COM MUN l C A T l 0 N
Use of Gamma Radiation to Control Algae Stephen D. Morton and Philip H. Derse Wisconsin Alumni Research Foundation, Madison, Wis.
53701
rn Gamma radiation is one possible method of controlling objectionable growth of algae. As a first step in determining its feasibility, the bloom-forming blue-green algae, Anabaena, Aphanizonienon, and Microcystis. the green alga, Chlorella, and the diatom, Cornphonemu. were irradiated with a cobalt-60 source. Viabilities as a function of radiation dosage decreased with increasing dosage, though usually not linearly. The dosage-viability results were not dependent on the concentration of algae irradiated tip to about 200 mg. per liter and only slightly dependent on the algal species. in contrast to chemical treatments. For significant control of algal growth under ideal laboratory conditions. a dosage of about 100 t o 150 krads is needed. The possible use of spent nuclear fuel from nuclear power plants in large scale applications is discussed.
T
he control of objectionable growths of algae in lakes, streams, cooling towers, reservoirs, and other areas appears t o be a problem that may exist for some time. Much research has been done on the use of gamma radiation t o control bacterial and fungal growth in foods. Gamma radiation should be considered as a possible method of algae control, as it appears to have few residual effects and the dosage needed is independent of the algal concentration. Considerable work has been done on radiation of the green alga, Ch/orellu. Ozenda (1965) found 70 to 100 krads was a 100 lethal dosage for Chlorell~i and Scenedesmus. Zakharov and Tugarinov (1964) found a n L D S oof 14.5 krads and no growth at 80 krads for Chlorelku. Gileva, Timofeeva, et a/. (1965) could not grow Chlorellu above 25 krads. Posner (1965) and Posner and Sparrow (1964) found a n L D g 0 for Chlorella of 25 krads; chronic radiation of 7 krads per day reduced growth t o 5 0 x . Few radiation studies have been made on the more economically significant forms of algae, such as the obnoxious bloomforming blue-greens.
The study described here was undertaken as the first step in determining the feasibility of using gamma radiation t o control algal growth in various problem areas. Procedure
Approximately 1.2 cc. of algal suspension is irradiated in a 12 x 35 mm. plastic-capped glass bottle. A 500-curie cobalt-60 source is used, which provides 2 X IO6 rads per hour of approximately 1-m.e.v. gamma radiation adjacent t o the source. Because of the small size of the sample bottles. the radiation can be considered homogeneous throughout the sample. All radiations are done at room temperat tire, approximately 23" C. Sample radiation dosage is controlled by the time of exposure. Within one hour after irradiation, 1.0 cc. of each sample is put into 24 cc. of medium, and grown under continuous fluorescent lighting. In all experiments a control was carried through all procedures except irradiation. The controls are the zero dosages in the data. Spectrophotometric measurements are made at 600 mp, using a Bausch & Lomb Spectronic 20 with sample tubes of 12-mni. diameter. At an absorbance of 0.10, the algae or diatom concentration is about 5,000,000 cells per m]., about 50 mg. per liter. An absorbance of about 0.02 (1,000,000 cells per ml. or 10 mg. per liter) corresponds to a heavy algal bloom in a eutrophic lake. The algae used were Anabaena circinalis (Wis. 1038), Aphanizoinenon ~70saquae (Wis. 1062), Chlorella pyrenoidosa (Wis. 2005), Gotnphonema sp. (Wis. 1134), and Microcj.stis aeruginosa (Wis. 1036). Allen's (1952) medium (pH 7) is used for Chlorella and Gorham's (Hughes, Gorham, et at., 1958) medium (pH 8 to 9) is used for the others. Both media are essentially dilute, inorganic, aqueous solutions. Results and Discussion
I n the data summarized in Tables I and 11, the initial absorbance refers to the absorbance of the original irradiated algae. The absorbance attained is that of the growing cultures, which are 1.0 cc. of the irradiated algae added t o 24 cc. of the medium.
Volume 2, Number 11, November 1968
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Table 1. Growth of Chlorella and Aphunizomenon from Suspensions of Different Initial Absorbances (I.A.) after 1-M.E.V. Gamma Irradiation Absorbances Attained
-__
Dosage, Krads
0.12 I.A.
0.035 I.A.
0.015 L.4.
0.0046 1.4.
Chlorekia 0 37 75 100 150 250 500
0 25 50 100 200 500 b
0.11 0.09 0.07 0.07 0.02 0.00 0.00 I days
0.11 1..4. 25 dags
0.22 0.19 0,065 1.8 >1.8 >1.5 1 . o 0.050 0.01 0.00 0.00
-
8 days
0 0 0 0 0 0
075 040 00 00 00 00
10 07 04 00
0.09 0.07 0.03 0.02 0.00 0.00 0 00
00 00 00
0.022 I.:t. 25 days
0 0 0 0 0 0
24 24 053 00 00 00
0.0080 I..%. 25 days 31 days
0 080 0.078
