Dynamics of microbial communities on marine snow aggregates: colonization,growth, detachment,and grazing mortality of attached bacteria

We studied the dynamics of microbial communities attached to model aggregates (4-mm-diameter agar spheres) and the component processes of colonization, detachment, growth, and grazing mortality. Agar spheres incubated in raw seawater were rapidly colonized by bacteria, followed by flagellates and ciliates. Colonization can be described as a diffusion process, and encounter volume rates were estimated at about 0.01 and 0.1 cm(3) h(-1) for bacteria and flagellates, respectively. After initial colonization, the abundances of flagellates and ciliates remained approximately constant at 10(3) to 10(4) and approximately 10(2) cells sphere(-1), respectively, whereas bacterial populations increased at a declining rate to >10(7) cells sphere(-1). Attached microorganisms initially detached at high specific rates of approximately 10(-2) min(-1), but the bacteria gradually became irreversibly attached to the spheres. Bacterial growth (0 to 2 day(-1)) was density dependent and declined hyperbolically when cell density exceeded a threshold. Bacterivorous flagellates grazed on the sphere surface at an average saturated rate of 15 bacteria flagellate(-1) h(-1). At low bacterial densities, the flagellate surface clearance rate was approximately 5 x 10(-7) cm(2) min(-1), but it declined hyperbolically with increasing bacterial density. Using the experimentally estimated process rates and integrating the component processes in a simple model reproduces the main features of the observed microbial population dynamics. Differences between observed and predicted population dynamics suggest, however, that other factors, e.g., antagonistic interactions between bacteria, are of importance in shaping marine snow microbial communities.     

Protozoan Bacterivory and Escherichia coli Survival in Drinking Water Distribution Systems

The development of bacterial communities in drinking water distribution systems leads to a food chain which supports the growth of macroorganisms incompatible with water quality requirements and esthetics. Nevertheless, very few studies have examined the microbial communities in drinking water distribution systems and their trophic relationships. This study was done to quantify the microbial communities (especially bacteria and protozoa) and obtain direct and indirect proof of protozoan feeding on bacteria in two distribution networks, one of GAC water (i.e., water filtered on granular activated carbon) and the other of nanofiltered water. The nanofiltered water-supplied network contained no organisms larger than bacteria, either in the water phase (on average, 5 × 10⁷ bacterial cells liter⁻¹) or in the biofilm (on average, 7 × 10⁶ bacterial cells cm⁻²). No protozoa were detected in the whole nanofiltered water-supplied network (water plus biofilm). In contrast, the GAC water-supplied network contained bacteria (on average, 3 × 10⁸ cells liter⁻¹ in water and 4 × 10⁷ cells cm⁻² in biofilm) and protozoa (on average, 10⁵ cells liter⁻¹ in water and 10³ cells cm⁻² in biofilm). The water contained mostly flagellates (93%), ciliates (1.8%), thecamoebae (1.6%), and naked amoebae (1.1%). The biofilm had only ciliates (52%) and thecamoebae (48%). Only the ciliates at the solid-liquid interface of the GAC water-supplied network had a measurable grazing activity in laboratory test (estimated at 2 bacteria per ciliate per h). Protozoan ingestion of bacteria was indirectly shown by adding Escherichia coli to the experimental distribution systems. Unexpectedly, E. coli was lost from the GAC water-supplied network more rapidly than from the nanofiltered water-supplied network, perhaps because of the grazing activity of protozoa in GAC water but not in nanofiltered water. Thus, the GAC water-supplied network contained a functional ecosystem with well-established and structured microbial communities, while the nanofiltered water-supplied system did not. The presence of protozoa in drinking water distribution systems must not be neglected because these populations may regulate the autochthonous and allochthonous bacterial populations.     

Size-selective grazing of coastal bacterioplankton by natural assemblages of pigmented flagellates,colorless flagellates,and ciliates

Fluorescently-labelled bacteria (FLB) were used to study the feeding strategies of a natural assemblage of estuarine protozoans and to examine whether the protozoan grazing could account for the in situ size structure of the bacterioplankton. The FLB, DTAF-stained enterococci, ranging in volume from 0.01 to 0.30 × 10^–1 µm³, were added to a natural planktonic assemblage at a density of 5.5% of the natural bacterioplankton. Initial densities (individuals ml^–1) were as follows: total natural bacteria, 2.2 × 10⁶; FLB, 1.2 × 10⁵; pigmented flagellates, 300; colorless flagellates, 250; and ciliates, 30. FLB consumption rates were determined by examining the contents of protozoan food vacuoles, and the long-term effect of grazing (over a period of 100 hours) was determined by monitoring the decline in the FLB density in experimental vessels. The average consumption rates of FLB by pigmented flagellates were similar to those by flagellates that lacked chloroplasts (0.9 and 0.6 FLB protozoan^–1 hour^–1, respectively). The ciliates consumed bacteria at an average rate that was 17-fold higher (per cell) than flagellates, and they displayed a greater preference for larger bacteria than did the flagellates. FLB of the mid-size classes (0.025–0.100 µm³) were heavily grazed by the entire protozoan assemblage; the smallest (<0.025 µm³) and the largest (>0.100 µm³) FLB escaped protozoan grazing. This had a profound effect on the resulting size distribution of FLB. At the end of a 100-hour incubation, the percentage of mid-size FLB (0.025 to 0.100 µm³) decreased 2.0–2.2-fold, while the percentage of the smallest and the largest FLB increased 2.0–2.5-fold. Resultant densities of FLB were consistent with initial clearance rates determined for the protozoan groups. The grazing rates of protozoans on FLB were species-specific; whereas some species consumed FLB, others did not demonstrate bacterivory. The results suggest that protozoan grazing has a major effect on the size distribution of coastal bacterioplankton. By selectively feeding on a particular size-class of bacteria, planktonic ciliates may consume 15–90% day^–1 of the standing stock of largest size classes of bacterioplankton. Thus, ciliates, which were present in low abundance in the field, could not balance the production of the entire bacterial community, but they may strongly influence the portion of the bacterial community represented by the largest bacterial class. The direct effect of flagellates (e.g., grazing) was limited to smaller bacteria.Offprint requests to: M. P. Shiaris.     

Protistan bacterivory in an oligomesotrophic lake: Importance of attached ciliates and flagellates

Seasonal and depth variations of the abundance, biomass, and bacterivory of protozoa (heterotrophic and mixotrophic flagellates and ciliates) were determined during thermal stratification in an oligomesotrophic lake (Lake Pavin, France). Maximal densities of heterotrophic flagellates (1.9 × 10³ cells ml^–1) and ciliates (6.1 cells ml^–1) were found in the metalimnion. Pigmented flagellates dominated the flagellate biomass in the euphotic zone. Community composition of ciliated protists varied greatly with depth, and both the abundance and biomass of ciliates was dominated by oligotrichs. Heterotrophic flagellates dominated grazing, accounting for 84% of total protistan bacterivory. Maximal grazing impact of heterotrophic flagellates was 18.9 × 10⁶ bacteria 1^–1h^–1. On average, 62% of nonpigmented flagellates were found to ingest particles. Ciliates and mixotrophic flagellates averaged 13% and 3% of protistan bacterivory, respectively. Attached protozoa (ciliates and flagellates) were found to colonize the diatom Asterionella formosa. Attached bacterivores had higher ingestion rates than free bacterivorous protozoa and may account for 66% of total protozoa bacterivory. Our results indicated that even in low numbers, epibiotic protozoa may have a major grazing impact on free bacteria. Correspondence: C. Amblard.     

Seasonal cycle of the microbial plankton in Crooked Lake,Antarctica

Summary Changes in the abundance of the components of the microbial plankton between July 1990 and March 1991 in Crooked Lake, one of the largest and deepest freshwater lakes in Antarctica, are described. Chlorophyll a concentration is low (0.2–0.4g·1^–1) and there is no discernable spring increase. The phytoplankton is largely dominated by flagellates. Bacterioplankton exhibits a seasonal pattern of abundance ranging from 1.0 × 10⁸·1^–1 in July to 3.25 × 10⁸·1^–1 in September. Changes in bacterial abundance probably relate to temperature and grazing by heterotrophic and mixotrophic flagellates. Total flagellated protozoan concentrations ranged between 25–136 × 10²·l^–1. Autotrophic and heterotrophic flagellate abundances were coupled and peaks in their abundance oscillated with peaks in bacterioplankton concentration. Four species of ciliated protozoa, dominated by oligotrichs, particularly the plastidic Strombidium, inhabit the lake. The plankton is characterised by the presence of floes which act as loci for bacteria, flagellates and amoebae and feeding sites for the ciliates and the two sparce metazoan components of the plankton. Crooked Lake is extremely oligotrophic but nonetheless supports a plankton community with a low species diversity and simple trophodynamics.     

Viruses as Partners in Spring Bloom Microbial Trophodynamics

Population sizes of algae, bacteria, heterotrophic flagellates, and viruses were observed through the 1989 spring diatom bloom in Raunefjorden in western Norway. The culmination of the diatom bloom was followed by a peak in the concentration of bacteria and an increase in the concentration of heterotrophic flagellates, a pattern consistent with the concept of a food chain from photosynthetically produced organic material, through bacteria, to bacterivorous flagellates. The concentration of viruses varied through the spring bloom from 5 × 10⁵ in the prebloom situation to a maximum of 1.3 × 10⁷ viruses ml⁻¹ 1 week after the peak of the diatom bloom. Coinciding with the collapse in the diatom bloom, a succession of bacteria and viruses was observed in the mucous layer surrounding dead or senescent diatoms, with an estimated maximum of 23% of the total virus population attached to the diatoms. The dynamic behavior observed for the virus population rules out the possibility that it is dominated by inactive species, and the viruses are suggested to be active members of the microbial food web as agents causing lysis in parts of the bacterial population, diverting part of the bacterial production from the predatory food chain.     

Trophic interactions among heterotrophic microplankton,nanoplankton, and bacteria in Lake Constance

A considerable portion of the pelagic energy flow in Lake Constance (FRG) is channelled through a highly dynamic microbial food web. In-situ experiments using the lake water dilution technique according to Landry & Hasset (1982) revealed that grazing by heterotrophic nanoflagellates (HNF) smaller than 10 µm is the major loss factor of bacterial production. An average flagellate ingests 10 to 100 bacteria per hour. Nano- and micro-ciliates have been identified as the main predators of HNF. If no other food is used between 3 and 40 HNF are consumed per ciliate and hour. Other protozoans and small metazoans such as rotifers are of minor importance in controlling HNF population dynamics.Clearance rates varied between 0.2 and 122.8 nl HNF^–1 h^–1 and between 0.2 and 53.6 µl ciliate^–1 h^–1, respectively.Ingestion and clearance rates measured for HNF and ciliates are in good agreement with results obtained by other investigators from different aquatic environments and from laboratory cultures. Both the abundance of all three major microheterotrophic categories — bacteria, HNF, and ciliates — and the grazing pressure within the microbial loop show pronounced seasonal variations.     

Dynamics of Bacterial Community Composition and Activity during a Mesocosm Diatom Bloom

Bacterial community composition, enzymatic activities, and carbon dynamics were examined during diatom blooms in four 200-liter laboratory seawater mesocosms. The objective was to determine whether the dramatic shifts in growth rates and ectoenzyme activities, which are commonly observed during the course of phytoplankton blooms and their subsequent demise, could result from shifts in bacterial community composition. Nutrient enrichment of metazoan-free seawater resulted in diatom blooms dominated by a Thalassiosira sp., which peaked 9 days after enrichment (≈24 μg of chlorophyll a liter⁻¹). At this time bacterial abundance abruptly decreased from 2.8 × 10⁶ to 0.75 × 10⁶ ml⁻¹, and an analysis of bacterial community composition, by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments, revealed the disappearance of three dominant phylotypes. Increased viral and flagellate abundances suggested that both lysis and grazing could have played a role in the observed phylotype-specific mortality. Subsequently, new phylotypes appeared and bacterial production, abundance, and enzyme activities shifted from being predominantly associated with the <1.0-μm size fraction towards the >1.0-μm size fraction, indicating a pronounced microbial colonization of particles. Sequencing of DGGE bands suggested that the observed rapid and extensive colonization of particulate matter was mainly by specialized α-Proteobacteria- and Cytophagales-related phylotypes. These particle-associated bacteria had high growth rates as well as high cell-specific aminopeptidase, β-glucosidase, and lipase activities. Rate measurements as well as bacterial population dynamics were almost identical among the mesocosms indicating that the observed bacterial community dynamics were systematic and repeatable responses to the manipulated conditions.     

Complex interactions and different possible pathways among functional components of the aquatic microbial world in Farasan Archipelago,Southern Red Sea,Saudi Arabia

This work aims to outline the dynamics of trophic links between the three main microbial components (bacteria, nanoflagellates, and ciliates) of the Farasan Archipelago in order to establish a baseline for future research in this area. The Farasan Archipelago lies along the southwestern coast of the Saudi Arabia, southern Red Sea between 16°20′–17°10′N and 41°30′–42°30′E and had been declared as marine and terrestrial reserve by the year 1996. Three different sites were chosen for this study, with each site visited bimonthly for 18 months from September 2016 to February 2018. Bacteria, nanoflagellates and ciliates were enumerated in order to explore the complex interactions between the main microbial categories in sea waters of the Farasan Archipelago. High abundances were recorded during the present study for bacteria (8.7 × 10⁶ bacteria ml⁻¹), nanoflagellates (3.7 × 10⁴ TNAN ml⁻¹) and ciliates (40.4 ciliates ml⁻¹). The paper discusses the various potential pathways controlling the complex interactions between these microbial groups in this part of the southern Red Sea. It is concluded that a linear trophic chain consisting of bacteria; heterotrophic nanoflagellates; filter feeding ciliates is a major route by which the production of bacteria is transferred to the higher consuming levels, thereby confirming the high importance of t bottom-up control (food supply), alongside top-down control (predation) in regulating bacterial abundances in the Farasan Archipelago. During the present investigation, each nanoflagellate ingested between 11 and 87 bacteria in one hour, while each ciliate consumed between 20 and 185 nanoflagellates every hour. These calculated grazing rates of protistan eukaryotes confirmed the role of heterotrophic nanoflagellates as the main consumers of bacteria, and the role of ciliates as the major control for the heterotrophic nanoflagellate population dynamics, and thus the top predators within the microbial plankton assemblage in the Farasan Archipelago.     

Benthic Bacterial and Fungal Productivity and Carbon Turnover in a Freshwater Marsh

Heterotrophic bacteria and fungi are widely recognized as crucial mediators of carbon, nutrient, and energy flow in ecosystems, yet information on their total annual production in benthic habitats is lacking. To assess the significance of annual microbial production in a structurally complex system, we measured production rates of bacteria and fungi over an annual cycle in four aerobic habitats of a littoral freshwater marsh. Production rates of fungi in plant litter were substantial (0.2 to 2.4 mg C g⁻¹ C) but were clearly outweighed by those of bacteria (2.6 to 18.8 mg C g⁻¹ C) throughout the year. This indicates that bacteria represent the most actively growing microorganisms on marsh plant litter in submerged conditions, a finding that contrasts strikingly with results from both standing dead shoots of marsh plants and submerged plant litter decaying in streams. Concomitant measurements of microbial respiration (1.5 to 15.3 mg C-CO₂ g⁻¹ of plant litter C day⁻¹) point to high microbial growth efficiencies on the plant litter, averaging 45.5%. The submerged plant litter layer together with the thin aerobic sediment layer underneath (average depth of 5 mm) contributed the bulk of microbial production per square meter of marsh surface (99%), whereas bacterial production in the marsh water column and epiphytic biofilms was negligible. The magnitude of the combined production in these compartments (~1,490 g C m⁻² year⁻¹) highlights the importance of carbon flows through microbial biomass, to the extent that even massive primary productivity of the marsh plants (603 g C m⁻² year⁻¹) and subsidiary carbon sources (~330 g C m⁻² year⁻¹) were insufficient to meet the microbial carbon demand. These findings suggest that littoral freshwater marshes are genuine hot spots of aerobic microbial carbon transformations, which may act as net organic carbon importers from adjacent systems and, in turn, emit large amounts of CO₂ (here, ~870 g C m⁻² year⁻¹) into the atmosphere.     

Microbial Community Structure and Dynamics in the Largest Natural French Lake (Lake Bourget)

We investigated the dynamics and diversity of heterotrophic bacteria, autotrophic and heterotrophic flagellates, and ciliates from March to July 2002 in the surface waters (0–50 m) of Lake Bourget. The heterotrophic bacteria consisted mainly of “small” cocci, but filaments (>2 μm), commonly considered to be grazing-resistant forms under increased nanoflagellate grazing, were also detected. These elongated cells mainly belonged to the Cytophaga-Flavobacterium (CF) cluster, and were most abundant during spring and early summer, when mixotrophic or heterotrophic flagellates were the main bacterial predators. The CF group strongly dominated fluorescent in situ hybridization–detected cells from March to June, whereas clear changes were observed in early summer when Beta-proteobacteria and Alpha-proteobacteria increased concomitantly with maximal protist grazing pressures. The analysis of protist community structure revealed that the flagellates consisted mainly of cryptomonad forms. The dynamics of Cryptomonas sp. and Dinobryon sp. suggested the potential importance of mixotrophs as consumers of bacteria. This point was verified by an experimental approach based on fluorescent microbeads to assess the potential grazing impact of all protist taxa in the epilimnion. From the results, three distinct periods in the functioning of the epilimnetic microbial loop were identified. In early spring, mixotrophic and heterotrophic flagellates constituted the main bacterivores, and were regulated by the availability of their resources mainly during April (phase 1). Once the “clear water phase” was established, the predation pressure of metazooplankton represented a strong top-down force on all microbial compartments. During this period only mixotrophic flagellates occasionally exerted a significant bacterivory pressure (phase 2). Finally, the early summer was characterized by the highest protozoan grazing impact and by a rapid shift in the carbon pathway transfer, with a fast change-over of the main predators contribution, i.e., mixotrophic, heterotrophic flagellates and ciliates in bacterial mortality. The high abundance of ciliates during this period was consistent with the high densities of resources (heterotrophic nanoflagellates, algae, bacteria) in deep layers containing the most chlorophyll. Bacteria, as ciliates, responded clearly to increasing phytoplankton abundance, and although bacterial grazing impact could vary largely, bacterial abundance seemed to be primarily bottom-up regulated (phase 3).     

Rates of Digestion of Bacteria by Marine Phagotrophic Protozoa: Temperature Dependence

The effect of temperature on length of time for digestion of bacteria was evaluated, by using fluorescently labeled bacteria (FLB), for phagotrophic flagellates and ciliates isolated from coastal northwest Mediterranean waters. Accumulation of FLB in protozoan food vacuoles was followed until a plateau of FLB per cell occurred; then after a 1:10 dilution of FLB with unlabeled bacteria, disappearance of FLB in food vacuoles was monitored. For both 3- to 5-μm flagellates and 10- to 40-μm ciliates, the absolute linear slopes of FLB uptake and disappearance were nearly identical in individual experiments over a temperature range of 12 to 22°C. We inferred from these results that the leveling off of the uptake curves resulted when equilibrium between ingestion and digestion of bacteria was attained. The time to leveling off then represented the average time needed for complete digestion of the bacteria ingested at the start of the experiment, and the inverse of this time represented a bacterial digestion rate. The digestion rate increased exponentially from 12 to 22°C for both a mixed flagellate assemblage and the oligotrichous ciliate Strombidium sulcatum, with a Q₁₀ of 2.8 for the flagellates and 2.0 for the ciliate. Although bacterial ingestion rates varied greatly, depending on protozoan cell size, total bacterial abundance, and temperature, digestion times appeared to be significantly influenced only by protozoan cell size (or type of protozoan) and by temperature.     

Short-term variations of virus-like particles in a tropical lake: relationship with microbial communities (bacteria, ciliates and flagellates)

The short-term dynamics of virus-like particles (VLPs) abundance, bacterioplankton, ciliates and flagellates were analyzed in a small tropical lake, during a rainy day (June 9–10, 2003) and a dry day (February 18–19, 2004), with intervals of 3 h between the samplings. Frequent sampling in intervals of 15 min were conducted. During the rainy day, the VLP mean abundance was 7.0×108 mL⁻¹ and bacterial density was 5.75×107 mL⁻¹. During the dry day, VLP and bacterial mean were 5.78×108 and 4.1×107 mL⁻¹, respectively. The virus/bacterium rate (VBR) varied from 11 to 18 on the rainy day and from 4 to 22 on the dry day. The density of VLP was higher during the night, especially on the dry day, suggesting a virucidal action of the solar radiation on them. When registered in intervals of 15 min, the densities were not associated with the fluctuations of bacteria or chlorophyll a (Chl a), but a strong negative correlation between VLP and protozooplankton was observed (Spearman: R=−0.71; p=0.04), possibly associated with the occurrence of viral lyses on these organisms. The variations of VBR in the system, indicate that the elevated densities and fluctuations of VLP is suggestive of an active and important participation of these biological entities in the dynamics of the microbial communities in the studied environment.     

Development of ice biota in a temperate sea area (Gulf of Bothnia)

A study of sea ice biota was carried out in the Gulf of Bothnia (northern Baltic Sea) during the winter of 1989–1990. Samples (ice cores) were taken at a coastal station at regular time intervals during the ice season. Chlorophyll a concentration, algal species distribution, bacterial numbers, and primary and bacterial production were measured. Colonization of the ice began in January when daylight was low. As the available light increased, the algae started to grow exponentially. The vertical chlorophyll a distribution changed and algal species composition and biomass changed during the season. During the initial and middle phase of colonization, ice-specific diatoms, Nitzschia frigida and Navicula pelagica, dominated the algal biomass. Nutrients (PO₄ ^3– and NO_3j) were found to be depleted during the time of algal exponential growth. The maximum algal biomass exceeded 800 g C 1^–1. The primary production supplied food for heterotrophic organisms. The presence of heterotrophic organisms of different trophic levels (bacteria, flagellates, ciliates and rotifers) indicated an active microbial food web.     

Sea Ice Microbial Communities: Distribution, Abundance, and Diversity of Ice Bacteria in McMurdo Sound, Antarctica, in 1980

An abundant and diverse bacterial community was found within brine channels of annual sea ice and at the ice-seawater interface in McMurdo Sound, Antarctica, in 1980. The mean bacterial standing crop was 1.4 × 10¹¹ cells m⁻² (9.8 mg of C m⁻²); bacterial concentrations as high as 1.02 × 10¹² cells m⁻³ were observed in ice core melt water. Vertical profiles of ice cores 1.3 to 2.5 m long showed that 47% of the bacterial numbers and 93% of the bacterial biomass were located in the bottom 20 cm of sea ice. Ice bacterial biomass concentration was more than 10 times higher than bacterioplankton from the water column. Scanning electron micrographs showed a variety of morphologically distinct cell types, including coccoid, rod, fusiform, filamentous, and prosthecate forms; dividing cells were commonly observed. Approximately 70% of the ice bacteria were free-living, whereas 30% were attached to either living algal cells or detritus. Interactions between ice bacteria and microalgae were suggested by a positive correlation between bacterial numbers and chlorophyll a content of the ice. Scanning and transmission electron microscopy revealed a close physical association between epibacteria and a dominant ice alga of the genus Amphiprora. We propose that sea ice microbial communities are not only sources of primary production but also sources of secondary microbial production in polar ecosystems. Furthermore, we propose that a detrital food web may be associated with polar sea ice.     

Benthic Bacterial Production and Protozoan Predation in a Silty Freshwater Environment

The interrelation of heterotrophic bacteria with bacterivorous protists has been widely studied in pelagic environments, but data on benthic habitats, especially in freshwater systems, are still scarce. We present a seasonal study focusing on bacterivory by heterotrophic nanoflagellates (HNF) and ciliates in the silty sediment of a temperate macrophyte-dominated oxbow lake. From January 2001 to February 2002 we monitored the standing stock of bacteria and protozoa, bacterial secondary production (BSP, ³H-thymidine, and ¹⁴C-leucine incorporation), and grazing rates of HNF and ciliates on bacteria (FLB uptake) in the oxic sediment of the investigated system. BSP ranged from 470 to 4050 µg C L^–1 wet sediment h^–1. The bacterial compartment turned out to be highly dynamic, indicated by population doubling times (0.6–10.0 d), which were comparable to those in the water column of the investigated system. Yet, the control mechanisms acting upon the bacterial population led to a relative constancy of bacterial standing stock during a year. Ingestion rates of protozoan grazers were 0–20.0 bacteria HNF^–1 h^–1 and 0–97.6 bacteria ciliate^–1 h^–1. HNF and ciliates together cropped 0–14 (mean 4)% of BSP, indicating that they did not significantly contribute to benthic bacterial mortality during any period of the year. The low impact of protozoan grazing was due to the low numbers of HNF and ciliates in relation to bacteria (1.8–3.5 × 10⁴ bacteria HNF^–1, 0.9–3.1 × 10⁶ bacteria ciliate^–1). Thus, grazing by HNF and ciliates could be ruled out as a parameter regulating bacterial standing stock or production in the sediment of the investigated system, but the factors responsible for the limitation of benthic protistan densities and the fate of benthic BSP remained unclear.     

Limnology of Crater Lakes in Los Tuxtlas,Mexico

Investigations on the abundance, biomass and position of heterotrophic flagellates (HF) in the benthic microbial food web of a melt water stream on King George Island, Antarctic Peninsula, were undertaken during the Antarctic summer from 23rd December 1997 until 13th March 1998. Abundance and biomass of potential HF resources (picophotoautotrophic and non-photoautotrophic bacteria) as well as potential predators on HF (ciliates and meiofauna) were also investigated. HF abundance ranged from approximately 9 × 10³ to 81 × 10³ cells cm^–3, values which fall into the same range as those found in lower latitudes. Numerically important benthic HF were euglenids, kinetoplastids, thaumatomastigids and especially chrysomonads. Most species identified have been shown to have a worldwide distribution. Abundance of the benthic ciliates ranged from 27 to 950 cells cm^–3. Mean bacterial abundance was 1.9 × 10⁷ and 5.2 × 10⁸ cells cm^–3 for picophotoautotrophic and non-photoautotrophic benthos, respectively. The well-developed microbial community was able to support the large number of nematods, gastotrichs, tardigrads and rotifers with abundances reaching more than 1000 individuals cm^–3. The largest portion of heterotrophic biomass was formed by the meiofauna with a mean of 63 g C cm^–3, followed by that of the heterotrophic bacteria with 4.80 g C cm^–3. Picophotoautotrophic bacteria contributed a mean of 1.37 g C cm^–3. HF and ciliates mean biomass was 0.61 and 1.99 g C cm^–3, respectively, with the HF biomass comprising between <10 and 70% of the total protozoan biomass. The data obtained in this study identify the melt water stream as a hot-spot of heterotrophic microbial and meiofaunal activity during the austral summer. The HF in the melt water stream formed a diverse group in terms of taxa and potential feeding types. Chrysomonads, kinetoplastids, euglenids and thaumatomastigida were the most abundant taxa. A classification into feeding types identified an average of 34% of the total HF as bacterivorous while all others were able to utilise other, larger organisms as resources. Potential trophic interactions between HF and bacteria and higher trophic levels are discussed.     

Nitrogen, Carbon, and Sulfur Metabolism in Natural Thioploca Samples

Filamentous sulfur bacteria of the genus Thioploca occur as dense mats on the continental shelf off the coast of Chile and Peru. Since little is known about their nitrogen, sulfur, and carbon metabolism, this study was undertaken to investigate their (eco)physiology. Thioploca is able to store internally high concentrations of sulfur globules and nitrate. It has been previously hypothesized that these large vacuolated bacteria can oxidize sulfide by reducing their internally stored nitrate. We examined this nitrate reduction by incubation experiments of washed Thioploca sheaths with trichomes in combination with ¹⁵N compounds and mass spectrometry and found that these Thioploca samples produce ammonium at a rate of 1 nmol min⁻¹ mg of protein⁻¹. Controls showed no significant activity. Sulfate was shown to be the end product of sulfide oxidation and was observed at a rate of 2 to 3 nmol min⁻¹ mg of protein⁻¹. The ammonium and sulfate production rates were not influenced by the addition of sulfide, suggesting that sulfide is first oxidized to elemental sulfur, and in a second independent step elemental sulfur is oxidized to sulfate. The average sulfide oxidation rate measured was 5 nmol min⁻¹ mg of protein⁻¹ and could be increased to 10.7 nmol min⁻¹ mg of protein⁻¹ after the trichomes were starved for 45 h. Incorporation of ¹⁴CO₂ was at a rate of 0.4 to 0.8 nmol min⁻¹ mg of protein⁻¹, which is half the rate calculated from sulfide oxidation. [2-¹⁴C]acetate incorporation was 0.4 nmol min⁻¹ mg of protein⁻¹, which is equal to the CO₂ fixation rate, and no ¹⁴CO₂ production was detected. These results suggest that Thioploca species are facultative chemolithoautotrophs capable of mixotrophic growth. Microautoradiography confirmed that Thioploca cells assimilated the majority of the radiocarbon from [2-¹⁴C]acetate, with only a minor contribution by epibiontic bacteria present in the samples.     

Rotating Magnetic Field-Assisted Reactor Enhances Mechanisms of Phage Adsorption on Bacterial Cell Surface

Growing interest in bacteriophage research and use, especially as an alternative treatment option for multidrug-resistant bacterial infection, requires rapid development of production methods and strengthening of bacteriophage activities. Bacteriophage adsorption to host cells initiates the process of infection. The rotating magnetic field (RMF) is a promising biotechnological method for process intensification, especially for the intensification of micromixing and mass transfer. This study evaluates the use of RMF to enhance the infection process by influencing bacteriophage adsorption rate. The RMF exposition decreased the t₅₀ and t₇₅ of bacteriophages T4 on Escherichia coli cells and vb_SauM_A phages on Staphylococcus aureus cells. The T4 phage adsorption rate increased from 3.13 × 10⁻⁹ mL × min⁻¹ to 1.64 × 10⁻⁸ mL × min⁻¹. The adsorption rate of vb_SauM_A phages exposed to RMF increased from 4.94 × 10⁻⁹ mL × min⁻¹ to 7.34 × 10⁻⁹ mL × min⁻¹. Additionally, the phage T4 zeta potential changed under RMF from −11.1 ± 0.49 mV to −7.66 ± 0.29 for unexposed and RMF-exposed bacteriophages, respectively.     

Succession of Protists on Estuarine Aggregates

Abstract Colonization by and succession of bacteria and bacterivorous protists on laboratory-made aggregates were determined over a period of 14 days during winter and spring in 1997. Aggregates were generated from natural water from the limnetic zone of the Elbe Estuary using a tilting tube roller system. Within 1 h after the beginning of the experiments, macroaggregates started to form. Aggregates reached a maximum size of 1 mm with a tendency toward large sizes at the end of the experiment after the 10th day. On the first day, high bacterial densities of more than 10⁹ cells ml⁻¹ were detected within the aggregates. The abundances of flagellates and ciliates within aggregates were also two or three orders of magnitude higher than in the surrounding water. Densities of aggregate associated organisms are comparable to those occuring in sediments. The first protistan colonizers on the aggregates were small heterotrophic flagellates, such as choanoflagellates and small euglenids. Later, beginning on the 4th day, small sarcodines and ciliates became abundant. The most abundant ciliates associated with aggregates were small species of the Hypotrichia, Cyrtophorida, and Hymenostomata. After 9 days, large omnivorous and carnivorous ciliates, such as large members of the Hypotrichia and the Pleurostomatida, occurred. In spring, large heterotrophic flagellates and amebae also appeared at this time. These findings indicated the existence of a succession of protists on newly formed aggregates and a microbial food net within the aggregates based on bacterial production. Additionally, most of the species observed during this study were adapted for living on surfaces. In natural environments they are more common in benthic than in pelagic environments. For them, aggregates are havens in the water column comparable to sediment communities. Received: 7 January 2000; Accepted: 15 May 2000; Online Publication: 28 August 2000