MICROSENSOR STUDIES OF OXYGEN AND LIGHT-DISTRIBUTION IN THE GREEN MACROALGA CODIUM FRAGILE

Scalar irradiance, oxygen concentration, and oxygenic photosynthesis w ere measured at 0.1 mm spatial resolution within the tissue of the sip honous green macroalga Codium fragile subsp. tomentosoides (van Goer) Silva by fiber-optic scalar irradiance microsensors and oxygen microel ectrodes, The scalar irradiance of visible light was strongly attenuat ed in the outer 0.2 mm of the tissue but was nearly constant for the s ubsequent 1.0 mm of photosynthetic tissue. Far-red scalar irradiance a t 750 nm increased below the tissue surface to a maximum of 200% of in cident irradiance at 1.2 mm depth due to multiple scattering in the me dullary tissue. The constant intensity of visible light below 0.2 mm w as thus a result of the combined effects of absorption and backscatter ing from the medulla. The oxygen exchange between the alga and the sur rounding water was diffusion-limited with a steep O-2 gradient inside and around the alga. In darkness, the tissue below 0.6 mm became anoxi c, and endophytic extracellular space provided an environment where an oxygenic microbial processes may occur. When illuminated at 160 mu mol photons.m(-2).s(-1), O-2 concentrations exceeded ambient levels throu ghout the thallus, with a maximum of 250% of air saturation just below the surface. The amplitude of oxygen variation was buffered by gas bu bbles formed in the medullary tissue.