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Abundance of picophytoplankton in the halocline of a meromictic lake, Lake Suigetsu, Japan
Authors:Mari Okada  Yukiko Taniuchi  Akio Murakami  Shinichi Takaichi  Shinya Ohtake  Kaori Ohki
Institution:(1) Department of Marine Bioscience, Faculty of Biotechnology, Fukui Prefectural University, 1-1, Gakuencho, Obama, Fukui 917-0003, Japan;(2) Kobe University Research Center for Inland Seas, Hyogo, Japan;(3) Biological Laboratory, Nippon Medical School, Kawasaki, Japan;(4) Present address: Sales Group, M&S Instruments Inc., Osaka 532-0005, Japan
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. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.
Keywords:Lake Suigetsu  Meromictic lake  Phycoerythrin-rich cyanobacteria  Picocyanobacteria  Picophytoplankton
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