Humic materials offer photoprotective effect to Escherichia coli exposed to damaging luminous radiation

The behavior of Escherichia coli immersed in aqueous systems amended with h umic acids, under PAR, UV-A, UV-B, and simulated solar radiation was examin ed. Culturability, ability to elongate, functioning of the electron transpo rt systems, and glucose uptake were assessed. Humic substances in the range from 1 to 50 mg L-1 protected cells from photoinactivation. Decrease in cu lturability and cellular activities was significantly (P < 0.05) less in th e presence of humic material. However, humic acids were not used as nutrien ts. Neither irradiated nor nonirradiated humic solutions (50 mg L-1) suppor ted the growth of 10(5) cells ml(-1). However, humic acids dissolved in 0.9 % NaCl efficiently absorbed light over wavelengths from 270 to 500 nm. Also , a photoprotective effect against simulated sunlight was observed when hum ic acids were not in contact with but rather enveloped the cellular suspens ions in double-wall microcosms. The protection afforded by humic acids agai nst luminous radiation likely derives from their ability to absorb these ra diations and hence reduces the amount of energy reaching the cells.