We measured photochemical mineralization of dissolved organic carbon in a humic lake in situ. At a depth of 1 cm, solar radiation mineralized 19 mmol C m Ϫ3 d Ϫ1 . The rate of mineralization decreased with increasing depth with an attenuation coefficient of 23 m Ϫ1 . Consequently, most photochemical mineralization in the water column (0.99 mmol C m Ϫ2 d Ϫ1 ) took place in the top 10 cm. The rate of photochemical mineralization was also modeled as a product of three spectra: (1) scalar photon flux density, (2) the apparent quantum yield ( ), and (3) the absorption of chromophoric dissolved organic matter. We described the spectrum for apparent quantum yield as ϭ c ϫ 10 Ϫ d , where c (dimensionless) and d (nm Ϫ1 ) are positive constants. Mathematical optimization for the best fit between the measured and the modeled photochemical mineralization resulted in of 7.52 ϫ 10 Ϫ0.0122 . The based on the measurements in situ agreed with determined in a laboratory at 320, 355, and 390 nm. Using the determined , we calculated that UV-B contributed 9%, UV-A 68%, and visible light 23% to the photochemical mineralization. Half of total photochemical mineralization was due to wavelengths Ͻ360 nm. Our method for the determination of is applicable in situ, improves the prediction of photochemical reaction rates in surface waters, and offers an alternative to the determination of quantum yields at discrete wavelengths. helsinki.fi). AcknowledgmentsWe thank Anne Ojala for the global radiation measurements, Jorma Keskitalo for many kinds of assistance, Heikki Haario for advice with the Matlab software, and Martti Heikinheimo for advice in the problems of radiative transfer in the atmosphere. The quantum yields at single wavelengths were determined in the laboratory of Helge Lemmetyinen under his supervision.

One hundred representative strains of Bacillus cereus were selected from a total collection of 372 B. cereus strains using two typing methods (RAPD and FT-IR) to investigate if emetic toxin-producing hazardous B. cereus strains possess characteristic growth and heat resistance profiles. The strains were classified into three groups: emetic toxin (cereulide)-producing strains (n = 17), strains connected to diarrheal foodborne outbreaks (n = 40) and food-environment strains (n = 43), these latter not producing the emetic toxin. Our study revealed a shift in growth limits towards higher temperatures for the emetic strains, regardless of their origin. None of the emetic toxin-producing strains were able to grow below 10 °C. In contrast, 11% (9 food-environment strains) out of the 83 non-emetic toxin-producing strains were able to grow at 4 °C and 49% at 7 °C (28 diarrheal and 13 food-environment strains). non-emetic toxin-producing strains. All emetic toxin-producing strains were able to grow at 48 °C, but only 39% (16 diarrheal and 16 food-environment strains) of the non-emetic toxinproducing strains grew at this temperature. Spores from the emetic toxin-producing strains showed, on average, a higher heat resistance at 90 °C and a lower germination, particularly at 7 °C, than spores from the other strains. No difference between the three groups in their growth kinetics at 24 °C, 37 °C, and pH 5.0, 7.0, and 8.0 was observed. Our survey shows that emetic toxin-producing strains of B. cereus have distinct characteristics, which could have important implication for the risk assessment of the emetic type of B. cereus caused food poisoning. For instance, emetic strains still represent a special risk in heat-processed foods or preheated foods that are kept warm (in restaurants and cafeterias), but should not pose a risk in refrigerated foods.