Low Temperature- and Light-Induced Flocculation of a Thermophilic Cyanobacterium Synechococcus vulcanus
Atsushi HIRANO*a, Shunji KUNITOa, Yorinao INOUEb, and Masahiko IKEUCHIc
a Energy and Environment R&D Center, TEPCO, 4-1 Egasaki,Tsurumi, Yokohama 230, Japan
b Photosynthetic Res. Lab., RIKEN, 2-1 Hirosawa, Wako, Saitama 351-01, Japan
c Dept. Life Sci., Univ. of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153, Japan
A thermophilic cyanobacterium Synechococcus vulcanus is an organism suitable for studies on photosynthesis, since its photosystem II is thermostable, while that in mesophilic organisms is most sensitive to high temperature among the photosynthetic components. S. vulcanus grows very fast at optimum temperature of 55℃ but loses viability at 25℃ or 60℃. We found that cell flocculation was induced by combination of a low temperature (30 - 40℃) and light illumination (80 - 240μEi/?/sec). The best flocculation was observed at 30℃ under illumination at 240μEi/?/sec. The flocculation was not induced in total darkness even at 30℃ but accelerated at higher light intensity. The photosynthetic inhibitor DCMU or protein synthesis inhibitor chloramphenicol suppressed the flocculation. The flocculated cells were reversibly dispersed when the growth temperature was returned to 55℃ in the light. Since photosynthetic organisms tend to get photoinhibitory stress at lower temperature than the optimal, our results seem to indicate that the flocculation is an active adaptation process against photoinhibitory stress. Further observation that the flocculated cells were much more resistant against photoinhibition than the dispersed ones supported this idea. Notably, a DNA gyrase inhibitor, nalidixic acid, which is known to induce SOS responses in some bacteria, enhanced the flocculation at 30℃ and induced a similar cell aggregation even at 55℃.
When the flocculated cells were mechanically dispersed by a moderate sonication, cells re-aggregated even in the dark or after treatment with formaldehyde. This suggests that the re-aggregation reflects physicochemical properties of the cell surface rather than physiological reactions. This re-aggregation of the dispersed cells were specifically suppressed by the addition of mannose, which is one of the major constituents of cell wall polysaccharides in S. vulcanus. Electron microscopic observation revealed some excreted substances only on the surface of flocculating cells. These suggest that the excretion of some polysaccharides induced by photoinhibitory stress at low temperature results in the cell flocculation, which confers resistance against the stress.
