Observations on enzymes of ammonia assimilation in two different strains of Cyanidium caldarium

Metabolism and metabolomics of eukaryotes living under extreme conditions

Treatises on extremophiles are frequently focused on organisms belonging to the Archaea and Eubacteria kingdoms. However, a significant number of eukaryotes, both unicellular and multicellular, have evolved to live and thrive in extreme environments. Although less is known about eukaryotic life in extreme environments in comparison to prokaryotic extremophiles, advances in genomics and in comprehensive, high-throughput metabolic profiling techniques have provided new insight into the metabolic adaptations of eukaryotes living under extreme conditions. In this review, we will provide an overview of extremophilic life forms with emphasis on eukaryotes and we will compare metabolic adaptations in different eukaryotic extremophiles to identify generalities and specializations in adaptation to life under extreme conditions. Special emphasis will be devoted to the thermoacidophilic unicellular red alga Galdieria sulphuraria (Cyanidiaceae) as one example of a eukaryotic extremophile.

The red algal-higher fungi phylogenetic link: the last ten years

The review of the red algal theory for ancestry of Ascomycetes and Basidiomycetes published 10 years ago by the author is updated. Criticisms are answered and new data are discussed. The production of choline sulfate, lenthionine and lanosol are added to the biochemical similarities between red algae and higher fungi. Distribution of polyols is shown to be in favour of the origin of higher fungi from parasitic red algae. As predicted, NADP-linked glutamate dehydrogenase has been found in red algae, and additional reports of chitin in various algae have been published. New supporting data come from the ultrastructure of red algae: mitosis outside the Ceramiales and ultrastructure of vegetative cells and tetrasporocysts of Corallinaceae. On the other hand, the discovery of proplastids in Holmsella makes it less fungus-like. However, no decisive argument has yet been produced for or against the theory. Further light should be expected from protein and nucleic acid sequences. Promising partial sequences of cytochrome c have indeed been published for red algae but the published 5 S ribosomal RNA sequences have not proven relevant to the problem. Sequences of the slower-evolving large rRNA and cytochrome c of red algae could provide convincing evidence and are urgently needed.

Depression of nitrate reductase in the presence of excess ammonium in a unicellular alga growing under conditions of phosphate limitation

Chemostat cultures of the unicellular alga Cyanidium caldarium have shown that under conditions of phosphate limitation nitrate reductase is completely derepressed even in cells growing in a large excess of ammonium, but that it occurs mainly in a catalytically inactive form. It is hypothesized that phosphate limitation contributes to maintaining intracellular level of glutamine suitable to stimulate inactivation but not repression of nitrate reductase. It is not excluded that in addition to variations in the intracellular level of glutamine, there are other metabolic events of the cell by which repression and inactivation of nitrate reductase could be differently influenced.

Studies on utilization of 2-ketoglutarate, glutamate and other amino acids by the unicellular alga Cyanidium caldarium

Two strains of Cyandium caladarium which possess different biochemical and nutritional characteristics were examined with respect to their ability to utilize amino acids or 2-ketoglutarate as substrates. One strain utilizes alanine, glutamate or aspartate as nitrogen sources, and glutamate, alanine, or 2-ketoglutarate as carbon and energy sources for growth in the dark. The growth rate in the dark on 2-ketoglutarate is almost twice as high or higher than that on glutamate or alanine. During growth or incubation of this alga on amino acids, large amounts of ammonia are formed; however, ammonia formation is strongly inhibited by 2-ketoglutarate. The capacity of the alga and develops fully only when the cells are grown or incubated in the presence of glutamate.