A sheet of 0.13-mm cellulose diacetate covered the plates to avoid medium dehydration. Spectrophotometric (Ocean Optics USB 2000 Spectroradiometer, Dunedin, FL) readings of the 290–750 nm output of the lamps that passed through the diacetate film plus the Petri dish lid were 5.4 W m−2 (Fig. 1), and the spectrum was almost identical to that of light passing through the diacetate, but without the Petri dish lid. Conidia were
also produced on a basal medium [minimal medium (MM)] 0.2% NaNO3, 0.1% K2HPO4, 0.05% MgSO4, 0.05% KCl, 0.001% FeSO4, and 1.5% Bacto agar (Becton, Dickinson and Co., Sparks, MD) adjusted to pH 6.9 under continuous dark, a condition that improves conidial tolerance of M. robertsii to UVB radiation and heat (Rangel et al., 2006a, b, PS-341 clinical trial 2008). Conidial suspensions (100 μL of 107 conidia mL−1) were inoculated evenly with a glass spreader onto agar media. The cultures were incubated at 28 ± 1 °C for 14 days. Three different batches of conidia were produced, one for each replication of the experiment.
The inoculum for each pair of treatments (UV and heat) was prepared for simultaneous exposures. Conidia were harvested after 14 days from colonies grown under continuous visible light or in the dark with a single pass of a microbiological loop through the spore layer of the fungal colonies without touching the HSP targets substrate, and the conidia immediately suspended in 10 mL of sterile Tween 80 solution (0.01% v/v) in 15-mL polystyrene tubes (Corning®, Corning, NY). The suspensions (c. 105 conidia mL−1) were shaken vigorously using a vortex; filtered through a polycarbonate membrane (25 mm diameter, 8-μm pore size, Whatman® Nucleopore®, Acton,
MA) to remove spore aggregates and mycelium; and the suspension was used immediately in the heat- and UVB-exposure experiments. UVB irradiation experiments were conducted at 28 ± 1 °C in a Percival growth chamber (Boone, IA), with two UVB fluorescent lamps (TL 20W/12 RS, Philips, Eindhoven, Holland), with Bay 11-7085 primarily UVB (peak at 313 nm) and minimal UVA radiation output. The Petri dish lids were removed during irradiation, but the plates were covered with cellulose diacetate filters (JCS Industries, Le Miranda, CA) to exclude UVC and short-wavelength UVB radiation. Spectral irradiance was measured as in Rangel et al. (2005a, b). The DNA damage (cyclobutane pyrimidine dimer formation) action spectrum developed by Quaite et al. (1992) and normalized to unity at 300 nm was used to calculate weighted UV irradiances in the chamber at sample height, which was 978 mW m−2. The total 2-h exposure afforded a dose of 7.04 kJ m−2.