The marine ciliate Strombidium sp. can bc raised through multiple generations on a monoxenic diet of the bacterium Vibrio natriegens, but Strombidium specific growth rates and yields are 3-4 times higher when a heterotrophic microllagellate is included as prey. In contrast to earlier studies we found that Strombidium grew inefficiently when feeding as a bacterivore, with gross growth efficiencies (K,) of 1 l-20% (determined on a nitrogen-, carbon-, and protein-specific basis). On the same bacterial diet K, of the scuticociliate Uronema sp. was 49-70%. Bacterivorous Uronema attained a 9-fold higher specific growth rate and 30-fold higher cell yield than Strombidium. Thus, the species composition of a ciliate assemblage can markedly influence the fate of microbial loop production.Cell volume of live Strombidium sp. varied > 3-fold during a growth cycle. Cell organic composition also varied: the C: N (mass) ratio decreased from 7.7 in stationary phase cells to 4.5 in exponentially growing cells, reflecting the ingestion of N-rich prey. In addition to these natural variations in cell volume, preservatives can shrink cells to half of their live volume, resulting in spurious values of ciliate growth efficiency if computed on a volume-specific basis.

Feeding by juvenile Antarctic krill Euphausia superba near South Georgia was assessed dunng the austral summer of 1995/1996. Gut fluorescence results were compared with those from incubat~ons in natural seawater and seawater enriched with phytoplankton and zooplankton. In natural seawater, with typically low food concentrations (median 56 mg C m-3) the median ration was 0.68% of kr~ll carbon d-'. Phytoplankton dominated carbon In the natural incubation water but d~noflagellates, ciliates and small calanoid copepods dominated the carbon ~n t a k e of krill. In both natural and enriched water, maximum clearance rates were on l to 3 mm calanoid copepods. Copepods larger than this (e.g. late copepodite stages of Calanoides acutus and Rhincalanus gigas) were cleared more slowly despite dominating the carbon in the enriched incubations. Oithona spp. were cleared more slowly than calanoids of similar size, despite their greater abundance and their similar contributions to available carbon These trends could reflect detectiodescape interactions between krill and copepods. With enriched food, copepods dominated krill diet, krill ratlons exceeded 10'X, of body carbon d.' and rations did not appear to reach a plateau even at food concentratlons of -1 g C m-3 This suggests that krill could feed rapidly during periodic encounters with layers or patches of zooplankton. Gut fluorescence revealed gut passage times of 3.7 to 6.3 h and a n algal carbon ration of 0.43% d-l, thus supporting the low algal carbon rations denved from the incubations. Pubhshed acoustic values of mean krill biomass north of South Georgia that summer of 8.3 g dry mass m-* were combined with their clearance rates to give estimates of krill removing daily 0.2% of phytoplankton standing stocks, 0.6% of protozoans and 1.6% of small calanoid copepods. This impact on copepods is much higher than previous estimates from Antarctic amphipods and chaetognaths. The long generation times of Antarctic copepods mean that krill were potentially important predators of small copepods during our study.

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Biofouling communities contribute significantly to aquatic ecosystem productivity and biogeochemical cycling. Our knowledge of the distribution, composition, and activities of these microbially dominated communities is limited compared to other components of estuarine ecosystems. This study investigated the temporal stability and change of the dominant phylogenetic groups of the domain Bacteria in estuarine biofilm communities. Glass slides were deployed monthly over 1 year for 7-day incubations during peak tidal periods in East Sabine Bay, Fla. Community profiling was achieved by using 16S rRNA genes and terminal restriction fragment length polymorphism (T-RFLP) of 16S rRNA genes in combination with ribotyping, cloning, and sequencing to evaluate diversity and to identify dominant microorganisms. Bacterial community profiles from biofilms grown near the benthos showed distinct periods of constancy within winter and summer sampling periods. Similar periods of stability were also seen in T-RFLP patterns from floating biofilms. Alternating dominance of phylogenetic groups between seasons appeared to be associated with seasonal changes in temperature, nutrient availability, and light. The community structure appeared to be stable during these periods despite changes in salinity and in dissolved oxygen.Biofilms develop on all surfaces in aquatic environments and are defined as matrix-enclosed microbial populations adherent to each other and/or surfaces (1, 32). A substantial part of the microbial activity in nature is associated with surfaces (12). Surface association (biofouling) is an efficient means for bacteria to proliferate in both favorable and sometimes hostile environments. By adopting a sessile mode of life, microbes can achieve several advantages over their planktonic counterparts (38), including the ability to capture and concentrate nutrients from the water column in the existing exopolysaccharide matrix, cometabolic interactions with neighboring microorganisms (17), and resistance to harmful chemicals (2) and environmental stress.Biofilm-associated microbes, because of their ubiquity, diverse metabolic capabilities, and high enzymatic activity, play a crucial role in biogeochemical cycling. Direct observations show that biofilm-associated organisms (photo-and heterotrophic) account for a major part of ecosystem processes, both numerically and metabolically (12). Biofilm communities in nature play a key role in the production and degradation of organic matter, the degradation of environmental pollutants, and the cycling of limiting nutrients.Biofilm formation and persistence in estuarine environments is governed by a suite of complex physical, chemical, and biological processes (4,44,45). Many of these parameters can vary significantly over different time scales. For example, nutrient availability can vary over diel light cycles, daily tidal cycles, and with rainfall events and seasonal change. Patterns in particulate and dissolved nutrient input to estuarine systems may influence shifts in biofilm bact...

The locomotory and feeding responses of a Euplotes sp. to attached populations of Vibrio natriegens and Pseudomonas fluorescens in a continuous flow system were analyzed by computer image analysis of video microscopy recordings. Upon entry into the chamber, the ciliates moved in long continuous arcs 300 µm in length during which time no bacteria were consumed. As feeding began, the average path length shortened, the arcs became tighter, and the ciliates changed direction more frequently. The feeding activity of the Euplotes appeared to be gregarious, being concentrated in patches within the biofilm of attached bacteria. It was also noted that the feeding effort targeted patches previously visited by other Euplotes, despite reduced bacterial density relative to the surrounding field of attached bacteria. This focused and intense feeding activity resulted in localized zones of nearly complete clearance within the attached bacterial populations. Loss of bacteria and averaged ciliate presence within feeding patches were determined from digitized time series images and discrimination thresholds for particle size. These data were used to determine grazing rates indicating that Euplotes sp. removed 120 V. natriegens cells·ciliate–1·h–1 and up to 882 P. fluorescens cells·ciliate–1·h–1. However, surface clearance rates for Euplotes sp. grazing on V. natriegens and P. fluorescens were 0.02 and 0.03 mm2·ciliate–1·h–1, respectively, indicating that surface grazing pressure was fairly consistent within the patches of intense feeding activity. The effect of such intense localized feeding behaviour on attached or biofilm bacteria would be to increase spatial and temporal heterogeneity within biofilms. Key words: digital image analysis, Euplotes, grazing, biofilms.

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We characterized microbial biofilm communities developed over two very closely located but distinct benthic habitats in the Pensacola Bay estuary using two complementary cultivation-independent molecular techniques. Biofilms were grown for 7 days on glass slides held in racks 10 to 15 cm over an oyster reef and an adjacent muddy sand bottom. Total biomass and optical densities of dried biofilms showed dramatic differences for oyster reef versus non-oyster reef biofilms. This study assessed whether the observed spatial variation was reflected in the heterotrophic prokaryotic species composition. Genomic biofilm DNA from both locations was isolated and served as a template to amplify 16S rRNA genes with universal eubacterial primers. Fluorescently labeled PCR products were analyzed by terminal restriction fragment length polymorphism, creating a genetic fingerprint of the composition of the microbial communities. Unlabeled PCR products were cloned in order to construct a clone library of 16S rRNA genes. Amplified ribosomal DNA restriction analysis was used to screen and define ribotypes. Partial sequences from unique ribotypes were compared with existing database entries to identify species and to construct phylogenetic trees representative of community structures. A pronounced difference in species richness and evenness was observed at the two sites. The biofilm community structure from the oyster reef setting had greater evenness and species richness than the one from the muddy sand bottom. The vast majority of the bacteria in the oyster reef biofilm were related to members of the ␥-and ␦-subdivisions of Proteobacteria, the Cytophaga-Flavobacterium -Bacteroides cluster, and the phyla Planctomyces and Holophaga-Acidobacterium. The same groups were also present in the biofilm harvested at the muddy sand bottom, with the difference that nearly half of the community consisted of representatives of the Planctomyces phylum. Total species richness was estimated to be 417 for the oyster reef and 60 for the muddy sand bottom, with 10.5% of the total unique species identified being shared between habitats. The results suggest dramatic differences in habitat-specific microbial diversity that have implications for overall microbial diversity within estuaries.Estuarine environments are among the most productive on earth, creating and processing more organic matter each year than comparably sized areas of reefs, forest, grassland, or agricultural land. This productivity supports the majority of the world's commercial and recreational fishery catch. In addition, these ecosystems are surrounded by high densities of human development that threaten the various estuarine habitats (i.e., salt marsh, sea grass, oyster reef) that contribute to this high productivity. Within each habitat, there are species that are characteristic to each and others that are less habitat specific. Thus, within-habitat diversity, or ␣-diversity, is determined by both species endemic to that habitat and the number of more widely distributed species suppo...