomotion, circadian rhythms or learning. Rhodopsin-mediated photostimulation of pacemaker cardiomyocytes in embryonal zebrafish hearts has been used to modulate the rhythmicity of cardiac contractions.
The creation of chimeric proteins such as light-gated GPCRs is also feasible, and this further expands the spectrum of biological functions that can be manipulated. Optogenetic technology is also envisaged for biomedical therapies of neurological diseases (e.g. ), but the problems of viral transfection remain unsolved.
The extensive ethical issues connected to such applications are obvious (e.g. The use of viral vectors, the manipulation of human subjects) and will need close attention.
Overall, these applied technologies are generating biotechnological strategies that open up novel realms of application even as they generate further basic insights into biological processes. The capacity for fields to shift between applied and basic research and back again calls into question standard understandings of the relation between science and technology and suggests that a better conceptualization of their dynamic interaction is needed.
Technological applications have also been the focus of public perceptions of microbial rhodopsin research over the last three decades. The New York Times, for example, speculated in 1977 that the purple membrane ‘may yield a scientific goldmine’ due to its possible uses in desalinization and solar energy generation. Many other articles from popular science magazines, journals or newspapers adopt a similar angle, highlighting in particular the potential of bacteriorhodopsin as an optical data storage medium (e.g. Recently, optogenetics has attracted a great deal of similar media attention (e.g. Conclusion The history of research on halophiles and the photosensitive proteins of their membranes spans more than a century of the life sciences, from the first microbiological examinations of salt-fish to recent developments in nanotechnology and the neurosciences. Our study highlights the contributions of research fields such as general microbiology, the physiology of vision, membrane biology and metagenomics, and emphasizes the contribution of microbial rhodopsin research to the molecular life sciences. The twentieth century should thus be seen as not just the ‘century of the gene’, which began with the rediscovery of Mendel's laws and led to the determination of DNA structure and the rise of genetic engineering , but as a century in which several major strands of molecular research in biology were woven together.
Over the last hundred years, numerous fields, including general microbiology and membrane research, have successfully contributed to the molecular life sciences. The example of optogenetics shows how different approaches can merge into an integrated and increasingly technological mode of treating life, but still increase knowledge about its basic processes. The history of rhodopsin research also reveals that productive interactions between different fields of science occur not only through the adaptation of theories, concepts or models, but very much at the level of materials or experimental systems. In the microbial rhodopsin case, entire regimes of experimentation were repeatedly transferred between research fields.
In the early days of purple membrane research, for example, an established body of chemical techniques from vitamin and visual rhodopsin research was imported to detect the retinal cofactor. When genetics was imported into microbial rhodopsin research in the 1980s, techniques and expectations were not imposed piecemeal or occasionally, but as a system of inquiry focused on the genetic material of the phenomenon. And in the birth of proteorhodopsin research, the techniques so successful in bacteriorhodopsin research were deliberately and systematically imported into the investigation of proteorhodopsin structure and function.
The history of microbial rhodopsin also calls into question distinctions commonly made between basic and applied science, or hypothesis-driven and data-driven science. Microbial rhodopsin research shows how scientific fields can originate in highly applied contexts from which questions and findings are ‘translated’ into more traditional laboratory-based and natural history activities. As bioenergeticist Efraim Racker (1913–1991) said: ‘If there is no money available for fundamental research start working on a project of applied research. If you proceed logically, you will soon be doing basic research’ (, p.
Our history also illuminates how techniques are transferred from one research context to very different settings, and that in this dynamic and often iterative process the originating field can benefit. The proteorhodopsin research story can be interpreted as a challenge to the common notion that science advances exclusively by testing hypotheses. The discovery of microbial rhodopsins in proteobacteria was not made on the basis of a separation between hypothesis testing and data gathering, but through the integrative combination of approaches that is similar to the process said to be essential for successful systems biology. A final consideration is what the history of microbial rhodopsin research might have to say for the relationship between the history of science and science itself. First, it needs to be borne in mind that the history of science can be written in many ways. Although we have largely left out the broader cultural and social framework of science (e.g. The creation or modification of institutions, and the interplay between overarching technological developments and expectations), this is not to say that these factors did not play significant roles in rhodopsin research.
Discussing them would require a different paper, however. Our account, which focuses on materials and experimental systems, has depicted an array of highly creative and inventive scientific activities. New fields of research were opened up when researchers pursued unusual phenomena, such as the colour of brines and salt-fish, or the colour changes of the purple membrane.
Curiosity, intuition and the ability to connect very different pieces of knowledge have played indisputably major roles in this process of discovery. Individual scientific projects did not follow prescribed routes of inquiry with specific goals. From a longer historical view, halophile and microbial rhodopsin research illustrate the adventitious and even meandering road to findings of great impact. In an era of focused funding and narrow top-down research agendas, the microbial rhodopsin story will be worth recalling as an example of how science proceeds from one influential finding to another, and in the process, transforms all it illuminates. Acknowledgements We thank Norbert Hampp (Marburg), Peter Hegemann (Berlin), Richard Henderson (Cambridge), Janos Lanyi (Irvine) and Dieter Oesterhelt (Martinsried) for discussions on the subject.
Norbert Hampp, Richard Henderson, Dieter Oesterhelt and W.H. Freeman & Co/Worth Publishers (New York) gave permission to reproduce the figures.
We are grateful to our referees and the Editor, who helped us clarify several important points in our discussion. Our research was funded by the ESRC Centre for Genomics in Society (Egenis) at the University of Exeter, UK, and the Max Planck Institute for the History of Science, Berlin. S contribution was completed with Australian Research Council Future Fellowship funding at the University of Sydney. Ancillary Article Information.
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