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remained quite small. Moreover, it has been observed from the microcopy images that the bioflm community of the Pacifc Ocean was dominated by bacteria, as the average cell size was only 0.83 micrometers. For a more detailed assessment of the experiment, the relative abundance of major bacterial groups was accomplished by fuorescence in-situ hybridization. Common bacterial oligonucleotide probes revealed that the bacterial bioflm composition was dominated by Gammaproteobacteria, which a GAM42a probe detected to be about 44 percent of bioflm; Alphaproteobacteria, which an ALF968 probe detected to be about 40 percent; and Betaproteobacteria, which a BET42a probe detected to be less than 2 percent. Overall, a strong linearity, 0.93, has been found between bacterial cell numbers of the reference subsamples and the bioflm sensor fuorescence data. This also demonstrates that natural bacterial flms in the feld, despite their changing and heterogeneous composition, exhibit linear signal response and that tryptophan fuorescence can be used as a universal measure for bacterial abundance. Between 2010 and 2011, other test experiments have been performed in the Baltic Sea with longer settling periods. These experiments yielded a saturation level of bacterial coverage and partly revealed diurnal signal patterns. These results show that interesting new aspects of bioflm formation dynamics wait to be uncovered, and their detailed investigation requires the sensor���s quasi-continuous measurement capabilities. Conclusion The developed sensor enables the detection of bacterial bioflms in the marine environment by detecting intrinsic tryptophan fuorescence of bacteria. It has been shown that bioflm fuorescence excited by a UV-LED at a wavelength of 280 nanometers and detected at 350 nanometers reveals a linear relationship between fuorescence intensity and bacteria cell number. The sensor was calibrated with two marine bacteria strains. A detection range of approximately 4,000 to 50 million cells per square centimeter enabled monitoring of bioflms from initial attachment of bacteria cells up to fully developed complex bioflms. 52 st / FEBRUARY 2013 The results of the feld experiment represent the frst quasi-continuous data set of bacterial bioflm establishment in the Pacifc Ocean. Further upgrades of the sensor will allow simultaneous recording of biomass and metabolic activity of bacteria, which will add novel applications. Acknowledgments The authors would like to thank Annett Klemm of the University of St. Andrews for helpful discussions and Ulf Riebesell for providing us with the opportunity to collect the probes during the cruise. This research was supported by grants from the German Science Foundation in the framework of the Cluster of Excellence ���The Future Ocean��� (DFG-EC 80). References For a list of references, contact Gernot Friedrichs at fried richs@phc.uni-kiel.de. n Matthias Fischer is a researcher with a background in biomedical engineering. He is conducting his Ph.D. research at the Helmholtz Centre for Ocean Research (GEOMAR) in Kiel, Germany. He has developed an optical bioflm sensor for quasi-continuous, in-situ measurements of bioflm formation dynamics in marine environments. Martin Wahl is a professor of marine benthic ecology at Helmholtz Centre for Ocean Research (GEOMAR). His research focuses on community structures and dynamics of life on living and nonliving surfaces, stress, chemical and global change ecology in marine habitats. He is the founder of an international training program that combines applied research with training for young scientists. Gernot Friedrichs is a professor of physical chemistry at Kiel University with a focus on the application of laser spectroscopic methods in the feld of chemical reaction kinetics and ocean surface chemistry. Within the framework of The Future Ocean Cluster of Excellence, he promotes the use of optical detection technology in marine environments. www.sea-technology.com