Hydrogen metabolism of green algae: discovery and early research - a tribute to Hans Gaffron and his coworkers.

The detection of hydrogen metabolism in green algae more than 60 years ago by Hans Gaffron dispelled the widely accepted dogma at that time that this feature was unique to prokaryotic organisms. Research on this unexpected aspect of algal physiology has continued until today because of its evolutionary implications and possible practical significance. This minireview focuses on the work of Gaffron and his collaborators, whose experiments provided most of the information about the mechanism of hydrogen metabolism in algae during the 35 years following its discovery. It is shown that the emergence of our present mechanistic concepts was closely linked to the changing perception of the process of photosynthetic water oxidation. Whereas the mechanism of 'photoreduction,' i.e., the photoassimilation of carbon dioxide with hydrogen as the electron donor, was well understood already by Gaffron's group as being a reaction mediated by Photosystem I only, a clear concept of the mechanism of light-dependent hydrogen production has been more difficult to establish. Gaffron and his collaborators provided ample evidence, however, that 'photohydrogen' evolution can be fueled by reducing equivalents derived from a photolysis of water as well as by an oxidation of internal and external organic molecules. The presently prevailing view embraces this concept of multiple pathways, but the relative contribution of each of them, and the regulatory mechanisms determining it, remain a matter of debate.