Even though the monolayers were sampled only from compression cycles and were not compared with the samples from expanson cycles, we believe that the partlton of fluorescent molecules had reached equibrum at the moment of transfer. The interval between compression and transfer (several minutes) was enough time for the distributon of fluorescent molecules to reach equlibrium through diffusion. The diffusion coefficient of DPPC in the LE domain IS lo-' to 1 0-8 cm2/s. 16. Averaged half-decay lengths (the lateral distances where the fluorescent intensities f a to half those of the maxima) determined from the cursor profiles across the domain boundaries for the monolayers sampled at T = 7,10,20, and 30 mN/m were 661 + 133, 181 i-38, 65 + 15, and 37 i 8 nm, respectively, in DPPC/Bodipy-PC samples and 345 i 31, 143 i 33,57 ? 17, and 56 i-13 nm, respectvely, in the samples with an additional 0.5 m o % of gangiosde G,,17. E. Betzig and R. J. Chichester, Science 262, 1422 (1 993). 18. The peak signal from a snge ipophlic carbocyanine dye, di-C,,(3) (Molecular Probes, D-282), molecule in a transferred DPPC monolayer was -250 counts per second (cps) per nanowatt of tip power, which is -20% of the typical value for a single molecule embedded in polymethylmethacrylate ( 7 7 ) This implies a quantum yield of -0.2 for a Bodipy in the monolayer, resulting n the equivalent emission slgnal of 1200 cpslnW, In DPPC/Bodipy-PC samples, average Intensities of the LE phase are -35,000 cps/nW at T = 4 mN/m and -75,000 cps/nW at T = 7 mN/m. The calculated number densities of the dye molecules in this phase are -7.4 x lo3 and -1.3 x lo4 molecules/~m2, respectively. 19. J. Hwang, E. Betzlg, M. Edidin, R. J. Chichester, Biophys. J. 66, A277 (1 994). 20. Dark spots at the center were caused by the dwell of the probe at one spot during the character~zation of the near-field signal before scanning began. 21. We thank R. Chichester and R. Pagano for their valuable helo and suaaestions and H. McConnel for helpful discussions--Supported by NIH grants A11 4584, DK44375 (M.E.), and A130557 (L.K.T.).

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International audienceSilicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO(2)). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO(2). The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO(2) in the coastal zone are to be expected over the course of this century