Induction and synthesis of nitrate reductase in Chlorella vulgaris

Heterotrophic cultures of microalgae: metabolism and potential products

This review analyzes the current state of a specific niche of microalgae cultivation; heterotrophic growth in the dark supported by a carbon source replacing the traditional support of light energy. This unique ability of essentially photosynthetic microorganisms is shared by several species of microalgae. Where possible, heterotrophic growth overcomes major limitations of producing useful products from microalgae: dependency on light which significantly complicates the process, increase costs, and reduced production of potentially useful products. As a general role, and in most cases, heterotrophic cultivation is far cheaper, simpler to construct facilities, and easier than autotrophic cultivation to maintain on a large scale. This capacity allows expansion of useful applications from diverse species that is now very limited as a result of elevated costs of autotrophy; consequently, exploitation of microalgae is restricted to small volume of high-value products. Heterotrophic cultivation may allow large volume applications such as wastewater treatment combined, or separated, with production of biofuels. In this review, we present a general perspective of the field, describing the specific cellular metabolisms involved and the best-known examples from the literature and analyze the prospect of potential products from heterotrophic cultures.

Possible role for mRNA stability in the ammonium-controlled regulation of nitrate reductase expression

Ammonium, or a metabolite of ammonium, represses the expression of nitrate reductase (NR) in Chlorella vulgaris. The removal of ammonium and addition of nitrate (induction) resulted in a rapid (20 min) peaked synthesis of NR mRNA. Nitrate reductase protein and activity increased at a much lower rate, reaching their maxima by 8 h. Ammonium added to nitrate-grown cells resulted in a dramatic decrease in NR mRNA from a steady-state level to undetectable levels within 15 min of ammonium addition. Nitrate reductase activity and protein levels decreased to 20% and 40% of initial levels respectively over 8 h. The half-life for NR mRNA under these conditions was estimated to be less than 5 min, compared with 120 min for NR protein. Such rapid decreases in NR mRNA suggested a degradation and/or cessation in NR mRNA transcription. No apparent difference in NR mRNA-specific RNAase activity of crude cell extracts (NR-induced or repressed) was observed. However, a significant difference in the susceptibility to degradation of NR mRNA from long-term nitrate-grown cells compared with the NR mRNA isolated from short-term induced cells (20 min in nitrate) was observed. NR mRNA isolated from long-term-nitrate-grown cells was completely degraded by RNAases in cell extracts under conditions in which the NR mRNA isolated from short-term induced cells was resistant to degradation. These results suggest that mRNA stability may be an important factor in the metabolic regulation of assimilatory nitrate reductase in Chlorella.