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Abstract

Scalar irradiance, oxygen concentration, and oxygenic photosynthesis were measured at 0.1 mm spatial resolution within the tissue of the siphonous green macroalga Codium fragile subsp. tomentosoides (van Goer) Silva by fiber-optic scalar irradiance microsensors and oxygen microelectrodes, The scalar irradiance of visible light was strongly attenuated in the outer 0.2 mm of the tissue but was nearly constant for the subsequent 1.0 mm of photosynthetic tissue. Far-red scalar irradiance at 750 nm increased below the tissue surface to a maximum of 200% of incident irradiance at 1.2 mm depth due to multiple scattering in the medullary tissue. The constant intensity of visible light below 0.2 mm was thus a result of the combined effects of absorption and backscattering from the medulla.
The oxygen exchange between the alga and the surrounding water was diffusion-limited with a steep O-2 gradient inside and around the alga. In darkness, the tissue below 0.6 mm became anoxic, and endophytic extracellular space provided an environment where anoxygenic microbial processes may occur. When illuminated at 160 mu mol photons.m(-2).s(-1), O-2 concentrations exceeded ambient levels throughout the thallus, with a maximum of 250% of air saturation just below the surface. The amplitude of oxygen variation was buffered by gas bubbles formed in the medullary tissue.