The Function of Acetate in Photosynthesis by Green Bacteria

Abstract

Our present knowledge of the physiology and biochemistry of green sulfur bacteria (genus Chlorobium) is largely based on the studies of van Niel' and of Larsen.2 Their work showed that the members of this genus are strictly anaerobic, obligate photolithotrophs, which perform a typical bacterial photosynthesis using reduced inorganic sulfur compounds as electron donors. The physiology of these photosynthetic bacteria is accordingly similar to that of the purple sulfur bacteria, with one significant exception: all members of the latter group so far studied in pure culture can also use a variety of simple organic substrates for photosynthesis in the absence of a reduced inorganic sulfur compound. The possible participation of organic compounds in the photometabolism of green bacteria was examined in detail by Larsen,2 using Chlorobium thiosulfatophilum. He found that none of the simple organic compounds commonly used as substrates for photosynthesis by purple sulfur bacteria could support the photosynthetic growth of C. thiosulfatophilum. However, manometric experiments suggested that resting cells could bring about a slow and limited transformation of organic acids anaerobically in the light. Only with propionate was a rapid metabolism observed. More detailed study showed that C. thiosulfatophilum performs a unique photosynthetic reaction with this substrate, grossly expressed by the equation:CH3CH2COOH + CO2 light HOOCCH2CH9COOH.(1)A large part of the propionic acid metabolized is excreted into the medium as succinic acid. The physiological function of this reaction is obscure. Larsen2 commented: "it can be safely concluded that the formation of succinic acid can hardly be considered as an important stage in the normal assimilatory process." However, the existence of the reaction revealed one very important fact: the failure of green bacteria to utilize organic compounds for growth cannot be attributed to the failure of such compounds to enter the cell.Recently, Mechsner3 has reported that a new species of Chlorobium, C. chiorochromatii, can grow well photosynthetically at the expense of peptone and malate. He confirmed, however, that the two species studied by Larsen, viz. C. limicola and C. thiosulfatophilum, are unable to utilize organic compounds for growth. This paper reports certain new observations on the utilization of organic substrates by C. limicola.Materials and Methods-Organisms and culture conditions: Several strains of C. limicola were isolated from local mud samples using the procedures described by Larsen2 for enrichment and purification. Larsen's mineral medium,"z which contains 0.024 M NaHCO3 and 0.0042 M Na2S, was used routinely for cultivation. In this medium, sulfide is the growth-limiting nutrient, as shown by preliminary experiments in which the sulfide content was varied, other components being kept constant. There is strict proportionality between cell yield and sulfide concentration 1328

Reevaluation of Chloropseudomonas ethylica Strain 2-K

“…Ethanol or a series of organic acids and sugars did not enhance growth in the presence of C 0 2 . These data agree with the acetate enhancement data found in Chlorobiurn thiosulfatophilum (17). No growth occurred in liquid cultures lacking C02 or in any culture incubated in the dark.…”

Photoassimilation of organic compounds by autotrophic blue-green algae

“…Unfortunately comparatively few studies have been made of the assimilation of organic compounds by obligate autotrophs, and it is still far from clear why such organisms are unable to grow on organic compounds. In green sulphur bacteria of the genus Chlorobium, inability to grow on acetate as sole or major carbon source is probably attributable to the inability of these organisms to oxidize acetate, as was first suggested by Sadler & Stanier (1960). In Chlorobium, acetate assimilation is not only light dependent, but requires carbon dioxide and an inorganic reductant such as hydrogen sulphide which is the essential source of reducing power for cell synthesis.…”

Both Forward and Reverse TCA Cycles Operate in Green Sulfur Bacteria

“…Requirement of CO 2 for Growth of C. tepidum with Organic Carbon-In contrast to heterotrophs, acetate or pyruvate can enhance but cannot be used as a sole carbon source without including CO 2 or HCO 3 Ϫ during the growth of GSBs (2,5). The enzymatic activities of four enzymes responsible for CO 2 assimilation in the RTCA cycle were reported in the GSB C. thiosulfatophilum (3,34) (now Chlorobaculum thiosulfatiphilum) (1), and we also detected the enzymatic activities of P-enolpyruvate carboxylase and P-enolpyruvate carboxykinase (25-40 nmol/min⅐mg protein) in cell extracts of the autotrophic and mixotrophic cultures of C. tepidum.…”