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The question of how electromagnetic fields--static or low to high frequency--interact with biological systems is of great interest. The current discussion among biologists, chemists, and physicists emphasizes aspects of experimental verification and of defining microscopic and macroscopic mechanisms. Both aspects are reviewed here. We emphasize that in certain situations nonthermal interactions of electromagnetic fields occur with cellular systems.

Microwaves near 42 GHz are found to influence the growth of Saccharomyces cerevisiae. The growth is measured photometrically in stirred aqueous culture. The microwave effect occurs and saturates above a threshold intensity « 10 mW/cm 2 , excluding any explanation based on microwave heating. A surprisingly strong frequency dependence is observed, with resonances as narrow as 8 MHz. These results confirm the existence of a nonthermal resonant microwave sensitivity in biology; they suggest yet unknown tuned systems triggering yet unknown biological actions.The existence of a nonthermal microwave sensitivity in biological systems has not been generally accepted. Experimental evidence has been scarce. Thus low-intensity millimeter waves were reported to affect microbial growth and metabolism 1 " 6 or to reduce x-ray sensitivity of bone marrow cells in mice. 7 Not all of these reports, however, can convincingly exclude that a slight heating might have caused the observed effects. Theoretically, nonthermal microwave effects were conjectured by Frohlich. 8 He discusses a thresholdlike behavior in the metabolic excitation of large-amplitude vibrations; this leads to a storage of energy and a resonant sensitivity to external radiation. 9 Here we report evidence of nonthermal resonant action of millimeter microwaves on the growth of yeast cultures, corroborating our earlier findings. 5 The experiment emphasizes high-frequency stability in order to resolve the unexpectedly narrow resonances; it furthermore provides the use of two greatly differing irradiation geometries to rule out directly artifacts from the microwave system. The procedure 10 used diploid, homozygot, and isogene wild type Saccharomyces cerevisiae grown on agar for three days at 30 °C, then stored at 4°C. Liquid suspensions with starting concentrations near 3x 10 5 cells/ml were held in glass cuvettes equipped both with mechanical stirrers and with submersible Teflon antennas for coupling in microwaves.One such cuvette was placed in the measuring arm each of a Beckman Acta CIII and a Beckman 24 double-beam spectrometer set at 550 nm, while the reference cuvettes in both instruments were filled with plain growth medium. The optical-density output signals V OD were amplified in logarithmic amplifiers and continuously recorded as InVOD vs time t, to give straight lines in case of exponential rise of the signals. This proved to be the case during a growth period of roughly 3 h, so that an exponential growth rate | Li = r i ln7 OD could be read off the plots with ± 1% uncertainty.Care was given to temperature stabilization and power measurements. The cuvette housings were thermostatted with flowing water at 30.7 ±0.1°C. The microwave system included a 1-mlong waveguide which coupled source to sample via an isolator, two directional couplers to monitor forward and backward running power, and finally an impedance transformer adjusted to minimize reflections. Standard procedures ascertained that standing-wave resonances of the system did not exhibit spectral widths ...

Weak microwave irradiation of aqueous yeast cultures was found to affect their growth rate in a frequency-selective manner. Depending on frequency (near 42 GHz), both increases and de­ creases of the growth rate were observed. The resonance bandwidths are of the order of 0.01 GHz. Simple thermal effects can be excluded. These findings support theoretical predictions of coherent molecular oscillations activating metabolic processes.

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