Characterization of swimming motility in a marine unicellular cyanobacterium

Abstract

The structural mechanism, behavior, energetics and functional significance of the unique swimming motility displayed by some oceanic isolates of the cyanobacterium Synechococcus was investigated. A variety of analytical techniques confirmed that these strains swam through liquids without flagella or flagellar-like appendages. No extracellular structures were observed in a broad range of cell preparations examined by transmission electron microscopy (TEM), or by high-intensity dark field microscopy. The possibility that a structure might be present that eluded visualization was eliminated by the lack of motility-dependent amplitude spectra, the absence of discrete circulation of microspheres around the cell body and the inability of shearing forces to arrest motility. TEM and gel electrophoretic analysis of spheroplasts, cell wall-enriched fractions from motile and nonmotile strains, and cell material collected following the application of a flagellar hook-basal body complex isolation technique to a motile Synechococcus strain provided no further evidence of a structure or protein unique to motile strains. The motile Synechococcus strains represent the only cyanobacterium reported to date capable of swimming rather than gliding motility. Swimming behavior was characterized by several features: between 50 - 80% of cells were actively motile during loyarithmic phase of growth, with speeds that ranged from 5 - 40 um s-1 the average speed was 13 um s-1. Swimming patterns were entirely random, consistent with the absence of bacterial flagella. Synechococcus motility resembled flagellar-mediated motility in that thrust (forward motion) was accompanied by torque (cell rotation) as demonstrated by i) dividing cells which swam with the daughter cells at an angle, ii) individual cells that were sometimes seen to rotate end over end at a rate of 3 to 5 rev s-1, iii) polystyrene beads attached to the cell body served as a point of reference as the cell rotated concomitant with translocation and iv) cells attached to the coverslip or slide spun about one pole at an average rate of 1 rev s-1. When observed in the same plane of focus, 50% of the cells spun clockwise and 50% spun counterclockwise, but unlike flagellated cells, Synechococcus was never seen to change direction of rotation, as would be predicted if the cell body were rotating as a single unit and the motility apparatus were incapable of reversing direction of rotation. This motility apparatus appeared to operate at a constant torque, as indicated by the relationship between swimming speeds and the fluidity of the surrounding medium. Investigation of the energetics of motility in Synechococcus WH8ll3 demonstrated that swimming was sodium coupled. There was a specific sodium requirement such that cells were immotile at external sodium concentrations below 10 mM, with speeds increasing with increasing sodium to a maximum speed at 150 to 250mM sodium, pH 8.0 to 8.5. The sodium motive force increased similarly, but other energetic parameters including proton motive force, electrical potential, and the proton and sodium diffusion gradients lacked correlation to levels of motility. When components of the sodium motive force were diminished by monensin or carbonyl cyanide m-chlorophenyl-hydrazone, motility was arrested. Motility was independent of the magnitude of internal ATP pools, which were depleted to 2% of control values without affecting cell motility. These results suggest that the direct source of energy for Synechococcus motility is a sodium motive force, and that the devise driving motility is located in the cytoplasmic membrane, as is the case for flagellated bacteria. The ecological role of Synechococcus motility was explored and several lines of evidence indicated that cells lacked behavioral photoresponses but were able to detect and respond to very low concentrations of simple nitrogenous compounds. When 23 compounds were tested in spatial gradients established in blind well chemotaxis chambers, cells displayed positive chemoresponses only when placed in gradients of NH4Cl, NaN03, urea, glycine and alanine. Cells also failed to respond in chambers which lacked gradients due to the presence of only seawater or an equal distribution of chemoeffector, demonstrating that a gradient was required to elicit a response. The apparent threshold levels of 10-10 M - 10-9 M for Synechococcus chemoresponses are 4 to 5 orders of magnitude lower than those for most other bacteria and place them in the range of ecological significance. The presence of chemotaxis in this oceanic cyanobacterium may help support the notion that nutrient enriched microaggregates may play an important role in picoplankton nutrient dynamics.