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tryptophan concentration demonstrating the wide dynamic range, from nanomolar to millimolar. Further calibration measurements in combination with epifuorescence microscopic counts of bacteria cell numbers with two common marine bacteria strains���Pseudoalteromonas carrageenovora and Bacillus subtilis���and a Monitoring of bacterial bioflm growth in Pacifc Ocean natural bacterial bioflm water (solid line). Analysis of bacteria cell density using community confrmed DAPI-stained subsamples by optical microscopy (squares). a low detection limit of Error bars represent standard error means (n = 20) of the about 4,000 bacteria cells bacterial cell density. The inset shows average percentages per square centimeter. of dominant bacterial groups. Taking into account the sensor���s geometrical factors, it can be fve times, and its averaged value is concluded from these measurements subtracted from the total fuorescence that approximately 30 million trypintensity. To ensure a stable light outtophan molecules per cell contribute put, the UV-LED is illuminated one to the measured signal, which agrees second before launching the subsewith reported protein content of bacquent bioflm detection sequence. teria. Typically, fve fuorescence measurements, with integration times of Field Experiment 10 milliseconds each, are performed. The feld sensor prototype was apA high-precision digital thermometer plied hourly to quasi-continuously takes a measurement after each detecmonitor bioflm formation dynamics tion cycle. The background and total on a ship cruise in December 2011 fuorescence intensity values, setup about 2 nautical miles south of Hawaii. parameters of the bioflm sensor, time The sensor unit was installed on deck and temperature are recorded on the of the vessel in an outdoor mesocosm SD memory card. Possible damage to with steady exchange of Pacifc Ocean the DNA of the microbial community, water. Over 11 days, the intrinsic biowhich is expected to occur for a proflm fuorescence intensity was mealonged UV exposure, is prevented by sured hourly, together with daily measwitching off the UV-LED immediately surements of several physicochemical after the fuorescence measurements. water parameters. The average water Finally, the light shield opens to allow temperature was 25.2�� C, the salinity for bioflm formation under natural was 53.5 millisiemens per centimeter, light conditions. the pH value was 8.4 and the oxygen A complete measurement cycle content was 5.8 milligrams per liter. takes about 12 seconds. Repeated Reference settling substrates were measurements at preset intervals, placed inside the mesocosm under typically every 15 to 60 minutes over the same hydrodynamic conditions several weeks, yield quasi-continuous to quantify the corresponding accusampling of the bioflm growth dynammulated bacterial cell density of the ics. bioflm. The bacteria were stained by a DNA-binding fuorescent dye, DAPI, Detection Range and were quantifed daily by epifuoThe sensor performance was tested rescence microscopy. in April 2010 in the laboratory by placTwenty random images of the subing a UV transparent cell culture dish strate were captured, and the bacteon the sensor head flled with L-trypria cell numbers were counted by an tophan solution in artifcial seawater. ImageJ software program. The sensor Dilution series in the concentration (c) readouts revealed an exponential trend range of 5 x 10���9 moles per liter ��� c in marine bioflm growth. Neverthe��� 1 �� 10���4 moles per liter yield linear less, with 2.5 percent, even after 11 correlations between sensor signal and days, the area covered by the bioflm www.sea-technology.com Sea & Sun Marine Tech Sea & Sun Technology FEBRUARY 2013 / st 51