Photochemistry and Photobiology, 2004, 80: 78-83 zyxwvut Effects of UV-B Irradiation on a Marine Microecosystemq Roberto Marangoni*, Nicola Messina, Domenico Gioffre and Giuliano Colombetti lstituto di Biofisica del C.N.R., Area della Ricerca di Pisa, Via G. Moruzzi 1, Pisa, Italy Received 14 August 2003; accepted 10 March 2004 A6STRACT zyxwvutsr Purpose of this work was to study the effect of UV irradiation on a microecosystem consisting of several interacting species. The system chosen was of a hypersaline type, where all the species present live at high salt concentration; it comprises different bacteria; a producer, the photosynthetic green alga zyxwvut Dunaliella salina; and a consumer, the ciliated protozoan Fabrea zyxwvutsr salina, which form a complete food chain. We were able to establish the initial conditions that give rise to a self- sustaining microecosystem, stable for at least 3 weeks. We then determined the effect of UV irradiation on this micro- ecosystem under laboratory-controlled conditions, in partic- ular by measuring the critical UV exposure for the two main components of the microecosystem (algae and protozoa) under UV-B irradiances comparable to those of solar irradiation. In our experiments, we varied irradiance, total dose and spectral composition of the actinic light. The critical doses at irradiances of the order of zyxwvuts 56 kJ/m2 (typical average daily irradiance in a sunny summer day in Pisa), measured for each main component of the microecosystem (algae and ciliates), turned out to be around 70 kJ/m2 for ciliates and 50 kJ/mz for D. salina. By exposing microecosystems to daily UV-B irradiances of the order of 8 W/m2 (typical average daily irradiance in a sunny winter day in Pisa), we found no effect at total doses of the order of the critical doses at high irradiances, showing that the reciprocity law does not hold. We have also measured a preliminary spectral-sensitive curve of the UV effects, which shows an exponential decay with wavelength. INTRODUCTION There is a complex dynamic interaction of solar radiation, atmosphere composition and living organisms. The first oxygen- free atmosphere (under which the early organisms had to possess strong defense strategies against UV irradiation, see Ref. 1) was changed by the emergence of life (2,3) into the present, oxygen- containing atmosphere, impermeable to UV-C and partly also to UV-B. There is now the danger that the atmosphere could become more permeable to UV-B because of the reduction of the ozone 7Posted on the website on 7 April 2004 *To whom correspondence should be addressed Istituto di Biofisica del C.N.R., Area della Ricerca di Pisa, zyxwvutsrqp Via G. Moruzzi 1, 56124 Pisa, Italy. Fax: 39-050-3 152760; e-mail: roberto.marangoni@ibf.cnr.it Abbreviation: ANOVA, analysis of variance. zyxwvutsr 0 2004 American Society for Photobiology 0031-8655/04 zyxwvutsrqp $5.00+0.00 layer (the so-called ozone hole problem) caused by anthropic activities (4-5). UV-B radiation is known to affect living organisms because of its ability to interact with the most important biological macro- molecules, such as nucleic acids, proteins and lipids (6-8). In the past 10-15 years, the problem of ozone depletion has followed two main research lines, one devoted to the monitoring of either the stratospheric ozone concentration zyxw (e.g. NASA/TOMS project [9,10]) or the solar radiation reaching the biosphere (e.g. EU ELDONET project 111-131) and the other oriented toward the assessments of the biological effects of UV-B radiation on cellular and subcellular apparatus. By putting together the information derived from these different approaches, one can possibly draw realistic predictions on how an increased UV-B permeation could alter the global ecosystem and in which direction. This would require the monitoring of ecosystems and establish- ing a relationship between their changes and the UV-B doses received, a very complex task to accomplish. Only a few studies have been performed in zyxwv situ, and most of them are focused on only a few species (see, for instance, 14-21). On the contrary, there is a large number of laboratory studies that are concerned with the effects of UV on selected organisms, from lower (bacteria, algae and protozoa) to higher (plant and animals, including humans) ones. The studies on microorganisms are generally based on a similar experimental approach: a small volume of a single-species culture is exposed to UV irradiation, and some fitness parameters (from cell survival to motility, from photoresponsiveness to photosynthetic efficiency, etc.) are mea- sured before and after UV exposure. A quantitative evaluation of the UV-induced damage can be obtained (for a general review, see Refs. 22-24) by a comparison of the values determined before and after irradiation. These studies have demonstrated the influence of UV-B radiations on a large variety of biological processes, from thymine dimer formation (25,26) to a reduced photosynthetic efficiency (27-29) to an impairment of motile and photomotile functions (24,30-33). The results of these studies have some drawbacks. First, the critical UV doses for cell survival (or for the damage caused to important cellular functions), deduced from dose- damage curves determined in single-cell studies, are low if compared with the UV doses recorded in a normal sunny day (13,24). This implies that the exposure conditions used in these experiments are far from those of a real ecosystem. Second, it is difficult to predict the behavior of a complex ecosystem starting from studies focused on only a single species. Studies on single species have, however, demonstrated that the resistance against UV varies widely among different microorgan- isms. This suggests that UV-resistant species might take advantage 78