6.3.1 GROUP I: TREATMENTS INVOLVING MAGNETIC FIELD PREDOMINANCE

Experiments involving a predominant magnetic field have been conducted on a range of microorganisms that represent both prokaryotes (eubacteria, archaea) and eukaryotes (algae, fungi, protozoa).

6.3.1.1 GROWTH

Growth is a physiological response of an organism and a positive effect on growth indicates that some of the biosynthetic pathways are being stimu­lated. Erygin et al. [15] grew a gram-positive bacterium Bacillus mucilagi­nous in a magnetic field of ~0.26 T under different media compositions and compared it with unexposed control cultures. The magnetically treated liquid medium consisting of ferromagnetic salts showed rapid growth of the bacterium over control in 3 h. Similarly magnetically treated dry whey medium yielded three times higher cell count than the untreated medium. However, there was an overall increased response from the exposed dry whey illustrating how the culture medium composition may influence the effect of magnetic field.

Moore [17] studied five strains of bacteria and a yeast under a magnetic flux of 5-90 mT and reported maximum stimulation of growth at 15 mT (at 0.3 Hz) and maximum inhibition at 30 mT. Experiments with varying time especially using oscillating magnetic fields have uncovered new ef­fects related to resonant phenomena in the living systems. The biostimula­tion of a denitrifying gramnegative bacterium Pseudomonas stutzeri by a magnetic field of 0.6-1.3 mT pulses via inductively coupled Helmholtz coils for 8-10 h resulted in a proliferation of biomass that was 10-30% more than the control [18].

Other than the medium conditions, magnetic flux and type of magnetic field, the exposure time is another major factor that governs the intensity of response. Justo et al. [8] observed that the growth of Escherichia coli could be stimulated or inhibited by exposure to an oscillating 100 mT extremely low frequency (ELF) magnetic field for 6.5 h. Exposed cells had 100 times greater viability than unexposed cells, however the viability varied with duration of exposure. It was suggested that the effect was a result of magnetic field driven alteration of membrane permeability and availability of ions in the culture medium.

Research groups in Japan and China have focused on investigating ways to improve the cultivation of the cyanobacterium Spirulina platensis for production of nutraceuticals using permanent magnetic fields. Hirano et al. [26] reported a significantly higher specific growth rate of 0.22 d-1 in S. platensis exposed to 10 mT magnetic field when compared to 0.14 d-1 for untreated culture. The growth of S. platensis was maximum when it was cultured phototrophically at lower light intensities; but did not show improvement under heterotrophic conditions.

Magnetic field induced growth stimulation in S. platensis has also been reported by Li et al. [27]. They observed a 47% increase in dry biomass on the sixth day of cultivation, and a 22% increase over control by day eight under the exposure of a 250 mT homogeneous magnetic field from a Helmholtz coil.

Chlorella vulgaris is another algal strain of interest for its nutraceutical value and is a promising producer of starch-glucose. This microalga can yield starch to the tune of 60 t ha-1 yr-1 which is 7.7 times more than that of traditional corn [50]. Takahashi et al. [31] used magnetic flux densities of 6-58 mT for cultivating Chlorella sp. and obtained facilitative growth at 40 mT. The specific growth rate of Chlorella vulgaris almost doubled from

0. 07 to 0.12 d-1 under magnetic field generated using a dual-yoke electro­magnet, which concentrates a magnetic field into a small cross-sectional area [30].

The static magnetic field strengths ranging from 0 to 230 mT on Du — naliella salina were used by Yamaoka et al. [32]. They observed an im­provement in growth rate that peaked at 10 mT with the addition of 1 mg L-1 of Fe-EDTA. A ~0.26 T magnetic field exposure using different growth media for the yeast Saccharomyces fragilis showed that rapid growth (27- 36% over the control in 3 h) occurred on magnetic treatment when a dry whey nutrient medium was used, but it turned inhibitory on using a liquid nutrient medium [15]. On the other hand Fiedler et al. [36] used an oscil­lating magnetic field generated by a Helmholtz coil via inductive coupling to produce 0.28-12 mT magnetic field at 50 Hz for 9 h to treat S. cerevisi — ae. They observed a maximum growth of 8 g L-1 of biomass under 0.5 mT magnetic field exposure and 6.8 g L-1 of biomass for the cells untreated.