Assimilation of carbon dioxide by hydrogen bacteria.

Autotrophic organisms can use carbon dioxide as their sole source of carbon. Within this group, the chemosynthetic bacteria secure energy for carbon dioxide assimilation by the oxidation of simple inorganic substances, although the photosynthetic organisms obtain this energy from light. The phylogenetic relationships of the chemosynthetic bacteria to the photosynthetic and to the heterotrophic organisms have been a matter for much speculation (1,2). The demonstration of biochemical similarities or differences among the organisms of these three groups could offer clues to their interrelationships. Recently, the main pathway of carbon dioxide fixation by photosynthetic tissue has been clarified. It has been demonstrated that PGAl is the first stable photosynthetic product (3, 4) and that ribulose diphosphate is the primary carbon dioxide acceptor (5). The carboxylating enzyme involved has been termed carboxydismutase. Similar reactions have been demonstrated in several chemoautotrophic bacteria. Santer and Vishniac (6) showed that ribulose diphosphate greatly stimulated the uptake of Cl402 by cell-free extracts of Thiobacillus thioparus and that radioactive PGA was formed. Trudinger (7) demonstrated a similar system in extracts of Thiobacillus denitri$cans. He was able to show that this organism potentially was capable of synthesizing hexose phosphates from carbon dioxide by a cyclic mechanism similar to that described for photosynthetic tissue by Calvin (8). However, the quantitative significance of the carboxydismutase system for the steady state fixation of carbon dioxide during the growth of these organisms remained uncertain. More recently, Suzuki and Werkman (9) reported