Michael D’Amico (University of Hawaii at Hilo)

1. Quartz Dark Quartz + DOM Dark + DOM Seawater with Tricho DOM 0.2um filtered seawater NH4+ NO2- PO43- Figure 3. Concentrations of NH4+ in containers at 3 and 12 hr for two replicate experiments. Notice a higher concentration in the quartz tubes than in the dark bottles for all treatments (except dark and quartz at 3 hr in Exp.1). Dark bottle concentrations decreased over time in each experiment. Karenia Brevis Trichodesmium spp. DPA Photoproduction of Labile Substrates from Dissolved Organic Matter Produced by Trichodesmium Michael D’Amico (University of Hawaii at Hilo) Deborah A. Bronk, Marta P. Sanderson, (College of William and Mary, Virginia Institute of Marine Science) Abstract The nitrogen-fixing cyanobacterium Trichodesmium (Tricho) spp. releases labile dissolved organic compounds that are potential nitrogen substrates for the surrounding plankton. One process that may be key to converting this dissolved organic matter (DOM) into labile forms is photochemical breakdown by UV light. In July 2003, rates of photoproduction of ammonium, nitrite, dissolved primary amines (DPA), and phosphate were measured in the oligotrophic waters of the Gulf of Mexico off St. Petersburg. Two sets of experiments were conducted with bulk water from the chlorophyll maximum, with and without DOM, produced by Tricho, added. Samples in quartz tubes were exposed to ambient sunlight with photoproduction rates measured at 3 hr and 12 hrs. Ammonium and phosphate photoproduction was observed with rates being higher when Tricho DOM was added. In contrast, significant decreases in nitrite concentrations were observed. No consistent pattern was seen in DPA concentrations. These data indicate that photoproduction is one mechanism converting DOM into labile nutrient forms in the Gulf of Mexico. DOM Figure 1. Schematic showing photoproduction of labile compounds though photolysis of DOM produced by Trichodesmium. NH4+ NO2- DPA PO43- Table 1. Rates of photoproduction of labile substrates between quartz and dark containers from DOM produced by Trichodesmium. Results Results continued 1. Measurable photoproduction was observed in all samples with Tricho DOM added and in three of four ambient water samples. It is also possible that NH4+ consumption occurred as well, but was balanced by rates of photoproduction. 2. Concentrations of NO2- decreased over time in seven of eight samples indicating photoconsumption. 3. Significant PO43- photoproduction was observed in all samples. The high rates observed where observed when Tricho DOM was added suggesting that this recently released material could be a source of PO43- as well as nitrogen. 4. No consistent trends for DPA were observed. Background Blooms of the toxic dinoflagellate, Karenia brevis, are common in the Gulf of Mexico near the Florida shelf (Mulholland et al. In press). It is believed that the nitrogen-fixing cyanobacterium, Trichodesmium spp., releases forms of DOM that are potential substrates for K. brevis (Mulholland et al. In press). One process that may be key to converting this DOM into utilizable forms is the production of compounds resulting from photolysis of DOM by UV light. This photoproduction of labile substrates may facilitate uptake of DOM by K. brevis, hence supporting the observed blooms. It was hypothesized that samples with water containing Tricho DOM would have higher rates of photoproduction than those without. Conclusions Data suggest that photochemical transformations of DOM are important in the production of labile substrates. This work also suggests that the N2-fixing cyanobacterium, Trichodesmium, can produce DOM that undergoes photochemical transformation producing labile compounds. Phytoplankton and bacteria can then use these compounds as a source of nitrogen and phosphorus. Photolysis of DOM is therefore one likely mechanism fueling the large blooms of Karenia brevis in the Gulf of Mexico. Methods Water from the chlorophyll maximum was collected and filtered through a 0.2um Supor filter and placed into six 125mL quartz tubes (3 quartz tubes with ambient water and 3 quartz tubes with water containing Tricho DOM); quartz allows UV light to pass through (Fig. 2). Six 125mL bottles were wrapped in aluminum foil as a dark control (3 bottles with ambient water, and 3 bottles with water containing Tricho DOM). Containers were placed in a flowing seawater incubator (to maintain constant temperature). Samples were removed from the incubator at 3 hr (late morning) and ~12 hr (dusk) and frozen for later colorimetric analysis of DPA, NH4+, NO2-, and PO43- concentrations. To produce Tricho DOM, Tricho colonies were picked from a net tow (using a 64um mesh net) and then incubating the Tricho for 24 hr in 0.2µm Supor filter water. At the end of the incubation, the sample was refiltered (0.2µm Supor) and refrigerated until use. References Bushaw, K. L., R. G. Zepp, M. A. Tarr, D. Schulz-Jander, R. A. Bourbonniere, R. E. Hodson, W. L. Miller, D. A. Bronk and M. A. Moran (1996). Photochemical release of biologically available nitrogen from aquatic dissolved organic matter. Nature 381: 404-407. Bushaw-Newton, K. L. and M. A. Moran (1999). Photochemical formation of biologically available nitrogen from dissolved humic substances in coastal marine systems. Aquatic Microbial Ecology 18: 285-292. Glibert, P. M. and D. A. Bronk, (1994) Release of dissolved organic nitrogen by marine diazotrophic cyanobacteria Trichodesmium spp. Applied and Environmental Microbiology 36: 3996-4000 Jankowski, J. J., D. J. Kieber, and K. Mopper (1999) Nitrate and Nitrite Ultraviolet Actinometers. Photochemistry and Photobiology 70(3): 319-328. Kieber, R. J., A. Li and P. Seaton, J. (1999). Production of nitrite from the photodegradation of dissolved organic matter in natural waters. Environmental Science and Technology 33: 993-998. Koopmans, D. J. and D. A. Bronk (2002). Photochemical production of dissolved inorganic nitrogen and primary amines from dissolved organic nitrogen in waters of two estuaries and adjacent surficial groundwaters. Aquatic Microbial Ecology 26: 295-304. Mulholland, M. R., C. A. Heil, D. A. Bronk, J. M. O’Neil, P. Bernhardt. Does nitrogen regeneration from the N2 fixing cyanobacteria Trichodesmium spp. Fuel Karenia Brevis blooms in the Gulf of Mexico? (in press). Figure 4. Concentrations of NO2- in containers at two time points (3 & 12 hr.) for two replicate experiments. In experiment 2, notice lower concentrations in the quartz vials compared to the dark containers, as well as a decrease of quartz vial concentrations over time. 1. 0.2um filtered seawater 3. Concentrations measured at 3 and 12 hrs. 2. 2. Acknowledgements I thank K.C. Filippino, J. See, and P. Bradley for their guidance in lab etiquette and for sharing their knowledge of chemical analyses. I thank the R/V Pelican for the use of their equipment. I also thank L. Schaffner and R. Seitz for organizing a wonderful and beneficial REU program. Contact info: Mike D’Amico (amico@hawaii.edu) or Debbie Bronk (bronk@vims.edu) Figure 5. Concentrations of PO43- in containers at two time points (3 & 12 hr.) for two replicate experiments. Notice a higher concentration in the quartz tubes in each experiment, as well as an increase in quartz tube concentration over time. Dark concentrations stay approximately the same over time (except in Exp.1 12hr.). Figure 2. Experimental protocol for UV release Experiments.