M7 AM Metals in the Environment: Chemistry and Fate Issues
Monday, 14 November 2005: 8:00 AM - 11:40 AM in 327-329
(MOR-1117-574367) pH and Zn profiles in photosynthetic biofilm using ion-selective electrodes: An explanation for diel metal cycling?
Morris, J1, Farag, A2, Meyer, J1, 1 University of Wyoming, Laramie, Wyoming, USA2 U.S. Geological Survey, Jackson, Wyoming, USA
ABSTRACT- We investigated the driving mechanism(s) behind diel Zn uptake and release in photosynthetic biofilm by using miniaturized, ion-selective electrodes (300 
m tip diameter) to measure pH and Zn activity in the overlying water and within the matrix of a mixed community of photosynthetic biofilm. In darkness, the pH in a 1- to 2-mm thick layer of biofilm decreased from 9.7 to 7.3 in 51 min; and then, under relatively weak lighting (100 mol/m2/s PAR), the pH increased from 7.3 to 9.5 in 14 min. Concurrently, Zn activity in the biofilm layer increased from 0 to 2.6 x 10-5 M in darkness and subsequently decreased from 2.6 x 10-5 to 0 M in light. Additionally, a steep gradient existed between the relatively neutral pH in the water column (∼7.1) and the elevated pH in the diffusive boundary layer and within the biofilm matrix (∼9.7). These high and low pH values generally are only separated by 1 to 2 mm, and this gradient does not occur in darkness. Gradients of Zn activity are also steep, wherein Zn activity decreases from 7.6 x 10-6 to 0 M within 1 mm near photosynthesizing biofilm and increases from 1.2 x 10-5 to 1.9 x 10-5 M within 1 mm near non-photosynthesizing biofilm. Therefore, photosynthetic biofilm can decrease Zn activity very quickly in light that is 10 to 20x less intense than full sunlight, and a steep gradient of pH and Zn activity exists near the water-biofilm interface of photosynthesizing biofilm. This suggests that Zn adsorption or precipitation of Zn-hydroxides and/or Zn-carbonates within the photosynthesizing biofilm matrix could be driving diel metal cycling in mining-impacted streams.
Key words: biofilm, ion-selective electrodes, diel metal cycling, zinc