A new technique for determing the concentration of oxygen in a liquid sample uses a laser (coming from the green fiber, right) and an oxygen-sensitive sticker called an optode (pale spot) inside the sample bottle. When struck by the laser, the sticker fluoresces; the wavelength of the light it gives off indicates the concentration of oxygen in the fluid around it. WHOI chemist Ben Van Mooy used this method to monitor microbial activity in samples of water taken from within and outside the oil sl