Abstract-Water samples were collected from coastal waters near Cape Lookout, N.C., during March 1975 for direct count of bacteria and for adenosine triphosphate (ATP) analysis.Bacteria averaged 6.6 x lo" cells ml-l and 0.09 CL" cell-l. Bacterial carbon was 4 to 25% of the ATP estimate of total plankton carbon.

The bacterioplankton community of confined seawater at 25°C changed significantly within 16 h of collection. Confinement increased CFU, total cell number (by epifluorescence microscopy), and average cell volume of bacterioplankton and increased the turnover rate of amino acids in seawater sampled at Frying Pan Shoals, N.C. The bacterioplankton community was characterized by two components: differential doubling times during confinement shifted dominance from bacteria which were nonculturable to bacteria which were culturable on a complex nutrient medium. Culturable cells (especially those of the genera Pseudomonas, Alcaligenes, and Acinetobacter) increased from 0.08% of the total cell number in the seawater immediately after collection to 13% at 16 h and 41% at 32 h of confinement. Differential filtration before confinement indicated that particles passing through a 3.0-,um-, but retained by a 0.2-,m-, pore-size Nuclepore filter may be a major source of primary amines to the confined population. The 3.0-,um filtration increased growth rate and ultimate numbers of culturable cells through the removal of bacterial predators or the release of primary amines from cells damaged during filtration or both.

Rapid turnover of dissolved free amino acids (DFAA) by bacterioplankton in the Gulf of Mexico was observed with techniques designed to eliminate contamination of samples with trace metals and organic compounds. The mean turnover rate of DFAA, based on incorporation of 0.5 nM additions of a mixture of amino acids, was 4.9-d-l for high productivity neritic environments and 1.3.d-' for low productivity oceanic environments. These rates are faster than those in parallel samples measured by traditional techniques and are consistently faster than previously reported values. Data for multiple level (0.01-7 nM) additions of the mixed substrate were in accord with the Michaelis-Menten enzyme kinetics model. Kinetic parameters derived from this model (V,,,, K, + S,, and R), bacterial cell numbers, and V,,,/cell were highest at photic depths of the neritic zone, intermediate at photic depths of the oceanic zone, and lowest at aphotic depths of the oceanic zone. Estimates of secondary productivity by bacterioplankton (based on V,,,,,) on an equal water volume basis were 6.6 + 1.5% (+-SE, y1 = 5) of the light-saturated primary productivity at the maximum productivity depth. Estimated turnover time of the bacterioplankton community ranged from 2.4 d at 5 m at the highest productivity station to 130 days at 250 m at the lowest productivity station.

We used quantitative microscopy to examine feeding of Penilia avirostris on natural (< 1 pm) and cultured (0.5-2.0 pm) bacterioplankton, autotrophic phytoplankton, heterotrophic microflagellates (2-5 pm), and bacteria-sized (0.2-l .O pm) fluorescent beads. Natural and cultured bacterioplankton were not appreciably ingested, except for extremely high concentrations (>9.0 x lo6 cells ml-l) of clumped cells from bacterial cultures. Bacterivorous microflagellates were ingested. Most species of available natural phytoplankton (chain-forming or large diatoms) and Pseudoisochrysis paradoxa (5-6 pm) were not ingested, but the 4-6-pm diatom Thalassiosira pseudonana and the 1 O-l 2-pm Thalassiosira weiss$?ogiii were. Food choice and feeding rate appeared related to food concentration as well as cell size. Our results contrast with previous reports of bacterivory by P. avirostris and other cladocerans, possibly due to preferential bacterivory on large or aggregated bacteria, elevated bacterial abundance levels in cultures, or failure to recognize heterotrophic microflagellates as an unseen intermediate trophic step in studies with nonmicroscopic techniques.Although P. avirostris does not feed on free-living bacterioplankton, it may be an important component of the "microbial loop" between bacterioplankton and higher consumers because of its predation on bactcrivorous microflagellates.