No abstract

The role of photosynthetic sulfur bacteria as primary producers in stagnant lakes having hydrogen sulfide is described.Photosynthetic bacteria normally appear at the boundary layer of the oxidative and reductive zones, where H& is present and the light intensity is lower than 10% of the surface value. The water of this layer was milky green or pink due to dense populations of Thiorhodaceae or Chlorobacteriaceae.

Light utilization efficiency and quantum yield of phytoplankton in a thermally stratified temperate sea were evaluated. Underwater spectral irradiance was measured with a specially designed underwater irradiance meter and the specific absorption coefficient of phytoplankton was determined by a modified opal glass method. The light utilization efficiency of phytoplankton at each depth was derived from photosynthetically fixed energy divided by the energy penetrating into that depth, and quantum yield was estimated from photosynthetic rate divided by quanta absorbed by phytoplankton. Vertical profiles of in situ photosynthetic rates per unit volume of water showed two peaks, the first at the depth of about 30% light level (ca. 45,000 cal m-2 h-l) and the second at the depth of about 1.5% light level (ca. 2,500 cal m-2 h-r) with about the same rates. The light utilization efficiency was about 0.5% at the first peak and 5% at the second, The quantum yield was about 0.02 mol C Einst-' at the surface peak and 0.1 at the subsurface peak. The latter value was nearly the same as the maximum yield reported from culture experiments.

The photosynthesis-light curve for purple sulfur bacteria had a steeper inclination than that for green sulfur bacteria at low light intensities. Light saturation occurred at intensities of S-7 klux in the former, in the latter at 10-30 klux. Light inhibition was observed in purple sulfur bacteria but was negligible in green sulfur bacteria. The optimal temperature for photosynthesis of these bacteria is considerably higher than that of most phytoplankton or green plants. Photosynthetic sulfur bacteria appear ordinarily in the contact layer between oxidative and reductive zones of meromictic or stagnant holomictic lakes; the light intensity in this contact layer is usually less than 10% of that at the surface. On the assmuption that the photosynthetic rate of these bacteria is limited mainly by the interaction of hydrogen sulfide concentration with light intensity, their growth was analyzed with a mathematical model. The properties of the growth phase observed in lakes were similar to those calculated. The main factors determining the growth of photosynthetic sulfur bacteria in lakes are the II23 concentration in the upper layer and the light conditions in the deeper layer.