The Photo-assimilation of Acetate by Pyrobotrys (Chlamydobotrys) stellata

Abstract

SUMMARYAlcohols, amino acids, organic acids and sugars were tested as carbon sources for the growth of Pyrobotrys (Chlamydobotrys) stellata in the light and dark. Growth was only recorded with acetate in the light. A carbon balance sheet of 14C-acetate assimilation showed a greater incorporation of 14C into polysaccharide and less released as 14C02 in the light, compared with the dark. The primary products of 14C-acetate assimilation into the soluble fraction of the organisms were identified; after 10 sec. 40 % of the total 14C present in this fraction was in succinic acid, 15% in citric acid and 16 % in malic acid. The percentage of total 14C in this fraction present in succinic and malic acids decreased consistently with time, while that in citric acid initially increased before decreasing. After 10 sec. the specific activity of succinic acid was more than twice that of citric acid. 4 x M-monofluoroacetate (MFA) effectively inhibited the incorporation of 14C-acetate into tricarboxylic acid cycle intermediates and. related compounds, and markedly inhibited l4COZ evolution. ~-N1-(3-4,dichlorophenyl)-N,N-dimethyl urea (DCMU) didnot significantly inhibit 14C-metabolism. The key enzymes of the glyoxylate cycle, isocitrate lyase (E. C. 4.1 .3.1) and malate synthetase (E. C. 4.1.3.2) were found to be present in P. stellata, and did not disappear in the absence of acetate, but even so growth was not recorded on acetate in the dark.

An Obligately Autotrophic Mutant of Chlamydomonas dysosmos i a Biochemical Elucidation

“…This contrasts with the situation in algae like Pyrobotrys stellata, which apparently cannot grow on acetate in darkness in spite of the proven presence of both isocitrate lyase and malate synthetase in cells of this species (Goulding & Merrett, 1967). It seems that it is not possible to adduce a single common explanation for obligate autotrophy.…”

Enzyme Levels in Relation to Obligate Phototrophy in Chlamydobotrys

“…Isocitrate dehydrogenase was specific for NADP, no activity being detected with NAD. The reaction rate was not (10) so that obligate phototrophy cannot be explained by a dysfunction of the glyoxylate cycle as with a C. dysosmonas mutant (22).…”

“…Although impermeability to organic compounds may be one explanation for obligate autotrophy, it will not account for the failure of acetate to support heterotrophic growth while enhancing the photoheterotrophic growth of Chlamydobotrys stellata (10).…”

THE PHYSIOLOGY OF NEOSPONGIOCOCCUM OVATUM DEASON

“…Wild-type Chlamydomonas dysosomos grows heterotrophically on acetate and contains the glyoxylate cycle enzymes but a mutant isolated by Lewin (1954) lacked isocitrate lyase so Neilsen et al (1972) concluded that obligate autotrophy in this mutant resulted from the absence of a functional glyoxylate cycle. In contrast acetate-grown cells of Chlamydobotrys stellata contain high levels of glyoxylate cycle enzymes (Goulding and Merrett, 1967) but acetate will not support heterotrophic growth. The probable reason for obligate phototrophy in C. stellata is the absence of a-ketoglutarate dehydrogenase and succinate thiokinase so that the yield of adenosine triphosphate from acetate oxidation may be insufficient to support heterotrophic growth (Merrett, 1976).…”