[Show abstract][Hide abstract] ABSTRACT: Numerous (0.5 to 4.8 × 105 cells/ml), small phytoplankton (smaller than 0.5–1 × 1–2 μm in cell size, picophytoplankton) were distributed in the halocline
(depth 2–12 m, 4–14 practical salinity units) of the saline meromictic lake, Lake Suigetsu (35°35′ N, 135°52′ E), located
in the central part of the coast of Wakasa Bay along the Japan Sea in Fukui Prefecture, Japan. Vertical distribution of phytoplankton
revealed that the maximum number of picophytoplankton was always observed near or a little deeper than the oxic-anoxic boundary
layer (depth 5–6 m); they were dominant phytoplankton in the water layer deeper than the oxic-anoxic boundary from July to
late September 2005. Spectral analysis of autofluorescence emitted from the particle fractions smaller than 5 μm measured
with a spectrofluorometer and from individual cells measured with a microscope photodiode array detector revealed that the
major component of picophytoplankton was phycoerythrin-rich, unicellular cyanobacteria (picocyanobacteria). Eukaryotic phytoplankton
about 2.5 μm in diameter were also found, but the numbers were low. Fluorescence intensity of chlorophyll a at 685 nm (room temperature) emitted from the particle fractions smaller than 5 μm was increased by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea.
These observations indicated that at least some picophytoplankton had a functional photosystem II in the halocline where sulfide,
the potential inhibitor of oxygenic photosynthesis, was always present. The large abundance together with their physiological
potency suggest that picophytoplankton are one of the important primary producers in the halocline of Lake Suigetsu.