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Environmental applications of luminescence

Algae delayed luminescence dynamics altered by Zinc stress - preliminary data

Julya C. M. T. A. C. M. Gallep1

1LaFA/FT - UNICAMP, Limeira/SP - Brasil

E-mail: gallep@unicamp.br

This study compares a standard ecotoxicological method - algae colony growth (Raphidocelis subcapitata) over 72 hours - with a photonic technique: the delayed luminescence (DL) analysis, developed by Katsumata et al.., by evaluating its decay profile and comparing data from control and stressed samples, with results as fast as 1 hour after inoculation. The method demands less consumables and work-hours than the long, standard growth tests. The test procedure is: preparation of the oligo medium (10L) for stock and erlenmeyer flasks (50mL), followed by washing and sterilization of the material to be used and preparation of the inoculum; inoculation of the algae samples with hazard to be tested, and flaks completed with oligo solution and sterilized water (20mL for erlenmeyer flasks and 10mL for test tubes), all flasks are kept on a shaking table with constant light and temperature. The DL test is run using a photon counting chamber immediately after inoculation (0h), and at 1, 3, 6, and 24 hours after it; the standard test (ie. light absorption of algae sampels) are run after 72 hours, using a spectrophotometer.In this work, solutions of Zinc sulfate heptahydrate (ZnSO4.7H2O) were tested. It is an inorganic salt and very toxic to aquatic organisms. Thus, the concentrations used were: 0 (control), 0.05, 0.1, 0.25, 0.5 and 1 mg/L, and DL measured by a dedicated dark chamber (PMX6100 (IR), Hamamatsu Photonics KK), with the sample being exposed to controlled light excitation (white LED for 30s + IR@700nm for 1s), and immediately after (< 50ms) the photon counting is performed for the next 60s, counts every 0.1s, forming a characteristic curve for the control samples: rapid decay followed by the formation of a peak and then a slow decay. By comparing the DL time profile of stressed and control samples, it is possible to determine the inhibition of the DL curve (iDL) and correlate it with the toxicity of the solution.This DL is based on the combined response of two systems: the photosyntetic I and the photosyntetic II, with charge recombination and electron transport: the initial electron receptors are the quinones QA and QB, near the primary donor chlorophyll molecule at PS-II (P680), giving so the decay rapid component; subsequently, electron carriers such as the plastoquinone, the plastocyanin and the ferredoxin at PS-I, located further away, giving the the slow component of DL (slow decay). Some substances - NaCl, diuron, flumetralin, and ametryn - have already been tested, showing satisfactory results when compared to the conventional method. For the zinc salt discussed here, a noticeable larger peak was observed for the control group and for the lowest concentration of zinc sulfate just few hours after inoculation.

Keywords: luminescence, algae, toxicology, rapid testing.

Acknowledgments: São Paulo Research Foundation – FAPESP, Brazil [grants 04/10146-3, 07/00431-0, 14/04232-6, 15/11280-0, 16/50344-6, 18/05300-6]; National Research Council CNPq, Brazil [grant 301765/2022-7]

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