Microbes, single-celled organisms in the domains Bacteria, Archaea, and Eukarya, interact in complex food webs in terrestrial and aquatic habitats. Most research on microbial food webs has been in aquatic ecosystems, and especially in the ocean. Primary production by photosynthetic and chemosynthetic microbes forms the base of food webs in most aquatic habitats. A large fraction of primary production, up to 50% or more, is utilized by heterotrophic prokaryotes. Prokaryotes in oxic, hypoxic, and anoxic habitats in aquatic water columns and sediments carry out significant biogeochemical transformations, completing major elemental cycles in the biosphere. As major predators of prokaryotic and algal cells, as consumers of biomass at the base of food webs, as regenerators of inorganic nutrients from the organic matter of their prey, and as a food resource for larger consumers, phagotrophic protists play a central role in the structure and functioning of aquatic food webs. Microbes also form interspecific symbioses that facilitate the growth of the host and symbiont, and interact via chemical signals in the environment or in other microbial cells. Models of the roles of microbes and known pathways of interaction in microbial food webs are continuing to evolve.

We tested the idea that bacterial cells with high nucleic acid content (HNA cells) are the active component of marine bacterioplankton assemblages, while bacteria with low nucleic acid content (LNA cells) are inactive, with a large data set (>1700 discrete samples) based on flow cytometric analysis of bacterioplankton in the Northeast Pacific Ocean off the coast of Oregon and northern California, USA. Samples were collected in the upper 150m of the water column from the coast to 250km offshore during 14 cruises from March 2001 to September 2003. During this period, a wide range of trophic states was encountered, from dense diatom blooms (chlorophyll-a concentrations up to 43μgl−1) at shelf stations during upwelling season (March–September) to lower chlorophyll-a concentrations (0.1–5μgl−1) during winter (November–February) and at basin stations (>1700m depth). We found only weakly positive relations of log total bacterial abundance to log chlorophyll-a concentration (as a proxy for availability of organic substrate), and of HNA bacteria as a fraction of total bacteria to log chlorophyll-a. Abundance of HNA and LNA bacteria co-varied positively in all regions, although HNA bacteria were more responsive to high phytoplankton biomass in shelf waters than in slope and basin waters. Since LNA cell abundance in general showed responses similar to those of HNA cell abundance to changes in phytoplankton biomass, our data do not support the hypothesis that HNA cells are the sole active component of marine bacterioplankton.

Variability in both the abundance and phylogenetic diversity of biosynthetically active prokaryotes has implications for global carbon cycling. In the present study, our primary goal was to determine the extent of variability in phylogenetic diversity of biosynthetically active prokaryotes from 3 regions in the California Current System off the Oregon coast, ranging from eutrophic shelf to oligotrophic basin. Assimilation of H-3-leucine, as determined by microautoradiography, was combined with fluorescence in situ hybridization (MICROFISH) to identify biosynthetically active prokaryotes. Oligonucleotide probes targeted 2 domains (Bacteria and Archaea), and 4 groups within the Bacteria (Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Cytophaga-like cells). We found that the Alphaproteobacteria and Cytophaga-like cells comprised the largest proportion of bacterial cells assimilating leucine. Alphaproteobacteria was the only group in which the abundance of active cells was significantly correlated to in situ phytoplankton stocks. Archaea were present in low numbers in most samples. However, in deep (> 250 m) samples from the oligotrophic basin station, 43% of cells identified as Archaea were biosynthetically active. In general, we observed a similar change in the proportional abundance of cells assimilating leucine for all identified phylogenetic groups. Thus, at this phylogenetic level, our data set is evidence for tandem increase or decrease in biosynthetic activity by the whole prokaryotic community, rather than for shifts in activity by specific phylogenetic groups.

The balance between rates of protein synthesis (measured by incorporation of radioactively labeled leucine) and rates of DNA synthesis (measured by incorporation of radioactively labeled thymidine) has been used to evaluate growth state in marine bacterioplankton. Our objective was to determine if variability in leucine and thymidine incorporation could further elucidate ecological differences in the role of high nucleic acid (HNA) and low nucleic acid (LNA) cells in marine ecosystems. We report here the first data set in which cell-specific rates of leucine and thymidine incorporation have been compared for HNA and LNA bacterial cells in the open ocean. In general, HNA cells had higher cell-specific incorporation rates of both leucine and thymidine, and had higher leucine:thymidine (Leu:TdR) incorporation ratios than LNA cells. The higher Leu:TdR ratios for HNA cells suggest that increases in the ratio may be associated with increased metabolic activity by marine bacterioplankton. Leu:TdR incorporation ratios were significantly positively correlated to temperature, but not to chlorophyll a concentrations or phytoplankton biomass calculated from specific carbon:chlorophyll a ratios, indicating that in this region temperature had a greater effect on bacterial growth state than did substrate supply. The proportion of total heterotrophic bacterial activity attributable to LNA cells was significantly greater at slope and basin stations compared to the mesotrophic shelf station. Finally, the difference in incorporation rates between HNA and LNA cells was greater for protein synthesis than for DNA synthesis.

Patterns of spatial distribution of small-sized phytoplankton: coccoid cyanobacteria (Synechococcus, SYN) and small photosynthetic eukaryotes (PEUK) were investigated in the upwelling ecosystem off the coasts of Oregon and Northern California during nine cruises of the GLOBEC Northeast Pacific Long Term Oceanographic Program (LTOP), from March 2001 to December 2002. SYN cells were 1–2μm, and most PEUK cells <5μm, in size. We found a consistent pattern of lowest abundance of small-sized phytoplankton in shelf regions, despite high nutrient and chlorophyll concentrations in recently upwelled water. Seaward of the upwelling front, the region of transition between inshore vertical sigma-t surfaces characteristic of upwelling and offshore horizontal sigma-t surfaces characteristic of vertical stratification, there were high abundances of both SYN (5–58×104cellsml−1) and of PEUK (1–8.6×104cellsml−1) in the upper 50m of the water column. Log-linear plots of SYN and PEUK abundances against nitrate+nitrite concentration showed a negative relationship between abundance and nutrient concentration. At chlorophyll-a concentrations of <∼2μgl−1), small cells typically comprised most of the carbon biomass of the phytoplankton. The pattern of phytoplankton distribution found in this study suggests a dramatic spatial shift in the structure of pelagic food webs in the Oregon upwelling ecosystem, from shelf upwelling blooms dominated by large diatoms, to slope and basin food webs dominated by<5μm-sized phototrophic cells. Why the abundances of small-sized phytoplankton, and especially of coccoid cyanobacteria, were low in high-nutrient, high-chlorophyll shelf waters remains to be explained.

ABSTRACT We evaluated whether bacteria with higher cell-specific nucleic acid content (HNA) or an active electron transport system, i.e., positive for reduction of 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), were responsible for the bulk of bacterioplankton metabolic activity. We also examined whether the phylogenetic diversity of HNA and CTC-positive cells differed from the diversity of Bacteria with low nucleic acid content (LNA). Bacterial assemblages were sampled both in eutrophic shelf waters and in mesotrophic offshore waters in the Oregon coastal upwelling region. Cytometrically sorted HNA, LNA, and CTC-positive cells were assayed for their cell-specific [3H]leucine incorporation rates. Phylogenetic diversity in sorted non-radioactively labeled samples was assayed using denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA genes. Cell-specific rates of leucine incorporation of HNA and CTC-positive cells were on average only slightly greater than the cell-specific rates of LNA cells. HNA cells accounted for most bacterioplankton substrate incorporation due to high abundances, while the low abundances of CTC-positive cells resulted in only a small contribution by these cells to total bacterial activity. The proportion of the total bacterial leucine incorporation attributable to LNA cells was higher in offshore regions than in shelf waters. Sequence data obtained from DGGE bands showed broadly similar phylogenetic diversity across HNA, LNA, and CTC-positive cells, with between-sample and between-region variability in the distribution of phylotypes. Our results suggest that LNA bacteria are not substantially different from HNA bacteria in either cell-specific rates of substrate incorporation or phylogenetic composition and that they can be significant contributors to bacterial metabolism in the sea.

Physical and geochemical data collected weekly during the year-long 2800km drift of the CCGS des Groseilliers show that Canada Basin waters, and in particular the composition of the halocline, can no longer be viewed as laterally homogeneous and in steady state. The halocline was thinner over the Mendeleyev Abyssal Plain and northern Chukchi Plateau. Here, Pacific-origin upper and middle halocline waters occupied the upper 80m of the water column and underlying Atlantic-origin lower halocline waters were fresher, colder and much more ventilated than observed in the past. These new observations of a sub-surface oxygen maximum suggest that outflow from the East Siberian Sea now supplies the Canada Basin lower halocline. East of the Northwind Ridge the halocline was thicker and appeared relatively unchanged. Here Pacific-origin upper and middle halocline waters occupied the top 225m and Atlantic-origin lower halocline waters were identified by an oxygen minimum. The intensity of the Pacific-origin signal, characterized by a nutrient maximum, was strongest over the Chukchi Gap—the passage between the Chukchi Shelf and Plateau—and the Northwind Abyssal Plain and identified two winter-water spreading pathways. Atlantic-origin waters as much as 0.5°C warmer than the historical record were observed over the Chukchi Gap and also over the northern flank of the Chukchi Plateau. These observations signaled that warm-anomaly Fram Strait Branch (FSB) waters, first observed upstream in the Nansen Basin in 1990, had arrived downstream in the Canada Basin eight years later and also indicate two routes whereby FSB waters enter the southern Canada Basin. Although samples were collected throughout one annual cycle, seasonal effects were small and confined to the upper 50m of the water column. These data show Canada Basin waters are in transition, responding to the effects of upstream change in atmospheric and oceanic circulation.

Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124d in mid-winter suggested a respiration rate of ∼3.3nM O2h−1 and a carbon demand of ∼4.5gCm−2. Increase in 0–50m integrated stocks of DO during summer implied a net community production of ∼20gCm−2. Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0±10.6nM O2h−1) compared to summer (35.3±24.8nM O2h−1). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased ∼10-fold, and the cell-specific rate of leucine incorporation ∼5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50m were, however, low compared to other oceanic regions, averaging 0.52±0.47ngCl−1h−1 during winter and 5.1±3.1ngCl−1h−1 during summer. Total carbon demand based on respiration experiments averaged 2.4±2.3mgCm−3d−1 in winter and 7.8±5.5mgCm−3d−1 in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.12±0.12mgCm−3d−1 in winter and 1.3±0.7mgCm−3d−1 in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region.

As part of the SHEBA/JOIS drift experiment, we continually analysed abundance and biomass of autotrophic and heterotrophic microbes in the upper 120m of the water column of the ice-covered Central Arctic Ocean from November 1997 through August 1998. Microbial biomass was concentrated in the upper 60m of the water column. There were low but persistent stocks of heterotrophic and autotrophic microbes during the winter months. Phytoplankton biomass began increasing when winter snow melted from the ice-pack in early June, after which there was a progressive decline of nitrate and silicate in the euphotic zone. We observed three distinct blooms over the summer. The initial bloom consisted of diatoms and phytoflagellates, mainly 2μm-sized Micromonas sp.; the two subsequent blooms were dominated by the flagellated (non-colonial) Phaeocystis sp. The carbon:chlorophyll ratio of the phytoplankton was 31±11. Stocks of bacteria and heterotrophic protists approximately doubled during the growing season, increasing in tandem with increase in phytoplankton biomass. Increase in cell abundances of bacteria and of the phytoflagellate Micromonas over 40–50d periods during the initial bloom period yielded estimates of realised growth rate of 0.025d−1 for bacteria and of 0.11d−1 for Micromonas. Heterotrophic protists included flagellates, ciliates, and dinoflagellates, with biomass divided nearly evenly between nanoplankton (Hnano, 0–20μm) and microplankton (Hmicro, 20–200μm) size classes.

During the Ocean Margins Program, we obtained data on the abundances of bacterioplankton and heterotrophic flagellates, and on rates of bacterivory, across the mid-Atlantic continental shelf off Cape Hatteras, NC, during four spring and summer cruises from 1993 to 1996. Bacterial and grazing parameters were compared for inner, middle, and outer shelf regions. In 1996, we sampled during two seasons: early spring (March) and mid-summer (July), and in addition determined the fractions of in situ bacterioplankton that had visible nucleoids (NV cells), or that had highly active electron transport systems (ETS), i.e. that were positive for reduction of the fluorogenic formazan compound, 5 cyano-2,3 ditolyl tetrazolium chloride (CTC+ cells), as well as the volumetric concentration of organic detrital particles. Detrital volumes and abundances of bacterioplankton and of heterotrophic flagellates, varied by an order of magnitude, and decreased from inshore to offshore shelf regions. In 1996, bacterial abundances and percentages of CTC+ cells were higher across the shelf during the early spring bloom season (March) compared to the post-bloom season (July). In March 1996, percentages of bacterial cells with visible nucleoids varied between 20% and 70%, but showed little change across the shelf; while fractions of total bacteria with highly active ETS were lower and more variable (1–16% CTC+ cells), and on average were twice as high in the inner shelf region compared to the rest of the shelf. Percentages of CTC+ cells were also higher for particle-associated bacteria. There was a strong positive relationship between percent CTC+ cells and volume of organic detrital particles. However, % CTC+ cells and detrital volume were not consistently related to either bulk particulate organic carbon or chlorophyll. Bacterivory, assessed via rate of ingestion of fluorescently labeled bacteria, could remove 2–9% (4–18% accounting for motile cells) of total bacterial stocks per day. If bacterivores selectively grazed CTC+ bacteria, then bacterivory could result in turnover of CTC+ cells at rates on the order of 0.4–1.4d−1.

Variation in bacterial abundance and activity was assessed by sampling the upper 35 to 80 m of the water column during 2 to 5 d periods at 3 sites: eutrophic-mesotrophic midshelf, mesotrophic-oligotrophic slope, and oligotrophic gyre edge, off the Oregon coast in late summer 1997 and 1998. Bacterial abundances varied 10-fold, from 0.2 to 2.3 x 10(6) cells ml(-1), and leucine incorporation rates varied 160-fold, from 1.5 to 240 muM h(-1). During the strong El Nino event in 1997, bacterial abundances were similar at all 3 sites, but midshelf H-3-leucine incorporation rates were similar to6-fold higher than rates at the slope and gyre sites. In 1998, after relaxation of the El Nino, bacterial abundances were lower, and average 3H-leucine incorporation rates were only 2.5 times higher at the midshelf site than at the slope and gyre sites. There was a close correlation between estimates of bacterial cell production rate based on 3 H-leucine and on 3 H-thymidine incorporation rates for the midshelf and slope sites, but no relation between the 2 estimates for the gyre site. During both years, bacterial abundance varied inversely with depth, salinity, and macronutrients and positively with temperature. Bacterial activity varied positively with chlorophyll concentration, temperature, and bacterial biomass. Rates of bacterial H-3-leucine incorporation were most strongly related to chlorophyll concentrations at the midshelf site and less related at sites farther offshore. There was no significant relation of bacterial parameters with the concentration of dissolved organic carbon. Our results showed dynamic mesoscale variability, on scales of 10s of meters to 10s of kilometers, and on scales of hours to days in rates of bacterial activity, which was positively related to phytoplankton concentration as a proxy for trophic state of the water mass. We also found inter-annual differences in distribution of bacterial abundance and activity in this region, which appeared to be related to climatic variability.

Three staining methods were used to identify metabolically active bacteria in Lake Kinneret, northern Israel: CTC, DAPI staining followed by a propanol wash, and the Molecular Probes Live/Dead stain. Positive results from these methods purport to show, respectively, actively respiring bacteria (CTC+), cells with intact nucleoids (NuCC), and cells with intact membranes (MEM+). Concomitantly, bacterial metabolic activity was measured as electron transport system (ETS) flux, O-2 uptake, activities of peptidase, beta -glucosidase and Lipase, and rate of leucine incorporation in monthly samples taken for 2.5 yr at a pelagic lake station. Laboratory experiments followed changes during 22 or 40 h in the percentages of 'active' bacteria in GF/C-filtered lake water with or without substrate enrichment or antibiotic inhibitors of cell division, or with bacterivorous protists. In lake samples, each of the staining methods detected different aspects of cellular state or metabolic activity but all 3 indicated low percentages of 'active' bacteria relative to total bacterial abundance. CTC+ ranged from 1.0 to 27.3% (average 5.1%), NuCC from 1.4 to 42.9% (average 8.3 %) and MEM+ from 1.0 to 29.9 % (average 8.8 %), with no clear seasonal or spatial patterns. No significant correlations were found between the proportions of 'active' bacteria in lake water as determined by these methods, although such correlations were observed in the laboratory experiments. Significant correlations were obtained between ETS and O-2 uptake, peptidase and P-glucosidase, and between leucine incorporation and peptidase. ETS was significantly correlated with CTC+ and NuCC cell abundance, but not with total bacteria (DAPI counts). In contrast, peptidase activity correlated with total bacterial counts. Results of time course experiments indicated that some bacteria which initially appear to be inactive can become active when stimulated by substrate addition, even though cell division is inhibited. Grazing by protists increased the percentage of active bacteria, at least during the active predator-prey phase. Our data support the hypothesis that in natural waters usually only a small fraction (probably <20 %) of the entire bacterial assemblage is strongly active metabolically at any given time. This proportion may increase dramatically with localized substrate inputs. The concept of bacterial assemblages, heterogeneous not only in terms of phylotype, but also in terms of levels of metabolic activity will need to be considered in future aquatic ecosystem models.

The important thing is to not stop questioning , Albert Einstein. Our colleague Jaroslav Vrba raised a number of points regarding application of the fluorogenic substrate MUF- N -acetyl-β-d-glucosaminide (MUF-[GlcNAc]) in methods designed to address aspects of the ecology of bacterivorous protists in fresh and marine waters [1]. His lengthy comment and associated references, we would argue, serve to underscore the difficulties involved with the use of enzyme-based methods in natural systems. To clarify our and Vrba's discussion, a central issue is that both low-affinity and high-affinity hydrolytic enzymes are expressed by aquatic organisms. Low-affinity enzymes are saturated at high (>20–100 μM) substrate concentrations, while high-affinity concentrations are saturated at much lower (<1–3 μM) concentrations of substrate. In addition, hydrolytic enzymes expressed by various organisms show different patterns of activity with respect to pH; some hydrolytic enzymes have peak activity at neutral to slightly basic pH, while others have peak activity at acidic pH [2,3]. Two methods have been proposed which rely on the rate of enzymatic cleavage of MUF-[GlcNAc] as a proxy for estimating the amount of β-glucosaminidase (βGAM)-like activity in aquatic systems. Vrba et al. [4,5] developed a method for quantifying in situ protistan bacterivory based on the rate of cleavage of low concentrations (high-affinity enzyme) of MUF-[GlcNAc] in whole water samples at neutral pH. Zubkov and Sleigh [6] subsequently suggested that acid βGAM activity at higher substrate concentrations (low-affinity enzyme), determined for live plankton samples collected on glass fiber filters, could be used to estimate the biomass of bacterivorous protists. We demonstrated that low-affinity βGAM activity …

The majority of bacteria suspended in seawater do not appear to be metabolically active or in good physiological condition as assessed by various methods. We tested the idea that a large fraction of 'inactive' bacterial cells can become 'active' with respect to detectable cell-specific electron transport system (ETS) activity, determined by the ability of cells to reduce the fluorogenic tetrazolium salt, CTC, when incubated for periods of time with or without additional substrate. Aliquots of 1.0 mu m filtered seawater were amended with mixed antibiotics to inhibit DNA synthesis and thus cell division, and incubated at in situ (12.8 and 16.4 degrees C) temperature or at 20 degrees C. Additions included: phosphate (0.83 mM.P, 5.3 mgP l(-1)), ammonium (1.67 mM N, 23.4 mgN l(-1)), and organic carbon as glucose, mixed amino acids or yeast extract (8.33 mM C, 100 mgC l(-1)). At 20 degrees C, the addition of mixed amino acids and yeast extract resulted in a large increase of % ETS-active cells (CTC-positive [CTC+] cells), from 1.9-2.4% at 0 h to 55-87% CTC+ cells by 21 to 28 h. At in situ temperature, the increase in % CTC+ cells was less, and the glucose addition caused the greatest increase in % CTC+ cells. Under conditions of increased temperature and high concentration of organic substrate, a large proportion of the apparently 'inactive' bacteria can become highly ETS-active within a day, suggesting that these cells are in fact alive, and are capable of attaining significant metabolic activity. The different response patterns of the bacterial assemblages at 20 degrees C compared to those at 12.8 and 16.4 degrees C suggests that temperature can be an important factor in bacterioplankton response to increase in substrate concentration.

We evaluated the extent to which the average metabolic activity of open ocean bacterioplankton changed during 2 to 3 d incubations of 1 mu m filtered seawater at in situ temperature. Indices of bacterial activity during incubation experiments were compared to those of in situ bacterioplankton, which were repeatedly sampled at each oceanic site. Indices included: total and cell-specific incorporation rates of tritiated leucine ([H-3]-Leu) and thymidine ([H-3]-TdR), ratio of Leu to TdR incorporation, and percentage of highly active cells as determined by detectable reduction of the fluorogenic dye, 5-cyano-2,3-ditolyl tetrazolium chloride (CTC). Patterns of change in metabolic activity were similar for experiments with water collected in 3 regions: midshelf, slope, and offshore, off the Oregon coast, USA, and incubated at in situ temperatures of 9.5 to 18 degrees C. At the beginning of the experiments, cell-specific incorporation rates of [H-3]-Leu and [H-3]-TdR were lower compared to in situ rates, but after 19 to 28 h, incorporation rates of [H-3]-Leu and [H-3]-TdR increased dramatically, by more than an order of magnitude, compared to in situ activity. When scaled to the abundance of CTC positive (CTC+) cells, rates of Leu and TdR incorporation were 2.8- to 8.1-fold higher during the incubations compared to activity of in situ CTC+ cells. In this study, marine bacteria in 1 mu m, incubated filtrate exhibited a much larger Variation in metabolic activity than did in situ bacterioplankton. In addition, the proportion of CTC+ cells was closely related to cell-specific rates of Leu and TdR incorporation.

Activity at acidic pH (4.5) of β-glucosaminidase (βGAM) has been suggested as a quantitative marker for biomass of bacterivorous protists in aquatic ecosystems. βGAM is an enzyme that cleaves peptidoglycan, a major component of bacterial cell walls. Measuring the rate of cleavage of the fluorochrome methylumbelliferone (MUF) from the fluorogenic substrate MUF-N-acetyl-β-d-glucosaminide (MUF-[GlcNAc]) is a simple assay for in situ activity of βGAM. However, this approach is seriously compromised by three characteristics of the enzyme: (1) all classes of marine microbes tested: bacteria, protists, and phytoplankton, exhibit βGAM activity, (2) the pH maximum for activity of βGAM is in the range of 6–8 for all classes of marine microbes, and (3) some species of marine phytoplankton have relatively high cell-specific and volume-specific βGAM activities at pH 4.5 and/or pH 7. Based on these results, enzymatic cleavage of the MUF-[GlcNAc] substrate does not appear to be useful as a specific assay for in situ biomass of heterotrophic protists, although the method could be applied in defined culture experiments.

The redox dye 5-cyano-2, 3-ditolyl tetrazolium chloride (CTC) is used in aquatic sciences as a vital stain for enumeration of respiring bacteria in situ. Questions concerning its efficacy have been raised. We propose that the abundance of CTC-positive (CTC+) bacteria is a useful parameter in microbial ecology based on the following information: (1) Taxonomically diverse strains of aerobic, heterotrophic marine bacteria reduce CTC to its fluorescent product. (2) The proportion of CTC+ cells in laboratory cultures and in bacterioplankton assemblages varies in meaningful ways: the proportion of CTC+ cells is greatest for bacteria in log-phase growth, and lowest for bacteria in late stationary phase; particle-associated bacteria in various marine environments exhibit a higher percentage of CTC+ cells compared to bacteria freely suspended in the water column; the proportion of CTC+ cells in a bacterioplankton assemblage can be increased by an order of magnitude or more by addition of substrate, in the absence of net change in bacterial numbers. (3) Flow cytometric analysis shows a strong relationship between characteristics of CTC+ cells (abundance, size, red fluorescence) and rates of leucine incorporation by bacterial assemblages. We suggest that CTC+ cells represent those bacteria characterized by a high level of metabolic activity, and that cells which show no apparent reduction of CTC have either low or no metabolic activity. Some portion of CTC-negative (CTC-) cells may have sufficient RNA content, and/or ability to assimilate labile substrates at dilute concentrations, to be identifiable as 'active' via indices of cell-specific rRNA content or of microautoradiography. Quantitative differences in metabolism between 'highly active' CTC+ cells, and 'less active' CTC- cells have yet to be determined.

Distribution, general composition and activity of heterotrophic protists, as well as the distribution of bacteria, were assessed in the upper water column of the central Arctic Ocean during the Arctic Ocean Section, July–September 1994. Bacterial biomass varied from 5 to > 25 mg C 1−1, with the highest values occurring in the Chukchi Sea. Protist biomass was highest (5–107 mg Cl−1) in the upper 50 m of the water column. Higher integrated (0–50 m) protist biomass values (average 910±250 mg C m−2, range 580–1370 mg C m−2) were found in the Chukchi Sea, compared to the central Arctic Ocean (average 480±320 mg C m−2, range 120–1120 mg C m−2). Heterotrophic dinoflagellates were more abundant than ciliates in the >20 μm size class at all stations. In the central Arctic Ocean, the <20 μm size class was numerically composed of dinoflagellates (16%), choanoflagellates (4%) and other flagellates (80%). Choanoflagellates were slightly more abundant in the Chukchi Sea (9% of cell numbers), but were a large component of the flagellate assemblage (55% of cell numbers) at only one station, in the Nansen Basin. Bacterivory estimated via uptake of added fluorescently labeled bacteria ranged from 1·2 × 103 to 46 × 103 bacteria ml−1 day−1; the highest rate was found at the station with a high choanoflagellate abundance. Observation of food vacuole contents showed that all size classes and taxonomic types of protists ingested phytoplankton. Choanoflagellates, and monads as small as 1·5 μm in size, ingested picoplanktonic eukaryotic phytoplankton, which were abundant (103−104 cells ml−1) in the upper 50 m. Larger protists ingested cryptomonads and diatoms, as well as pico-autotrophs. Clearance rates of 10–100 μm sized ciliates and dinoflagellates, based on the uptake of 1–5 μm fluorescent microspheres, were similar to rates reported for herbivorous protists in temperate waters. In terms of ecosystem carbon flow, we infer that phagotrophic protists in the Arctic Ocean are important consumers of phytoplankton and bacteria, and may represent a significant food resource for zooplankton.

We investigated the report of Zweifel and Hagström that only a portion of marine bacteria contain nucleoids—the DNA‐containing regions of procaryotic cells—and that such bacteria correspond to the active or viable fraction of bacterioplankton. In Oregon coastal waters, 21–64% of bacteria had visible nucleoids; numbers of nucleoid‐visible (NV) bacteria were greater than numbers of metabolically active bacteria, based on cells with active electron transport systems (ETS) and intact cell membranes. During log growth of a marine isolate, proportions of NV and ETS‐active cells approached 100%. In stationary growth phase, the fraction of ETS‐active cells decreased rapidly, while that of NV cells remained high for 7 d. When starved cells of the isolate were resupplied with nutrient (50 mg liter‒1 peptone), total cell number did not increase during the initial 6 h, but the proportion of NV cells increased from 27 to 100%, and that of ETS‐active cells from 6 to 75%. In an analogous experiment with a bacterioplankton assemblage, a similar trend was observed: the number of NV cells doubled during the initial 6 h prior to an increase in total cell counts. These results show that some bacteria without visible nucleoids are capable of becoming NV cells, and thus have DNA in a nucleoid region not detectable with the method used here.