Physiological characteristics of picophytoplankton, isolated from Lake Kinneret: Responses to light and temperature

Two different phylogenetic groups of picophytoplankton, namely picocyanobacteria and picoeukaryotes, are represented in Lake Kinneret. Three species were isolated from the lake and identified as the picoeukaryote Mychonastes homosphaera and two picocyanobacteria, Synechococcus sp. A and B. Picocyanobacterial and M. homosphaera cultures grew well at light intensities up to 330 and 700 μmol photons m^-2 s^-1, respectively, but poorly below 10 μmol photons m^-2 s^-1. Picocyanobacterial and M. homosphaera cultures photoacclimated to low light by increasing their chlorophyll per cell through increase in photosynthetic unit (PSU) size and PSU numbers, respectively. Growth rates of Synechococcus A and B were higher at temperatures characteristic of summer-autumn in the epilimnion, when maximum abundances of picocyanobacteria occur. Growth rates of M. homosphaera were higher at 14°C, corresponding to lake water temperatures during their occurrence in winter-spring. Temperature is a dominant factor influencing the seasonal dynamics of both picocyanobacteria and picoeukaryotes in Lake Kinneret, while the vertical distribution is controlled by acclimation to different light conditions. Differences in temperature tolerance and photoacclimation suggest that Synechococcus A belongs to picocyanobacteria found in summer below surface waters, while Synechococcus B represents picocyanobacteria found throughout the year at all depths. Photoacclimation to high light as shown in M. homosphaera cultures, may account for the relatively high abundance of picoeukaryotes in surface waters in Lake Kinneret.