In situ reproductive rate of freshwater Caulobacter spp

Abstract

Electron microscope grids were submerged in Lake Washington, Seattle, Wash., in June 1996 as bait to which Caulobacter sp. swarmers would attach and on which they would then reproduce in situ. Enumeration of bands in the stalks of attached cells implied that the caulobacters were completing approximately three reproductive cycles per day. A succession of morphological types of caulobacters occurred, as well as an episode of bacteriovore grazing that slowed the accumulation of caulobacters and prevented the aging of the population.

FIG. 1

Stalked vibrioid caulobacter cells attached to grids at 2 (A to C) and 4 (D and E) days of submersion in Lake Washington, displaying one (A), two (B), three (C), four (D), and six (E) stalk bands (marked by arrows in panels A to C). PHB, granule with the appearance of poly-β-hydroxybutyrate; Pn, granule with the appearance of inorganic polyphosphate. Each marker is 1 μm.

FIG. 2

Stalked vibrioid caulobacter cells attached to grids at 4 days of submersion in Lake Washington, displaying eight (A) and nine (B) stalk bands. Each marker is 1 μm.

FIG. 3

Stalked caulobacter cells attached to grids at 7 days of submersion in Lake Washington, displaying various numbers of stalk bands. (A) cluster of vibrioid (V) cells; (B) cluster of predominantly fusiform (F) cells. SR, stalk remnant. Arrows indicate bands at the ends of damaged regions of stalks. Each marker is 1 μm.

FIG. 4

(A) Stalked caulobacter cells and stalk remnants attached to grids at 7 days of submersion in Lake Washington. (B to D) The stalk remnants are shown at higher magnification. The stalk band typically separates the intact region of the stalk remnant from the collapsed, gnawed end that was previously connected to the cell (arrows). The number of stalk bands remaining is indicated at the gnawed end of each remnant; the numbers (1, 3, 5, and 7) suggest that grazing was not selective for cells of a particular age. Each marker is 1 μm.

FIG. 5

Numbers of stalked caulobacter cells found attached to grids on each day of submersion in Lake Washington. Cell numbers were totaled for the two sites and normalized per grid for each day's collection. Each wide bar represents the total number of caulobacter cells, of all three types, and includes cells whose band numbers could not be determined (see text).