ORNITHINE CYCLE ENZYMES IN THE BLUE-GREEN ALGA NOSTOC MUSCORUM

Cell-free extracts of a bacteria-free culture of Nostoc muscorumwere used to demonstrate the occurrence of part of the KREBS-HENSELEITornithine cycle in blue-green algae. Evidence is presented forthe conversion of ammonia and bicarbonate to carbamyl phosphateand the coupling of carbamyl phosphate to ornithtne to yieldcitrulline. Attempts to demonstrate the conversion of citrullineto arginine were not successful. ¹Present address: Scripts Institute of Oceanography, Universityof California, La Jolla, California, U.S.A. ²Present address: Department of Biochemistry and Nutrition,The University of Texas Medical Branch, Galveston, Texas, U.S.A.     

Potential Sources of Error in the Measurement of Low Rates of Planktonic Photosynthesis and Excretion

THE introduction of liquid scintillation counting techniques to the study of aquatic productivity^1–3 has stimulated interest in the accurate determination of low rates of algal photosynthesis, such as occur in tropical waters or in deep living phytoplankton populations. Attention is also being directed to the measurement of soluble extracellular products of photosynthesis⁴. Our studies have revealed some potential sources of interference in the widely used ¹⁴C technique when used to study low rates: (1) retention of NaH¹⁴C0₃ by filters; (2) passive adsorption of radioactivity by filters and/or seston and (3) soluble and insoluble radioactive impurities in commercially available NaH¹⁴C0₃.     

ATP levels in algal cells as influenced by environmental conditions

The cellular content of adenosine triphosphate (ATP) relativeto cell size and cellular organic carbon has been investigatedin 30 different algal cultures representing 7 phyla. Duringexponential growth in batch culture, cellular contents of ATPremained at fairly uniform levels in all these unicellular algaeand averaged 0.35% of the cellular organic carbon content. Duringextreme nitrogen or phosphorus deficiency the cellular levelsof ATP decreased to 20–50% of that found in exponentially-growingcells, but these percentages may be low due to detrital carbonin the senescent cultures. The steady state levels of ATP in cells were similar in lightor in dark, although ATP concentrations fluctuated for a fewminutes upon any sudden change in light conditions. When thelight was turned on there was a rapid increase in ATP levels,followed by a slow decrease; when the light was turned off,there was a rapid fall in cellular ATP levels, which then rosewithin a few minutes to achieve the steady state concentration.The cellular concentrations of ATP in these algae and in othermicrobial groups are discussed relative to studies where ATPdeterminations are used to estimate microbial biomass. (Received June 2, 1970; )     

Effects of Mineral Salts on Short-term Incorporation of Carbon Dioxide inChlorella

The effects of various mineral salts have been studied, bothsingly and in combination, on the uptake and distribution ofphotosynthetically incorporated carbon-14 in Chlorella. Whenmagnesium sulphate, potassium nitrate, and potassium phosphatewere combined in the same concentrations as are found in thenormal nutrient solution, the uptake of carbon-14 was increasedand more radioactivity was incorporated into the amino-acids,with a decrease in the percentage of radioactivity in the sugarphosphates when compared to the controls in distilled water.The most striking effect of the addition of just one salt wasobtained with salts containing nitrogen, particularly ammoniumchloride, which in one experiment increased the radioactivityfound in the amino-acids from 9.9 per cent. (expressed as percentageof the total soluble radioactivity found in this group of compounds)in the controls to 57 per cent., while the radioactivity inthe sugar phosphates fell from 64 to 7 per cent., correspondingly.The total uptake of carbon dioxide by the cells supplied withammonium chloride was three times as great as that in the controlsin distilled water. The relevance of these findings to the methodology involvedin experiments dealing with the path of carbon in photosynthesisis discussed, with particular emphasis upon the concept of a‘steady state’ and how it is thought this may bestbe achieved.     

Formation of Radioactive Citrulline during Photosynthetic C14O2-Fixation by Blue-Green Algae

Citrulline has been isolated and identified from extracts ofNostoc muscorum. All members of the Cyanophyceae hitherto investigatedshow a relatively large amount of the CO₂ fixed during photosynthesisin citrulline (ranging as high as 20 per cent. in Nostoc) whencompared to the trace amounts found in the Chlorophyceae. Nostocalso has the ability to fix C¹⁴ in citrulline during dark fixation,but at a rate slower than in light. As no free urea or arginine was found in Nostoc, it is likelythat citrulline is functioning in reactions other than thoseleading to arginine and urea synthesis. Other possible functions for citrulline are briefly discussed.     

BIOTIN LIBERATION BY THE LICHEN ALGA COCCOMYXA SP. AND BY CHLORELLA PYRENOIDOSA

The unicellular green alga Coccomyxa, a component of the lichenPeltigera aphthosa, liberated about 7.2mµg biotin permg dry weight of cells into the culture medium during a growthperiod of 15–20 days. The corresponding figure for thefree-living alga Chlorella pyrenoidosa was 0.45mµg ofbiotin. Chromatographic analysis indicated that this was freebiotin and not a bound form of the vitamin. The biotin concentrationof rinsed Coccomyxa cells was 1.88mµg per mg dry weightof cells, of which less than 0.01mµg was extractable byhot water. Cells of Chlorella contained 0.16mµg of biotinper mg dry weight, of which 0.11mµg was extractable byhot water. The biotin content of Coccomyxa, which was about12 times that of Chlorella, is thus almost entirely in the boundform. The importance of biotin in the symbiotic interactionsbetween the alga and the fungus in Peltigera is discussed. ¹Present address: University Department of Agriculture, Oxford,England. ²Present address: Institute of Marine Resources, Universityof California, La Jolla, California, U.S.A.