Author: Ibon Cancio, UPV/EHU Associated Professor in Cell Biology; researcher in the ‘Cell Biology & Environmental Toxicology’ (CBET) research group of the Plentzia Marine Station (PiE-UPV/EHU); Spanish scientific representative in the EMBRC Committee of Nodes
Arguably, vision is the most important of human senses. A company that makes money from producing and selling microscopes (very good ones!) argues that 80% of our impressions are received thanks to our eyes. To see is to learn and, in order to learn in science, you need to travel and meet people. From one station to another, and this could include one marine station or another. Selig Hecht and George Wald were practitioners of this idea and, as such, made a significant contribution to our understanding of the mechanisms of vision.
Selig Hecht (1882-1947) was born in Glogow, an Austrian city that is now in Poland, and as a young child was taken to New York in the USA. As a student in zoology, he won a grant for a summer stay at the Bureau of Fisheries Station in Beaufort, North Carolina. This resulted in the publication of his first two papers, one on the weight to length relation in fishes, and the other one on calcium absorption during molting in the blue crab. After graduating, Selig moved to industry to work as a chemist in fermentation, analysing the reasons of beer deterioration under light. This would become his first contact with photochemistry (interestingly, there are quite a few histories in marine science related to beer). The job did not last for long and he returned to academia at Harvard where he met a lively group of physiologists. In pursuing his PhD, he spent summers at the Bermuda Biological Station, studying the physiology of Ascidia atra (now Phallusia nigra). His PhD came with his marriage to Celia Huebschmann, and their honeymoon would consist on at trip to the west coast to visit the Oceanographic Institute at La Jolla. There he obtained a fellowship to spend the summer, performing experiments on the light sensitivity of the sea squirt Ciona intestinalis. That is how the study of photochemical reactions on photoreceptor cells developed into a lifetime of work on the physiology of vision.
After taking a position as University assistant professor in Omaha, he began yearly summer trips to the Marine Biological Laboratory in Woods Hole, to perform some of his most significant research work. In the spring of 1924, another woman came into Hecht’s life: his daughter Maressa. The baby brought with her a trip to yet another marine station, this time Stazione Zoologica in Naples, where he carried out research on Ciona and the bioluminescent rock-boring clam, Pholas dactylus. Hecht spent one year there to understand the kinetics of dark adaptation in these simple organisms. After another year in Cambridge, the family returned to the USA in 1926, where Hecht assumed a professorship in biophysics at Columbia University. He then returned to his summer research trips at Woods Hole.
Hecht’s approach to the physiological research of vision was phylogenetically holistic as he considered that biochemical mechanisms of light-sensitivity were conserved across animals. In his research career, he drove his attention to the light reflex mechanisms and light dark adaptation of marine organisms such as tunicates and molluscs as well as insects with compound eyes and humans. Light reflexes in ascidians and clams are easy to study. These are readily manipulable organisms whose responses and reactions are simple and slow enough to be measured. In his experimental work with Mya arenaria, he would wait for the syphon to be extended to shine a light on the clam and measure how quickly it was retracted.
Hecht additionally demonstrated that low- and high-intensity light discrimination are handled by different kinds of cells in humans. Low-intensity light is processed by achromatic rod cells, while high-intensity light is processed by chromatic cone cells. In 1929, Hecht proposed that his photochemical theory of vision could explain how three types of cone cells could be used to account for the trichromatic theory of human colour vision. But Hecht above all used his studies of light and dark adaptation in invertebrates to propose an abstract theory for universal photochemical vision in all animals.
In 1877, Franz Christian Boll published an article on how he had extracted a substance from the retina of frogs that he called ‘visual purple’. Boll observed that visual purple bleached out from red to yellow, finally becoming colourless when exposed to light. The substance regenerated when placed back in the dark. Hecht, using his studies of light and dark adaptation in marine invertebrates, theorised a universal molecular basis for photochemical vision. He argued that light acted on a substance (S), that was broken into two products (P and A), bleached visual purple. During the process of dark adaptation, P and A would create S again. This was a theoretical solution to the problem of infinite exposure.
George Wald (1906-1997) was born in New York and joined New York University (NYU) to study law but came out with a degree in sciences instead in 1927. For graduate work, he joined Hecht’s laboratory at Columbia University, receiving his PhD in zoology in 1932. He always recognised Hecht’s impact on him, and he inherited Hecht’s holistic comparative conception of animal light sensitive photochemistry. In 1932, months before Hitler came to power, Wald moved to Germany for two years for post-doctoral research in the laboratory of the Nobel prize winner Otto Warburg. There he found vitamin A in the retina of frogs (afterwards he briefly visited the laboratory of Paul Kerrer, who had described the chemical structure of vitamin A, in Switzerland). Wald’s discovery provided the basis to understand why vitamin A deficiency results in blindness, and later to explain the theoretical proposition of his first professor.
Rhodopsin (the name for visual purple today) is the photopigment of the rod cells of most vertebrates. It is formed by a protein, opsin, bound to a cofactor, vitamin A, or rather its aldehyde, retinal (retinene, as it was first termed). Wald next studied the chemical events taking place in the photon receptor following exposure to light and the reversal of these effects during dark adaptation. Wald further demonstrated that rhodopsin, when exposed to light, undergoes a vitamin A conformation change from opsin bound 11-cis retinal to all-trans retinal released from opsin. In darkness, all-trans retinal binds to opsin again. Wald thus provided a molecular solution to the theoretical proposal of Hecht, showing that the precursors of rhodopsin, Hecht’s P and A, were also the products of rhodopsin, Hecht’s S.
Wald, as it was the case of his mentor, was a common summer visitor of MBL. Here he worked with different fish species and found that rhodopsin was not the only rod photopigment. While marine fish and land vertebrates use the rhodopsin system, freshwater fish use a different pigment in which other carotenoids replace vitamin A. Wald termed this pigment ‘porphyropsin’ due to, surprise, surprise… its purple colour (Porphyra is the Greek name for purple and for the sea snails of the Muricidae family from which the Tyrian purple dye was extracted)! This is very interesting considering, for instance, the morphological modification and pigments that eel eyes experience during ‘silvering’ in preparation for reproductive migration from continental waters into the sea. Many fish alter their expressed visual pigments during development and, in the case of anadromous fish species, in migration between water masses. Wad’s life-long exploration of the biochemical and molecular mechanisms of vision earned him the Nobel Prize in Physiology or Medicine, with yet another marine biologist, Haldan Keffer Hartline, in 1967.
Science, marine science is an elaborated form of social intercourse mainly based in collaborative relationships. Travel and maybe you will see the light!!