Size of Suspended Bacterial Cells and Association of Heterotrophic Activity with Size Fractions of Particles in Estuarine and Coastal Waters

The size of bacteria and the size distribution of heterotrophic activity were examined in estuarine, neritic, and coastal waters. The data indicated the small size of suspended marine bacteria and the predominance of free-living cells in numerical abundance and in the incorporation of dissolved amino acids. The average per-cell volume of suspended marine bacteria in all environments was less than 0.1 mum. Cell volume ranged from 0.072 to 0.096 mum at salinities of 0 to 34.3 per thousand in the Newport River estuary, N.C., and from 0.078 to 0.096 mum in diverse areas of the Gulf of Mexico. Thus, the free-living bacteria were too small to be susceptible to predation by copepods. In the Newport River estuary, ca. 93 to 99% of the total number of cells and 75 to 97% of incorporated tritium (from H-labeled mixed amino acids) retained by a 0.2-mum-pore-size filter passed through a 3.0-mum-pore-size filter. Although the amino acid turnover rate per cell was higher for the bacteria in the >3.0-mum size fraction than in the <3.0-mum size fraction, the small number of bacteria associated with the >3.0-mum size particles resulted in the low relative contribution of attached bacteria to total heterotrophic activity in the estuary. For coastal and neritic samples, collected off the coast of Georgia and northeast Florida and in the plume of the Mississippi River, 56 to 98% of incorporated label passed through a 3.0-mum-pore-size filter. The greatest activity in the >3.0-mum fraction in the Georgia Bight was at nearshore stations and in the bottom samples. Our data were consistent with the hypothesis that resuspension of bottom material is an important factor in influencing the proportion of heterotrophic activity attributable to particle-associated bacteria.