Photosynthesis in sediments determined at high spatial resolution by the use of microelectrodes

The present study investigated photosynthetic rates and their regulation by light within the upper 5mm of sediment in a tidal area of Niida River in Hachinohe, Japan. Steady-state concentration profiles of O(2), NH(4)(+), NO(2)(-), H(2)S, and pH in the sediment were measured with microelectrodes. Microzonation of O(2) respiration, denitrification and SO(4)(2-) reduction was found in the sediment. When light intensities exceeded 1050 micromol photons/m(2)/s, net photosynthetic activity was detected in the upper 0.5mm of the microbial mat colonizing on the sediment surface in the tidal area. In contrast, gross photosynthetic activity was detected in the upper 1.0mm of the microbial mat at 1900 micromol photons/m(2)/s. As light intensity increased, the net photosynthetic rate and O(2) penetration depth increased. The maximal net photosynthetic rate and O(2) penetration depth were 6.1 micromol O(2)/cm(3)/h and 2.2mm, respectively, at 1900 micromol photons/m(2)/s. Net photosynthetic rates in the microbial mat in the tidal area were lower than in the upstream sediment. The analysis of continuous O(2) concentration measurements in different layers of the microbial mat during artificial light-dark cycles demonstrated that the photosynthetic activity response to changes in light intensity was extremely fast (a few seconds) and the O(2) concentration in the microbial mat became stable within 200s. The measurement of physical and chemical parameters in river water revealed that the study site was relatively polluted and sunlight intensity significantly fluctuated temporally. These results suggested that the in situ microbial processes occurring in the sediment fluctuated in accordance with periodic fluctuations in sunlight intensity.