No abstract

Patrick, Sheila & Holding, A.J. 1985. The effect of bacteria on the solubilization of silica in diatom frustules. Journal of Applied Bacteriology59, 7–16. Natural bacterial populations in samples of water from Loch Leven and Lough Neagh increased the rate of solubilization of frustule silica from an axenic Cyclotella meneghiniana culture, compared with sterile autolysis, at 25d̀C. In the inoculated cultures 50–60% of the silica was solubilized over a period of 30 d. Bacterial populations in Loch Leven water also enhanced the solubilization of silica from non‐axenic cultures of Asterionella formosa, Tabellaria flocculosa, Navicula pellicu‐losa and C. meneghiniana, compared with control cultures sterilized with mercuric chloride. Similar results were obtained with Lough Neagh populations incubated with A. formosa. In comparison with untreated cells, the treatment of diatom cells with ultra‐sonication did not increase the release of silica. Pure cultures of bacteria from Loch Leven water enhanced the release of silica from non‐axenic A. formosa and axenic C. meneghiniana compared with sterile control treatments. The variation in the ability of cultures to solubilize the frustule silica appeared to be related to their potential to produce hydrolytic enzymes. Natural populations of Loch Leven and Lough Neagh water bacteria and certain bacterial cultures caused the diatoms to aggregate, which did not enhance the release of silica.

No abstract

Rhizosphere Root elongation Root hairs Trifolium repens White clover Summary Effects of factors associated with soil acidity (low pH, low calcium, high aluminium and high manganese) on the Trifolium repens-Rhizobium trifolii symbiosis were investigated under laboratory conditions using an axenic solution:culture technique. 200 uM manganese increased root elongation in the range pH 4.3-5.5, but had no effect on root hair formation, the number of Rhizobium in the thizosphere, or nodule formation. Root elongation and root hair formation were unaffected at pH 4.3 when 500 or 1000 #M calcium was supplied, whereas multiplication of Rhizobium in the thizosphere and nodulation were inhibited at pH 4.3 and 4.7.50-1000 #M calcium had no effect either on the multiplication of Rhizobium in the range pH 4.3-5.5, or on nodule formation in the absence of aluminium.50 #M aluminium inhibited root elongation and root hair formation at pH 4.3 and 4.7; the effect on root elongation was reduced by increasing the calcium concentration from 50 to 1000#M. 50#M aluminium also inhibited Rhizobium multiplication in the dfizosphere and reduced nodule formation at pH 5.5 (at which aluminium precipitated out of solution), but root elongation and root hair formation were unaffected. These effects of aluminium at pH 5 .5 may explain the poor response to inoculation by white clover in acid mineral soils after liming.

Nitrate added to Lake Mendota sediment samples incubated under anoxic conditions disappeared rapidly. When 15NO3 was added, 15N2 appeared in the atmosphere within 24 hours. Gas collected from sediments in situ contained from 24 to 50% N2 and from 45 to 75% CH4. Sediment samples from three sites in the lake varying in depth of water column were incubated under He in the laboratory. Production of N2 and CH4 increased with time at a low (2 ppm) level of NO3‐N addition, but CH4 production was inhibited for at least 27 days by addition of 100 to 1000 ppm NO3‐N. Production of N2 was less at 1000 than at 500 ppm NO3‐N added, probably due to suppression of the nitric oxide reductase enzyme system. Evidence was obtained that the denitrification pathway in lake sediments is the same as that reported for submerged soils. namely: NO3− → NO2− →→ N2O → N2.