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36 Storey'

s Way, Cambridge CB3 0DT, England. DISCUSSION Nucleopore membranes were used because bacteria pass through the cylindrical pores of these thin (lO #m) polycarbonate membranes more easily than across the complicated pore systems of thicker cellulosic filters. The back- washing enabled bacteria and algae which had settled on the membrane between the pores to be resuspended in the liquid culture. This gave a more efficient filtration of bacteria and lowered the bacteria-algal ratio. Several manufactures of membrane filters now produce filtration cells which incorporate a magnetic stirrer above the membrane to increase their efficiency. These simpler systems may provide a more convenient, though more expensive, filtration apparatus than the one described here. Both the Aphanizomenon and Asterionella cultures used here had cells with little or no mucilage, and with the exception of the heterocysts of Aphani- zomenon, epiphytic bacteria were not observed. Because bacteria were often found attached to the heterocysts, filaments with these cells were not isolated. Many species of algae, however, have large mucilagenous sheaths containing bacteria which would not be removed by filtration alone. This difficulty has

174 S. I. HEANEY AND G. H. M. JAWORSKI been overcome for certain mucilagenous blue-green algae by McCurdy &

Hodgson (1974) who used glass bead dispersion to separate contaminating bacteria from the algal mucilage. The procedure of isolating filaments on to agar culture medium has the advantage that the growth of individual filaments or cells may be followed, and bacterial contamination observed more readily. Several workers have tried to purify Asterionella but have failed to maintain pure clones for long periods (Provasoli, 1958;

Hughes &

Lund, 1962). Hughes &

Lund (t962) found that growth in their pure culture was erratic compared to growth in cultures with bacteria present. These difficulties have not been experienced with the clones obtained in this work and may be due to the improved culture medium. ACKNOWLEDGEMENTS We wish to thank A. Foster and P. Webster for technical assistance and G. Hall who performed the bacteriological tests with the Asterionella culture. We also thank Dr J. W. G. Lund, F.R.S. for his comments and criticisms of the manuscript. REFERENCES ALLEN,M. B., 1952.The cultivation of Myxophycae. Arch. Mikrobiol., 17: 24~,5. Cau, S., 1942. The influence of the mineral composition of the medium on the growth of planktonic algae. Part 1. Methods and culture media. J. Ecol., 30: 284-325. COLLINS,V. G. &

WILLOUGHBY, L. G., 1962.The distributionof bacteria and fungal spores in Blelham Tam with particular reference to an experimental overturn. Arch. MikrobioL, 43: 294-........