Presence of HCN in chlorella vulgaris and its possible role in controlling the reduction of nitrate

Production and release of selenocyanate by different green freshwater algae in environmental and laboratory samples

In a previous study, selenocyanate was tentatively identified as a biotransformation product when green algae were exposed to environmentally relevant concentrations of selenate. In this follow-up study, we confirm conclusively the presence of selenocyanate in Chlorella vulgaris culture medium by electrospray mass spectrometry, based on selenium's known isotopic pattern. We also demonstrate that the observed phenomenon extends to other green algae (Chlorella kesslerii and Scenedesmus obliquus) and at least one species of blue-green algae (Synechococcus leopoliensis). Further laboratory experiments show that selenocyanate production by algae is enhanced by addition of nitrate, which appears to serve as a source of cyanide produced in the algae. Ultimately, this biotransformation process was confirmed in field experiments where trace amounts of selenocyanate (0.215 ± 0.010 ppb) were observed in a eutrophic, selenium-impacted river with massive algal blooms, which consisted of filamentous green algae (Cladophora genus) and blue-green algae (Anabaena genus). Selenocyanate abundance was low despite elevated selenium concentrations, apparently due to suppression of selenate uptake by sulfate, and insufficient nitrogen concentrations. Finally, trace levels of several other unidentified selenium-containing compounds were observed in these river water samples; preliminary suggestions for their identities include thioselenate and small organic Se species.

Nitrate reductase of Chlorella fusca: Partial purification, cytochrome content and presence of HCN after in vivo inactivation

An inactivated nitrate reductase (EC 1.6.6.1) formed in vivo by the green alga Chlorella fusca Shihira and Kraus is shown to be a cyanide complex. The partially purified inactive enzyme releases 0.048 nmol of HCN per unit of enzyme activated. This compares with 0.066 nmol of HCN liberated in similar previous measurements with the inactivated enzyme from Chlorella vulgaris. The nitrate reductase from C. fusca has been purified to a level of 67 μmol nitrate reduced per min per mg enzyme. It contains a cytochrome b557, at a level 1.9-fold higher per unit of active enzyme, than the nitrate reductase from C. vulgaris.

The effect of cyanide and some other carbonyl binding reagents on glycolate excretion by Chlorella vulgaris

The excretion of glycolate by illuminated Chlorella vulgaris cells at low CO2 tension can be stimulated about tenfold by substituting O2 for air, or by addition of cyanide, hydroxylamine, hydrazine or semicarbazide to the cells in air. For each reagent there is a concentration range giving a maximum effect. It is proposed, as a working hypothesis, that the HCN formed internally when the cells are illuminated in O2, may cause the glycolate excretion.

Reversible inactivation of nitrate reductase in Chlorella vulgaris in vivo

The NADH-nitrate oxidoreductase of Chlorella vulgaris has an inactive form which has previously been shown to be a cyanide complex of the reduced enzyme. This inactive enzyme can be reactivated by treatment with ferricyanide in vitro. In the present study, the activation state of the enzyme was determined after different prior in vivo programs involving environmental variations. Oxygen, nitrate, light and CO2 all affect the in vivo inactivation of the enzyme in an interdependent manner. In general, the inactivation is stimulated by O2 and inhibited by nitrate and CO2. Light may stimulate or inhibit, depending on conditions. Thus, the effects of CO2 and nitrate (inhibition of reversible inactivation) are clearly manifested only in the light. In contrast, light stimulates the inactivation in the presence of oxygen and the absence of CO2 and nitrate. Since the inactivation of the enzyme requires HCN and NADH, and it is improbable that O2 stimulates NADH formation, it is reasonable to conclude that HCN is formed as the result of an oxidation reaction (which is stimulated by light). The formation of HCN is probably stimulated by Mn(2+), since the formation of reversibly-inactivated enzyme is impaired in Mn(2+)-deficient cells. The prevention of enzyme inactivation by nitrate in vivo is in keeping with previous in vitro results showing that nitrate prevents inactivation by maintaining the enzyme in the oxidized form. A stimulation of nitrate uptake by CO2 and light could account for the effect of CO2 (prevention of inactivation) which is seen mainly in the presence of nitrate and light. Ammonia added in the presence of nitrate has the same effect on the enzyme as removing nitrate (promotion of reversible inactivation). Ammonia added in the absence of nitrate has little extra effect. It is therefore likely that ammonia acts by preventing nitrate uptake. The uncoupler, carbonylcyanide-m-chloro-phenylhydrazone, causes enzyme inactivation because it acts as a good HCN precursor, particularly in the light. Nitrite, arsenate and dinitrophenol cause an enzyme inactivation which can not be reversed by ferricyanide in crude extracts. This suggests that there are at least two different ways in which the enzyme can be inactivated rather rapidly in vivo.