A suite of pneumatically operated sampling devices was employed to investigate distributional patterns of marine bacteria at the millimetre scale. Spatial heterogeneity in bacterial abundance, or patchiness, was expressed as a coefficient of variation, and ranged from 5.5 to 75%. Discrete regions of enhanced bacterial abundance, as well as clear gradients in abundance across entire sample arrays were observed, with changes in bacterial abundance of up to 16-fold observed across a distance of 32 mm. The role of turbulence in influencing bacterial distribution patterns was examined in a series of laboratory experiments. Levels of heterogeneity were found to be up to 6.5 times higher in stirred than unstirred water samples under laboratory conditions. The gradients in bacterial abundance observed here suggest that small-scale processes operate within the marine microenvironment to create and maintain spatial structure in the bacterioplankton community. We suggest that previously hypothesised nanoscale 'hot spots' and microzones exist only as maxima within a continuously variable distribution of bacteria within the marine environment.

The motility characteristics of natural assemblages of coastal marine bacteria were examined. Initially, less than 10% of the bacteria were motile. A single addition of tryptic soy broth caused an increase in the motile fraction of cells but only after 7 to 12 h. Motility peaked at 15 to 30 h, when more than 80% of cells were motile. These results support the proposal that energy limits motility in the marine environment. Cell speeds changed more than an order of magnitude on timescales of milliseconds and hours. The maximum community speed was 144 m s ؊1 , and the maximum individual burst velocity was 407 m s ؊1. In uniform medium, speed was an inverse function of tryptic soy broth concentration, declining linearly over 0.001 to 1.0%. In media where concentration gradients existed, the mean speed was a function of position in a spatial gradient, changing from 69 to 144 m s ؊1 over as little as 15 to 30 m. The results suggest that marine bacteria are capable of previously undescribed quick shifts in speed that may permit the bacteria to rapidly detect and keep up with positional changes in small nutrient sources. These high speeds and quick shifts may reflect the requirements for useful motility in a turbulent ocean.

Seawater enrichments of marine bacteria clustered in 20-to 50-m-wide bands near air-water interfaces. The cells within the band travelled at up to 212 m s ؊1 and at an average speed of 163 m s ؊1. Mean cell speeds peaked mid-run at 187 m s ؊1. At the end of the run, bacteria reversed direction rather than randomly reorienting. The duration of the stops during reversal was estimated at 18 ms, six to seven times shorter than that found in enteric bacteria. Cells hundreds of micrometers from the band travelled at half the speed of the bacteria in the band. The fastest isolate from the seawater enrichment was identified as Shewanella putrefaciens and had an average speed of 100 m s ؊1 in culture. Air-water interfaces produced no clustering or speed changes in isolates derived from enrichments. Salinity and pH, however, both influenced speed. The speed and reversal times of the seawater enrichments indicate that the bacteria in them are better adapted for clustering around small point sources of nutrients than are either enteric or cultured marine bacteria.

Natural communities of marine bacteria, an isolate (FMB-Bf3) from one marine community, and Escherichia coli were examined by video microscopy for the magnitude and uniformity of their speed. Natural communities formed tight microswarms that showed higher speeds (mean ‫؍‬ 230 m s ؊1) than did E. coli (15 m s ؊1) or FMB-Bf3 (mean ‫؍‬ 62 m s ؊1). Outside the microswarms, the marine bacteria slowed to 45 m s ؊1. Between turns, in mid run, and while travelling in straight lines, the natural-community bacteria accelerated up to 1,450 m s ؊2 while the cultured bacteria showed maximum accelerations of 70 and 166 m s ؊2. The frequency distribution of speed change for the marine bacteria was skewed towards a few large negative accelerations and a range of positive accelerations. The general pattern was one of relatively slow increases in speed followed by abrupt declines. The results indicate that the mechanical generation and energetic maintenance, as well as the environmental function, of bacterial motility need reappraisal. We conclude that the standard bacterial motility parameters of low and uniform speed, derived from culture-based studies, are not necessarily applicable to marine bacterial communities.

Retinal projections were mapped in Tasmanian devils which had one eye injected with 3H-proline. The retinal fibers terminate in seven regions in the brain. These are (1) dorsal lateral geniculate nucleus (LGNd), (2) ventral lateral geniculate nucleus, (3) lateral posterior nucleus, (4) pretectum, (5) superior colliculus, (6) hypothalamus and (7) accessory optic system. The pattern of retinal input to six of these regions is similar to that seen in other marsupials. The pattern of retinal projections to the LGNd, while basically similar to that observed in other polyprotodont marsupials, is much simpler than that seen in the related native cat, Dasyurus viverrinus. The LGNd of Sarcophilus presents the simplest cytoarchitectural organisation of any marsupial examined so far. Each LGNd receives overlapping projections from both eyes. Suggestions of an intermittent lamination are seen in the LGNd contralateral to an eye injection of 3H-proline. On the ipsilateral side there are two patches of label, a large lateral patch and a smaller medial patch, both of which occupy areas receiving contralateral input. The monocular segment, occupying the ventral 40% of the nucleus, is more extensive than has been reported in any other polyprotodont marsupial.