Ian Morris

The inhibition of nitrate assimilation by ammonium in chlorella

Chlorella vulgaris growing with ammonium nitrate as nitrogen source preferentially assimilates ammonium. Nitrate assimilation ceases completely when ammonium is added and recommences as soon as ammonium has disappeared. Ammonium does not inhibit nitrate reduction by cells unable to assimilate ammonium because they lack a carbon source. Thus the inhibition is not due to ammonium per se but is connected with its assimilation. The inhibition is not thought to result from competition for reduced pyridine nucleotide because nitrate reductase of Chlorella is specific for DPN while glutamic dehydrogenase is specific for TPN. Nitrite addition also inhibits nitrate assimilation completely but ammonium only partially inhibits nitrite assimilation.

The development of nitrate reductase in Chlorella and its repression by ammonium

SummaryChlorella vulgaris, grown with ammonium sulphate as nitrogen source, contains very little nitrate reductase activity in contrast to cells grown with potassium nitrate. When ammonium-grown cells are transferred to a nitrate medium, nitrate reductase activity increases rapidly and the increase is partially prevented by chloramphenicol and by p-fluorophenylalanine, suggesting that protein synthesis is involved. The increase in nitrate reductase activity is prevented by small quantities of ammonium; this inhibition is overcome, in part, by raising the concentration of nitrate. Although nitrate stimulates the development of nitrate reductase activity, its presence is not essential for the formation of the enzyme since this is formed when ammonium-grown cells are starved of nitrogen and when cells are grown with urea or glycine as nitrogen source. It is concluded that the formation of the enzyme is stimulated (induced) by nitrate and inhibited (repressed) by ammonium.

THE PATH OF CARBON IN PHOTOSYNTHESIS BY MARINE PHYTOPLANKTON12

Using cultures of a number of different marine algae (diatoms Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin, chrysophyte Isochrysis galbana Parke, green flagellate Dunaliella tertiolecta Butcher, dinoflagellate Gonyaulax tamarensis Lebour) the short‐term, pattern of 14CO2 assimilation has been investigated. In all except D. tertiolecta the labelling of amino acids and intermediates of the tricarboxylic acid (Krebs) cycle was significantly heavier than that of sugar phosphates. Over periods of 30–120 s labelling in amino acids and Krebs cycle intermediates accounted for 41–95% of the 14C fixed (depending on the alga). Over shorter times (< 10 s) the pattern in the 2 diatoms showed significant labelling of C4 acids (and related com‐pounds) and little labelling of sugar phosphates. The reverse wits seen with D. tertiolecta. Also, in the 2 diatoms and in G. tamarensis significant inhibition of photosynthesis by oxygen could only be achieved with 100% oxygen; atmospheric levels having little effect. Parallel measurements of 2 carboxylating enzymes showed that ribulose‐1,5‐diphosphate carboxylase (RuDPCase) was significantly greater than phospho (enol)pyruvate carboxylase (PEPCase) activity only in the green flagellate. It is suggested that photosynthesis in marine diatoms depends on an active PEPCase utilizing bicarbonate as a substrate and that a less active RuDPCase utilizes CO2. In D. tertiolecta the pattern more closely resembles that of a “Calvin (C3)” plant. The dinoflagellate and the chrysophyte appeared to show a mixed C3 and C4 photosynthesis.

EFFECTS OF ENVIRONMENTAL FACTORS ON PHOTOSYNTHESIS PATTERNS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE). I. EFFECT OF NITROGEN DEFICIENCY AND LIGHT INTENSITY1

Cultures of the marine diatom Phaeodactylum tricornutum Bohlin incorporated, a large proportion of the total fixed carbon (50% or more) into amino acids and amides during short periods of photo‐assimilation of 14C‐labelled carbon dioxide. Although increasing nitrogen limitation in a nitrate‐limited chemostat had little significant effect on the proportion of C incorporated into amino acids and amides combined, it did affect the distribution of radioactivity within individual compounds of this group. In particular, increasing degrees of N deficiency reduced the proportion incorporated into amides to almost undetectable levels, reduced the proportion in alanine and increased the proportion in glutamic acid. Also, increasing N limitation decreased the relative synthesis of sugar phosphates and increased the proportion of C assimilated into intermediates of the tricarboxylic acid cycle. Reduced light intensity did not have any significant effect on the proportion of C incorporated into the total amino acids and amides, but did cause a decrease in the radioactivity

EFFECTS OF ENVIRONMENTAL FACTORS ON PHOTOSYNTHESIS PATTERNS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE). II. EFFECT OF OXYGEN1

Oxygen inhibited the rate of light‐saturated photosynthesis of the marine diatom Phaeodactylum tricornutum Bohlin. However, inhibition could only be detected with O2 concentrations approaching 100%. Atmospheric concentrations of O2 (21%) had little effect on photosynthesis. In this, Phaeodactylum more closely resembles the so‐called C‐4 plants which show low rates of photorespiration. The results presented here agree with others in showing increased O2 inhibition at reduced bicarbonate concentrations. The biochemical mechanism of photorespiration in Phaeodactylum appears to be similar to that reported for other photosynthetic systems.

Photosynthetic Carbon Dioxide Assimilation by Rhodospirillum rubrum

SummaryAlthough Rhodospirillum rubrum, grown photoheterotrophically on malate, assimilates carbon dioxide less rapidly than it does when grown autotrophically, the difference is less marked than previously suggested.The rate of photoassimilation of carbon dioxide varies during batch culture on malate, reaching a maximum at about mid-exponential phase. It also varies with density and growth rate in a turbidostat continuous-flow culture on malate and increases with decreasing growth rate in a chemostat continuous-flow culture growing with limiting malate concentrations.The changing rates of carbon dioxide photoassimilation during photoheterotrophic growth under the various conditions are paralleled by changing activities of ribulose diphosphate carboxylase.Under conditions of maximum carbon dioxide fixation the rate by photoheterotrophic cultures approaches that shown by the bacterium growing autotrophically and is assimilated eight to ten times more slowly than is malate in chemostat cultures.The rate of carbon dioxide fixation also increases to that shown by autotrophic cells when photoheterotrophic cultures are deprived of malate, but without subjecting them to the conditions required for autotrophic growth.

The effects of 2,4-dinitrophenol and other uncoupling agents on the assimilation of nitrate and nitrite by Chlorella

Abstract 1. 2,4-Dinitrophenol and five other uncoupling agents (at concentrations of 1.0 mM) inhibit the utilization of nitrate by about 80–95%. The inhibitions can be relieved by increasing nitrate concentrations. 2. The uncoupling agents fail to inhibit the activity of nitrate reductase in cell-free extract. 3. Inhibition of nitrite utilization by the six uncoupling agents depends in the experimental conditions. Under heterotrophic conditions, all six inhibit. When cultures are illuminated (particularly in the absence of CO 2 ) three of the uncouplers are much less effective as inhibitors than they are under heterotrophic conditions.

Inhibition of nitrate and nitrite reduction by 2,4-dinitrophenol in ankistrodesmus

SummaryAs Kessler (1955, 1959) has shown, nitrite reduction by the green alga, Ankistrodesmus braunii is completely inhibited by 10-3m 2,4-dinitrophenol. However, although nitrite accumulates in the medium when cultures are supplied with nitrate and dinitrophenol, the reduction of nitrate is not completely insensitive to the inhibitor.Direct measurements show that 2,4-dinitrophenol inhibits nitrate disappearance from the medium by 65–80%. The degree of inhibition increases when the initial nitrate concentration is decreased.It is suggested that inhibition of nitrate assimilation by dinitrophenol is due to inhibition of an active uptake of nitrate by the cells, and that at high nitrate concentrations, a dinitrophenol-insensitive uptake process increases in importance.

The pathway of carbon dioxide assimilation in Rhodospirillum rubrum grown in turbidostat continuous-flow culture.

SummaryA comparison of light and dark short-term incorporation of [14C]-carbon dioxide by Rhodospirillum rubrum grown in turbidostat continuous-flow culture at two different steady states on medium containing malate has shown that the labelling of phosphate esters was the main light-dependent process. Thus, the reductive pentose phosphate cycle appears to be the major pathway of carbon dioxide assimilation in the light under these growth conditions.The labelling of glutamate was also light-dependent and was most marked in the most rapidly growing steady state culture.The assimilated [14C]carbon was transferred to metabolites of the tricarboxylic acid cycle, particularly C4-dicarboxylic acids, and the transfer involved additional carboxylations which were not light-dependent. The activity of these reactions accounted for initial high rates of carbon dioxide assimilation in the dark.In the dark assimilated [14C]carbon accumulated in succinate.

Some effects of chloramphenicol on the metabolism of chlorella

SummaryChloramphenicol at concentrations of 1–3 mg per ml does not show specific inhibition of protein synthesis in Chlorella. Thus, although incorporation of 14C-labelled phenylalanine, carbon dioxide and glucose into the protein fraction is inhibited by chloramphenicol, the antibiotic also prevents incorporation of 14C-carbon dioxide and 14C-glucose into polysaccharide, together with incorporation of 14C-adenine into RNA and DNA fractions.The inhibitory effects are not specific for macromolecule biosynthesis since incorporation of 14C-carbon dioxide and 14C-phenylalanine into an alcohol-soluble fraction, and that of 14C-adenine into a cold acid-soluble fraction are also inhibited.Incorporation of 14C-glucose into the alcohol-soluble fraction, and incorporation of 14C-uracil into the cold acid-soluble and RNA fractions are insensitive to the antibiotic. The respiration of 14C-glucose is inhibited by only 20% with a chloramphenicol concentration of 3 mg per ml.