Bacterial Colonization and Ectoenzymatic Activity in Phytoplankton-Derived Model Particles. Part II. Cleavage and Uptake of Carbohydrates

Abstract The bacterial colonization and development of the ectoenzymatic glucosidase activity and glucose uptake were followed together with bacterial growth (measured as thymidine incorporation) in laboratory experiments, using phytoplankton-derived particles incubated in rolling tanks. Bacterial colonization of the particles was rapid. In the particles, bacterial turnover rates (production/biomass) were low (0.02 to 0.14 d⁻¹). In the ambient water, turnover rates increased from 0.1 d⁻¹ to 23.3 d⁻¹, until the end of the experiment. In the control, lacking any particles, turnover of bacteria ranged from 0.3 to 7.6 d⁻¹. Similarly, glucose uptake rates, per bacterium, were 1 to 2 orders of magnitude lower for particle-attached bacteria than for their free-living counterparts. Generally, K_m values for glucosidase activity declined, over the incubation period, in particles and free-living bacteria until 168 h, and slightly increased, thereafter, to values of approximately 0.1 μM. Particle-attached bacteria exhibited significantly lower uptake rates of both thymidine and glucose, per bacterium, throughout the incubation. The per-cell ectoenzymatic activity was similar in particle-associated and free-living bacteria during the initial phase of the experiment, but was significantly higher after ≈200 h. Dissolved total (TCHO), as well as monomeric carbohydrates (MCHO), declined continuously in both particles and ambient water; they remained constant in the control; TCHO comprised about 50% of the dissolved organic carbon (DOC) in the particles. In ambient water TCHO contribution to DOC varied, with only one exception, between 25 and 45%; and in the control, between 20 and 50%. The shift detectable in the relation between ectoenzymatic activity and uptake of glucose between free-living and attached bacteria over the incubation period may reflect changes in the physiological status of the bacteria. Received: 3 February 1997; Accepted: 6 November 1997     

Diel and Seasonal Variations in Abundance,Activity, and Community Structure of Particle-Attached and Free-Living Bacteria in NW Mediterranean Sea

Diel and seasonal variations in abundance, activity, and structure of particle-attached vs free-living bacterial communities were investigated in offshore NW Mediterranean Sea (0–1000 m). Attached bacteria were always less abundant and less diverse but generally more active than free-living bacteria. The most important finding of this study was that the activity of attached bacteria showed pronounced diel variations in the upper mixed water column with higher activities at night. Under mesotrophic conditions, the contribution of attached bacteria to total bacterial activity increased from less than 10% at day time to 83% at night time. At high chlorophyll a concentration, the highest cell-specific activities and contribution to total bacterial activity were due to free-living bacteria at day and to attached bacteria at night. Under summer oligotrophic conditions, free-living bacteria dominated and contributed to the most important part of the bacterial activity at both day and night, whereas attached bacteria were much less abundant but presented the highest cell-specific activities. These diel and seasonal variations in activities were concomitant to changes in bacterial community structure, mainly in the upper layer. The number of attached ribotypes was fairly constant suggesting that particles are colonized by a relatively limited number of ubiquitous ribotypes. Most of these ribotypes were also free-living ribotypes suggesting that attached bacteria probably originate from colonization of newly formed particles by free-living bacteria in the upper layer. These results reinforce the biogeochemical role of attached bacteria in the cycling of particulate organic carbon in the NW Mediterranean Sea and the importance of diel variability in these processes.     

Experimental study of Microcystis-associated and free-living bacteria

In the experiment with water from the hypereutrophic Lake Frederiksborg Slotso (Denmark) sampled during the autumn peak of Microcystis growth, the quantity and production of free-living and cyanobacteria-associated heterotrophic bacteria were determined, as well as the extracellular enzymatic (aminopeptidase) activity. The functional diversities of associated and free-living bacterial communities were additionally compared using BIOLOG GN microplates to reveal the possible export of Microcystis-attached bacteria into ambient water. It has been shown that the cell size, production values, and growth rates of associated bacteria were less than those of free-living bacteria. At the same time, the potential aminopeptidase activity of associated bacteria was always higher than that of free-living bacteria. The experimental results have shown significant compositional differences in the structure of bacterial communities from different habitats.     

Phylogenetic analysis of particle-attached and free-living bacterial communities in the Columbia river, its estuary, and the adjacent coastal ocean.

The Columbia River estuary is a dynamic system in which estuarine turbidity maxima trap and extend the residence time of particles and particle-attached bacteria over those of the water and free-living bacteria. Particle-attached bacteria dominate bacterial activity in the estuary and are an important part of the estuarine food web. PCR-amplified 16S rRNA genes from particle-attached and free-living bacteria in the Columbia River, its estuary, and the adjacent coastal ocean were cloned, and 239 partial sequences were determined. A wide diversity was observed at the species level within at least six different bacterial phyla, including most subphyla of the class Proteobacteria. In the estuary, most particle-attached bacterial clones (75%) were related to members of the genus Cytophaga or of the alpha, gamma, or delta subclass of the class Proteobacteria. These same clones, however, were rare in or absent from either the particle-attached or the free-living bacterial communities of the river and the coastal ocean. In contrast, about half (48%) of the free-living estuarine bacterial clones were similar to clones from the river or the coastal ocean. These free-living bacteria were related to groups of cosmopolitan freshwater bacteria (beta-proteobacteria, gram-positive bacteria, and Verrucomicrobium spp.) and groups of marine organisms (gram-positive bacteria and alpha-proteobacteria [SAR11 and Rhodobacter spp.]). These results suggest that rapidly growing particle-attached bacteria develop into a uniquely adapted estuarine community and that free-living estuarine bacteria are similar to members of the river and the coastal ocean microbial communities. The high degree of diversity in the estuary is the result of the mixing of bacterial communities from the river, estuary, and coastal ocean.     

Experimental study of Microcystis-associated and free-living cyanobacteria

In the experiment with water from the hypereutrophic Lake Frederiksborg Slotso (Denmark) sampled during the autumn peak of Microcystis growth, the quantity and production of free-living and cyanobacteria-associated heterotrophic bacteria were determined, as well as the extracellular enzymatic (aminopeptidase) activity. The functional diversities of associated and free-living bacterial communities were additionally compared using BIOLOG GN microplates to reveal the possible export of Microcystis-attached bacteria into ambient water. It has been shown that the cell size, production values, and growth rates of associated bacteria were less than those of free-living bacteria. At the same time, the potential aminopeptidase activity of associated bacteria was always higher than that of free-living bacteria. The experimental results have shown significant compositional differences in the structure of bacterial communities from different habitats.     

Decomposition of protein compounds in an eutrophic lake: spatial and temporal distribution of exopeptidase and endopeptidase activities in various size fractions

Activities of exopeptidases and endopeptidases in various size fractions and the possible regulation of these enzymes in response to the changes in substrate concentrations were studied. The endopeptidase activities were determined using a protein substrate labelled with a fluorochrome. Among the enzyme activities studied only leucine aminopeptidase and endopeptidase activities were recorded. The protein compounds did not seem to cleave by synergistic endo and exoenzyme mechanisms. The inhibitory effect of ethylenediamine tetraacetate (EDTA) and phenylmethylsulfonile fluoride (PMSF) suggested a high proportion of metallo-proteases and serine-proteases. The inhibitory profile at 1 and 5 m indicated that planktonic organisms probably produced the same type of proteases, whereas enzymes present in the 0–1.2 and 1.2–100 μm fractions were not similar. The mean percentages of aminopeptidase activity at 1, 5 and 14 m in the dissolved fraction were 12.5, 12.7 and 18.4%. This enzyme activity was low in the 0.2–1.2 μm fraction (12.1% at 1 m, 13.3% at 5 m and 19.1%, at 14 m) compared to that measured in the 1.2–100 μm fraction, whereas the average percentages of endopeptidase activities in this fraction were 50.9% at 1 m, 50.1% at 5 m and 53.5% at 14 m. The bacteria attached to particles had a higher specific activity than free-living bacteria. It was 11.7 times higher than the enzyme activity associated with larger free-living bacteria and 112.3 times higher than the activity of small free-living bacteria. In this study, the specific activity of the aminopeptidase (activity in the fraction 0.2–100 μm per number of bacteria) was correlated with Chl a at 1 m (r = 0.65, P <0.01), 5 m (r = 0.78, P <0.001) and 14 m (r = 0.96, P <0.001). The production of protein compounds by the phytoplankton could therefore regulate aminopeptidase activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Particle-attached and free-living bacterial communities in a contrasting marine environment: Victoria Harbor, Hong Kong

Diversity of particle-attached and free-living marine bacteria in Victoria Harbor, Hong Kong, and its adjacent coastal and estuarial environments was investigated using DNA fingerprinting and clone library analysis. Denaturing gradient gel electrophoresis (DGGE) analysis of 16S rRNA genes showed that bacterial communities in three stations of Victoria Harbor were similar, but differed from those in adjacent coastal and estuarine stations. Particle-attached and free-living bacterial community composition differed in the Victoria Harbor area. DNA sequencing of 28 bands from DGGE gel showed Alphaproteobacteria was the most abundant group, followed by the Bacteroidetes, and other Proteobacteria. Bacterial species richness (number of DGGE bands) differed among stations and populations (particle-attached and free-living; bottom and surface). BIOENV analysis indicated that the concentrations of suspended solids were the major contributing parameter for the spatial variation of total bacterial community structure. Samples from representative stations were selected for clone library (548 clones) construction and their phylogenetic distributions were similar to those of sequences from DGGE. Approximately 80% of clones were affiliated to Proteobacteria, Bacteroidetes and Cyanobacteria. The possible influences of dynamic pollution and hydrological conditions in the Victoria Harbor area on the particle-attached and free-living bacterial community structures were discussed.     

Temporal variability of attached and free-living bacteria in coastal waters

The temporal variability of the abundance and the incorporation of ³H-thymidine and ¹⁴C-glucose by attached and free-living bacteria, as well as their relation with environmental factors, were analyzed in a coastal marine ecosystem during a year. Both communities were quantitatively very different. Attached bacteria represented only 6.8% of the total bacterial abundance, whereas free-living bacteria represented 93.2%. The environmental factors most closely linked to the abundance and activity of free-living bacteria were temperature and the concentration of dissolved nutrients. Moreover, the free-living community showed similar temporal variations in abundance and in activity, with lower values in the cold months (from October to May). The attached community did not present the same pattern of variation as the free-living one. The abundance of the attached bacteria was mainly correlated to the concentration of particulate material, whereas their activity was correlated to temperature. We did not find a significant correlation between the abundance and the activity of the attached community. On the other hand, the activity per cell of the two communities did not present a clear temporal variation. Attached bacteria were more active than free-living ones in the incorporation of radiolabeled substrates on a per cell basis (five times more in the case of glucose incorporation and twice as active in thymidine incorporation). However, both communities showed similar specific growth rates. The results suggest that the two aquatic bacterial communities must not be considered as being independent of each other. There appears to be a dynamic equilibrium between the two communities, regulated by the concentrations of particulate matter and nutrients and by other environmental factors.Offprint requests to: M. Unanue.     

Attached and free-living bacteria: Production and polymer hydrolysis during a diatom bloom

Abundance, production and extracellular enzymatic activity of free-living and attached bacteria were measured during the development and collapse of a spring bloom in a eutrophic lake. Free-living bacteria accounted for most of the total bacterial production during the first part of the bloom. Their production had a significant positive correlation to chlorophyll (P < .01) and polysaccharide concentration (P < .02) and to potential -glucosidase and aminopeptidase activity (P < .05), suggesting that algal release of dissolved polymeric compounds provided an important carbon source for bacterial production. As the bloom collapsed, we observed a change in the activity and structure of the microbial community. The mean contribution of attached bacteria to total bacterial production increased from 12% during the first part of the bloom to 26% at the end. Also, the extracellular enzymatic activity of attached bacteria increased as the bloom collapsed and constituted up to 75% of the total hydrolytic activity. An estimated disparity between hydrolytic activity and the corresponding carbon demand of attached bacteria suggested a net release of dissolved organic compounds from organic particles via polymer hydrolysis by attached bacteria. Correspondence to: M. Middelboe     

Comparison of extracellular enzyme activities and community composition of attached and free-living bacteria in porous medium columns

Free-living and surface-associated microbial communities in sand-packed columns perfused with groundwater were compared by examination of compositional and functional characteristics. The composition of the microbial communities was assessed by bulk DNA extraction, PCR amplification of 16S ribosomal DNA fragments, separation of these fragments by denaturing gradient gel electrophoresis, and sequence analysis. Community function was assessed by measurement of beta-glucosidase and aminopeptidase extracellular enzyme activities. Free-living populations in the aqueous phase exhibited a greater diversity of phylotypes than populations associated with the solid phase. The attached bacterial community displayed significantly greater beta-glucosidase and aminopeptidase enzyme activities per volume of porous medium than those of the free-living community. On a per-cell basis, the attached community had a significantly higher cell-specific aminopeptidase enzyme activity (1.07 x 10(-7) nmol cell(-1) h(-1)) than the free-living community (5.02 x 10(-8) nmol cell(-1) h(-1)). Conversely, the free-living community had a significantly higher cell-specific beta-glucosidase activity (1.92 x 10(-6) nmol cell(-1) h(-1)) than the surface-associated community (6.08 x 10(-7) nmol cell(-1) h(-1)). The compositional and functional differences observed between these two communities may reflect different roles for these distinct but interacting communities in the decomposition of natural organic matter or biodegradation of xenobiotics in aquifers.     

Specific uptake rates of amino acids by attached and free-living bacteria in a mesotrophic lake.

Seasonal and spatial patterns of specific uptake rates of amino acids by bacteria in Lake Constance were studied. The total bacterial population was divided into small (0.2- to 1.0-micron) and large (1.0- to 3.0-micron) free-living bacteria and attached bacteria by fractionated filtration. Data for attached bacteria, received by retention on 3.0-micron-pore Nuclepore filters, were corrected for free-living bacteria in this fraction. Specific uptake rates based on autoradiography were also recorded. Specific uptake rates for attached bacteria ranged from 9.41 X 10(-11) to 6.11 X 10(-8) ng of C h-1 cell-1 and were therefore significantly greater than those for free-living bacteria during most time periods. However, they were not significantly different from those for cells proven to be active by autoradiography. Specific uptake rates for small free-living bacteria ranged between 7.68 X 10(-11) and 4.60 X 10(-9) ng of C h-1 cell-1. They were nearly in the same range of those for large free-living bacteria (5.10 X 10(-11) to 1.07 X 10(-8) ng of C h-1 cell-1), although both fractions exhibited pronounced differences in their seasonal and vertical distributions.     

Specific uptake rates of amino acids by attached and free-living bacteria in a mesotrophic lake

Seasonal and spatial patterns of specific uptake rates of amino acids by bacteria in Lake Constance were studied. The total bacterial population was divided into small (0.2- to 1.0-micron) and large (1.0- to 3.0-micron) free-living bacteria and attached bacteria by fractionated filtration. Data for attached bacteria, received by retention on 3.0-micron-pore Nuclepore filters, were corrected for free-living bacteria in this fraction. Specific uptake rates based on autoradiography were also recorded. Specific uptake rates for attached bacteria ranged from 9.41 X 10(-11) to 6.11 X 10(-8) ng of C h-1 cell-1 and were therefore significantly greater than those for free-living bacteria during most time periods. However, they were not significantly different from those for cells proven to be active by autoradiography. Specific uptake rates for small free-living bacteria ranged between 7.68 X 10(-11) and 4.60 X 10(-9) ng of C h-1 cell-1. They were nearly in the same range of those for large free-living bacteria (5.10 X 10(-11) to 1.07 X 10(-8) ng of C h-1 cell-1), although both fractions exhibited pronounced differences in their seasonal and vertical distributions.     

Archaeaplankton in the Columbia River, its estuary and the adjacent coastal ocean, USA

PCR-amplified 16S rRNA genes from particle-attached and free-living Archaea in the Columbia River estuary, particle-attached Archaea in the river, and Archaea in the adjacent coastal ocean were cloned, and 43 partial sequences were determined. There was a high diversity of Archaea in the estuary, especially among the particle-attached Archaea, with representatives from four major phylogenetic clusters. Eighteen of 21 estuarine clones were closely related to clones from the river and the coastal ocean or to clusters of marine and soil clones identified in other studies. This contrasts with a similar study of the estuarine bacterial community that found 62% of bacterial 16S rRNA clones to be unique to the estuary. Archaea in the estuary were primarily allochthonous, and therefore, unlike the bacteria, probably do not form a native estuarine community.     

Community composition and activity of prokaryotes associated to detrital particles in two contrasting lake ecosystems

The composition, distribution and extracellular enzyme activities of bacteria attached to small (2-50 microm in size) transparent exopolymer and Coomassie-stained proteinaceous particles (TEP and CSP) were examined in two lakes of different trophic status located in the Massif Central of France. TEP concentrations (10(4)-10(6) particle per L) were significantly higher in the more productive lake and were significantly related to chlorophyll a concentrations. The majority of TEP and CSP were colonized by bacteria that constituted 2.6% and 7.4% of the total 4',6-diamidino-2-phenylindole-stained bacteria in lakes Pavin and Aydat, respectively. In both lakes, the composition of particle-associated bacteria was different from that of free-living bacteria, the Betaproteobacteria and Bacteroidetes (i.e. former Cytophaga-Flavobacteria group) being the dominant groups on particles. We also found that 2-5 microm TEP were more colonized than 2-5 microm CSP in the two lakes, and that TEP colonization was higher in the less productive lake. Measurements of Leucine aminopeptidase and alpha-glucosidase activities in fractionated lake water (0.2-1.2, 1.2-5 and >5 microm fractions) indicated that proteolytic activity was always higher and that particle-associated bacteria have higher enzymatic activities than free-living bacteria. The glycolytic activities in the 1.2-5 and >5 microm fractions were related to the abundance of TEP. We conclude that small freshwater detrital organic particles constitute microhabitats with high bacterial activities in pelagic environments and, undoubtedly, present significant ecological implications for the prokaryotic community structure and function in aquatic ecosystems.     

Characterisation and differentiation of oligotrophic waters by culturable particle-attached and free-living bacterial communities

This study reports the development of a tool to characterise and differentiate northern Adriatic waters, particularly oligotrophic, high-salinity waters, based on the cellular fatty acids of culturable heterotrophic bacterioplankton. The growth abilities and population dominance were observed for particle-attached and free-living bacteria cultured in three types of media: Marine Broth, diluted Marine Broth (1:10) and R2 broth. Three groups of water layers were distinguished by hierarchical clustering analysis: eutrophic, oligotrophic and oligotrophic nutrient-selected. Significant differences between the resulting groups were tested by two-way ANOVA (with replication). Eutrophic layers were characterised by readily culturable particle-attached and free-living fractions of the bacterial community in all three media, all dominated by fast-growing γ-Proteobacteria. In contrast, oligotrophic water layers with low productivity had a much weaker culturability and a different population dominance for the free-living community, as compared to their attached or growth-arrested counterparts, for all media. The free-living bacteria from strictly oligotrophic environments demonstrated minimum culturability in Marine Broth, while those from selective oligotrophic environments were culturable and were dominated either by Cytophaga–Flavobacter complex, α-Proteobacteria or γ-Proteobacteria. The conclusive evidence regarding the selective and refractory nature of organic compounds in these waters demonstrates the dominant culturability of the Cytophaga–Flavobacter complex and α-Proteobacteria in free-living communities in all growth-media. The response of fatty acid dominance ratios depends significantly on the trophic state and fraction (p < 0.05), although the effect of the trophic state is completely different in attached and free fractions. Both fractions were tested separately, demonstrating a significant influence of the trophic state (p < 0.05), while the effect of the media on the fatty acid response was not significant (p > 0.05). An interaction between media and trophic status was present in the attached fraction (p < 0.05), yet this was not observed in the free fraction (p > 0.05), indicating that any systematic difference between trophic states was the same for each media tested. Accordingly, the free-living fraction of bacterioplankton is a more informative attribute and can be used solely as an indicator of the water layer trophic condition.     

Pelagic Bacteria–Particle Interactions and Community-Specific Growth Rates in Four Lakes along a Trophic Gradient

Abstract The relationships between bacterial concentration, bacterial production, and cell-specific activity of both free and attached bacteria and environmental factors such as suspended solids, nutrients, and temperature were examined in four lakes, two in New Zealand and two in Switzerland. Estimates of cell-specific production were obtained by microautoradiographic counts of [³H]thymidine-labeled cells. Bacteria attached to particles accounted for only 1.3 to 11.6% of the total bacterial abundance, but showed overall 20-fold higher specific growth rates and were relatively more active than their free counterparts. On average, 80 to 100% of epibacteria were attached to organic particles. The abundance and production of free and attached bacteria were positively correlated; however, relationships between these fractions and some environmental variables differed. Cell-specific activities of active bacteria were not equivalent to mean cellular activities of the entire bacterial community and differed in their relationship to trophic state. [³H]Thymidine-positive bacteria were more tightly linked to chlorophyll a than were total bacteria. Our findings indicate that production by attached bacteria, fueled by phytoplankton carbon, supplies ``new' free bacteria to the bacterial community. Our results support the idea that particulate organic matter acts as a source of dissolved nutrients to free bacteria. Bottom-up control of bacterial biomass, as shown by regressions of biomass vs production, appeared to be stronger in two ultraoligotrophic lakes than in two more eutrophic ones. Received: 17 April 1998; Accepted: 24 August 1998     

Comparison of cell-specific activity between free-living and attached bacteria using isolates and natural assemblages

Marine snow aggregates are microbial hotspots that support high bacterial abundance and activities. We conducted laboratory experiments to compare cell-specific bacterial protein production (BPP) and protease activity between free-living and attached bacteria. Natural bacterial assemblages attached to model aggregates (agar spheres) had threefold higher BPP and two orders of magnitude higher protease activity than their free-living counterpart. These observations could be explained by preferential colonization of the agar spheres by bacteria with inherently higher metabolic activity and/or individual bacteria increasing their metabolism upon attachment to surfaces. In subsequent experiments, we used four strains of marine snow bacteria isolates to test the hypothesis that bacteria could up- and down-regulate their metabolism while on and off an aggregate. The protease activity of attached bacteria was 10-20 times higher than that of free-living bacteria, indicating that the individual strains could increase their protease activity within a short time (2 h) upon attachment to surfaces. Agar spheres with embedded diatom cells were colonized faster than plain agar spheres and the attached bacteria were clustered around the agar-embedded diatom cells, indicating a chemosensing response. Increased protease activity and BPP allow attached bacteria to quickly exploit aggregate resources upon attachment, which may accelerate remineralization of marine snow and reduce the downward carbon fluxes.     

Bacterial production and growth rate estimation from [h]thymidine incorporation for attached and free-living bacteria in aquatic systems

Production and specific growth rates of attached and free-living bacteria were estimated in an oligotrophic marine system, La Salvaje Beach, Vizcaya, Spain, and in a freshwater system having a higher nutrient concentration, Butron River, Vizcaya, Spain. Production was calculated from [methyl-H]thymidine incorporation by estimating specific conversion factors (cells or micrograms of C produced per mole of thymidine incorporated) for attached and free-living bacteria, respectively, in each system. Conversion factors were not statistically different between attached and free-living bacteria: 6.812 x 10 and 8.678 x 10 mug of C mol for free-living and attached bacteria in the freshwater system, and 1.276 x 10 and 1.354 x 10 mug of C mol for free-living and attached bacteria in the marine system. Therefore, use of a unique conversion factor for the mixed bacterial population is well founded. However, conversion factors were higher in the freshwater system than in the marine system. This could be due to the different trophic conditions of the two systems. Free-living bacteria contributed the most to production in the two systems (85% in the marine system and 67% in the freshwater system) because of their greater contribution to total biomass. Specific growth rates calculated from production data and biomass data were similar for attached and free-living bacteria.     

Phylogenetic Differences in Attached and Free-Living Bacterial Communities in a Temperate Coastal Lagoon during Summer,Revealed via High-Throughput 16S rRNA Gene Sequencing

Most of what is known about coastal free-living and attached bacterial diversity is based on open coasts, with high particulate and nutrient riverine supply, terrestrial runoffs, and anthropogenic activities. The Magdalen Islands in the Gulf of St. Lawrence (Canada) are dominated by shallow lagoons with small, relatively pristine catchments and no freshwater input apart from rain. Such conditions provided an opportunity to investigate coastal free-living and attached marine bacterial diversity in the absence of confounding effects of steep freshwater gradients. We found significant differences between the two communities and marked temporal patterns in both. Taxonomic richness and diversity were greater in the attached than in the free-living community, increasing over summer, especially within the least abundant bacterial phyla. The highest number of reads fell within the SAR 11 clade (Pelagibacter, Alphaproteobacteria), which dominated free-living communities. The attached communities had deeper phylum-level diversity than the free-living fraction. Distance-based redundancy analysis indicated that the particulate organic matter (POM) concentration was the main variable separating early and late summer samples with salinity and temperature changes also significantly correlated to bacterial community structure. Our approach using high-throughput sequencing detected differences in free-living versus attached bacteria in the absence of riverine input, in keeping with the concept that marine attached communities are distinct from cooccurring free-living taxa. This diversity likely reflects the diverse microhabitats of available particles, implying that the total bacterial diversity in coastal systems is linked to particle supply and variability, with implications for understanding microbial biodiversity in marine systems.