Biomass Of Heterotrophic Nanoflagellates Research Articles

Distribution, Size-Morphological Structure and Production of Heterotrophic Bacterioplankton in Gorky Reservoir

The abundance, biomass, size-morphological structure, growth rate and production of bacterioplankton, the intensity of primary phytoplankton production and dark fixation of CO2, as well as the abundance and biomass of heterotrophic nanoflagellates were determined in a large plain eutrophic reservoir (Gorky Reservoir, Middle Volga). The abundance, biomass, and production of bacterioplankton were relatively high and averaged 7.6 × 106 cells/mL, 117.9 mg C/m3, and 59.2 mg C/(m3 × day), respectively. Heterotrophic nanoflagellates reached a high level of quantitative development – 6.9 × 103 cells/mL, 47.9 mg C/m3. Their biomass averaged 41.6 ± 18.4% of the bacterioplankton biomass, which indicates that, in addition to bacteria, nanoflagellates used other food sources. Small rods and cocci dominated among the size-morphological bacterioplankton groups and accounted for 36.3 and 33.3% of its total abundance, respectively. Small rods averaged more than a half (56.2%) of the total biomass and were the most stable component of the community. The growth rate and production of bacterioplankton increased in those parts of the reservoir where the medium-sized cocci and coccobacilli accounted for from 18.2 to 29.3% of the total abundance. The highest bacterial activity was recorded in the area affected by the warm waste waters of the Volgorechensk State District Power Plant. Based on the data obtained, the reservoir divided on the upper river section and the lower lake section.

Bottom-Up Control of the Groundwater Microbial Food-Web in an Alpine Aquifer

Groundwater ecosystems are typically poor in organic carbon and productivity sustaining a low standing stock of microbial biomass. In consequence, microbial food webs in oligotrophic groundwater are hypothesized to be bottom-up controlled. To date, quantitative information on groundwater microbial communities, food web interactions, and carbon flow is relatively lacking in comparison to that of surface waters. Studying a shallow, porous alpine aquifer we collected data on the numbers of prokaryotes, virus-like particles and heterotrophic nanoflagellates (HNFs), the concentration of dissolved (DOC) and assimilable organic carbon (AOC), bacterial carbon production (BCP), and physical-chemical conditions for a 1 year hydrological cycle. The potential effects of protozoan grazing and viral lysis onto the prokaryotic biomass was tested. Flow of organic carbon through the microbial food web was estimated based on data from the literature. The abundance of prokaryotes in groundwater was low with 6.1 ± 6.9 × 104 cells mL–1, seasonally influenced by the hydrological dynamics, with higher densities coinciding with a lower groundwater table. Overall, the variability in cell numbers was moderate, and so it was for HNFs (179 ± 103 HNFs mL–1) and virus-like particles (9.6 ± 5.7 × 105 VLPs mL–1). The virus to prokaryotes and prokaryote to HNF ratios ranged between 2–230 and 33–2,084, respectively. We found no evidence for a viral control of prokaryotic biomass, and the biomass of HNFs being bottom-up controlled. First estimations point at carbon use efficiencies of 0.2–4.2% with prokaryotic production, and carbon consumed and recycled by HNFs and phages to be of minor importance. This first groundwater microbial food web analysis strongly hints at a bottom-up control on productivity and standing stock in oligotrophic groundwater ecosystems. However, direct measurement of protozoan grazing and phage mediated lysis rates of prokaryotic cells are urgently needed to deepen our mechanistic understanding. The effect of microbial diversity on the population dynamics still needs to be addressed.

Heterotrophic nanoflagellates in water column and bottom sediments of the Rybinsk Reservoir: Species composition, abundance, biomass and their grazing impact on bacteria

Abundance, biomass, and taxonomic composition of heterotrophic nanoflagellates (HNFs) have been determined in the water column and bottom sediments of the large lowland meso-eutrophic reservoir (Rybinsk Reservoir, Upper Volga) in summer. The role of HNFs in the consumption of the bacterial production is estimated. In the reservoir, 55 species from 15 large taxa, including 35 species from the plankton, are identified and 45 species are from benthos samples. The orders Kinetoplastida, Choanomonada, and Chrysomonadida are distinguished by the highest species diversity. Abundance and biomass of HNFs in the water column average 991 ± 326 cells/mL and 41.4 ± 14.1 mg/m3, while in the bottom sediments they are (236 ± 61) × 103 cells/mL and 10.7 ± 4.0 μg/mL, respectively. The biomass of HNFs average 11.2% of the bacterial biomass in the water column and only 0.8% of that in the sediments. Flagellates are found to be a major factor which control the development of bacterioplankton grazing, on average, 32.3% of its daily production, whereas their impact on bacteriobenthos is insignificant, as they consume, on average, only 2.0% of its production.

Trophic relationships between planktonic bacteria, heterotrophic nanoflagellates and viruses in a mesoeutrophic reservoir

The abundance, biomass, and production (РВ) of bacrerioplankton; the taxonomic composition, abundance, biomass of heterotrophic nanoflagellates (HNF) and the rate of consumption of bacteria by HNFs; and the abundance of virioplankton, frequency of visibly infected bacterial cells, virus-induced mortality of bacterioplankton, and viral production were estimated in the mesoeutrophic Rybinsk Reservoir. The rate of bacterial mortality due to viral lysis (7.8–34.1%, on average 17.2 ± 2.0% of daily РВ) was lower than the consumption of bacteria by the HNF community (15.4–61.3%, on average 32.0 ± 4.2% of daily РВ). While consuming bacteria, HNFs simultaneously absorbed a significant number of viruses residing on the surface and inside the bacterial cells.

Impact of the blue mussel Mytilus edulis on the microbial food web in the western Wadden Sea, The Netherlands

To study the impact of juvenile blue mussels Mytilus edulis on the microbial food web in the Dutch Wadden Sea, natural sea water was first exposed to mussel filtration. Subsequently, filtered plankton communities were used in a dilution experiment to establish mussel-induced changes in bacterial, pico- and nanophytoplankton growth rates as well as heterotrophic nanoflagellates (HNAN) and ciliate-induced grazing mortality rates. During the experimental period, from July to September, mussel filtration had a size-selective impact on the plankton community; on average, nanophytoplankton, HNAN and ciliates biomasses were removed at equal rates, while bacterial and picophytoplankton biomasses were affected to a much lower extent. The reduction in HNAN predators by mussels significantly lowered the grazing mortality rates for picophytoplankton. For bacteria, grazing mortality did not change, while specific growth rates almost doubled (from 0.65 to 1.16 d-1). There was an increase in HNAN biomass following the enhanced bacterial production. Single exposure to mussel filtration thus led to a stimulation of the bacterial-HNAN pathway. HNAN biomass, although seriously reduced by mussel filtration, recovered to pre-filtration levels within 24 h. Nanophytoplankton and ciliates did not recover completely within 24 h. The results from this study reveal potentially important effects of mussel filtration on the pelagic food web not disclosed when considering phytoplankton biomass alone.

Seasonal and annual variation of virioplankton abundance in rivers running through an industrial city (city of Cherepovets, Upper Volga)

In April to October 2009–2011, the species composition and seasonal changes in abundance and biomass of heterotrophic nanoflagellates were studied in the Sheksna River and in the small Yagorba and Serovka rivers running through a large industrial center. A total of 48 species and forms of colorless flagellates were recorded in the rivers. The abundance and biomass of heterotrophic nanoflagellates (average for 3 years) were 5788 cells/mL and 251.5 mg/m3, correspondingly, in the Yagorba River; 6164 cells/mL and 266.2 mg/m3 in the Serovka River; and 4450 cells/mL and 193.5 mg/m3, correspondingly, in the littoral zone of the Sheksna River. The maximal values (10.1–11.6 thousand cells/mL and 591–654 mg/m3) were more frequently recorded in the second half of summer. A high positive correlation was observed between the biomass of single bacterial cells and heterotrophic nanoflagellates. In 2010, in all rivers with extremely high water temperatures, the abundance and biomass of heterotrophic nanoflagellates in summer was significantly higher than in other years.

Weak coupling between heterotrophic nanoflagellates and bacteria in the Yellow Sea Cold Water Mass area

A study was carried out to investigate the grazing pressure of heterotrophic nanoflagellates (HNF) on bacteria assemblages in the Yellow Sea Cold Water Mass (YSCWM) area in October, 2006. The results show that the HNF abundance ranges from 303 to 1 388 mL−1, with a mean of 884 mL−1. The HNF biomass is equivalent to 10.6%–115.6% of that of the bacteria. The maximum abundance of the HNF generally occurred in the upper 30 m water layer, with a vertical distribution pattern of surface layer abundance greater than middle layer abundance, then bottom layer abundance. The hydrological data show that the YSCWM is located in the northeastern part of the study area, typically 40 m beneath the surface. A weak correlation is found between the abundances of HNF and bacteria in both the YSCWM and its above water layer. One-way ANOVA analysis reveals that the abundance of HNF and bacteria differs between inside the YSCWM and in the above water mass. The ingestion rates of the HNF on bacteria was 8.02±3.43 h−1 in average. The grazing rate only represented 22.75%±6.91% of bacterial biomass or 6.55%+4.24% of bacterial production, implying that the HNF grazing was not the major factor contributing to the bacterial loss in the YSCWM areas.

Heterotrophic organisms and viruses in the Oka River and Cheboksary Reservoir during the abnormally hot summer of 2010

In July 2010, abnormally high water temperature (25-29 degrees C), as well as increased biomass and phytoplankton production caused intensive development of heterotrophic bacteria and heterotrophic nanoflagellates. It was found that the abundance, biomass, and production of heterotrophic bacterioplankton, as well as the abundance and biomass of heterotrophic nanoflagellates, and the number of planktonic viruses, which were calculated on average for the reservoir under study, turned out to be higher in the years with lower water temperature (20-23 degrees C). The virus-induced mortality of bacterioplankton in the Oka River and the Cheboksary Reservoir averaged 25.4 +/- 3.4 and 22.4 +/- 2.7% of the daily bacterioplankton production.

The relative importance of autotrophic and heterotrophic microbial communities in the planktonic food web of the Bay of Quinte, Lake Ontario 2000–2007

The structure and function of the microbial and planktonic communities of the Bay of Quinte, Lake Ontario were studied for 8 years from 2000 to 2007. The bay has a long history of eutrophication and has undergone remediation efforts which included reductions in phosphorus loadings and the implementation of a long term research and monitoring program (1972–present) conducted by Fisheries and Oceans Canada along with other federal and provincial agencies. Microbial loop research was added in 2000 to the ongoing monitoring program which included nutrients, phytoplankton, zooplankton, benthos and fish so that a comprehensive picture of food web linkages would emerge. The structure of the microbial-planktonic food web was determined based on microscopic analysis of bacteria, autotrophic picoplankton (APP), heterotrophic nanoflagellates (HNF), ciliates, phytoplankton and zooplankton and was compared to a traditional grazing food chain consisting of phytoplankton and zooplankton. On a seasonal weighted mean basis, HNF biomass (fresh weight) of 1.7–8.4 g m−3 at Belleville and 1.1–6.3 C m−3 at Conway exceeded that of zooplankton in virtually all observations and was often greater than the combination of phytoplankton and zooplankton. Furthermore, the results showed that HNF contributed upwards to 85% of the organic carbon pool on a seasonal weighted mean basis. Various parameters in the upper bay relating trophic status to autotrophic communities were measured including: point source phosphorus loadings (>10 kg d−1); primary production (>300 g C m−2 y−1); chlorophyll a (>12 μg l−1) and phytoplankton biomass (>3 g m−3) which indicated that the upper bay remained eutrophic. This was also confirmed by the “battery of tests” strategy of ecological indicators developed in our laboratory to assess trophic status, health, and potential Beneficial Use Impairments. Based on our observations spanning 8 years, it was concluded that the microbial food web was dominated by heterotrophic communities which are still widely ignored in Great Lakes research and monitoring efforts. Our data clearly demonstrates that future studies and management strategies should include the “microbial loop” to obtain a holistic picture of the structure, function and dynamics of the lower food web.

Trophic linkages between grazers and ultraplankton within the microbial food web in subtropical coastal waters

To investigate the trophic linkages among different micrograzers and their preys and the possible occurrence of trophic cascades within the microbial food web, we combined the dilution technique and size-fractionation methods to monitor the grazing rates of different grazer assem- blages on various ultraphytoplankton cells in subtropical waters. The mortality rates of Synechococ- cus and picophytoeukaryotes (<2 µm) were often higher in dilution experiments set up with 5 µm pre- filtered seawater (containing only heterotrophic nanoflagellates (HNFs)) than in the <20 µm treatments (including both HNFs and small ciliates), indicating the effect of trophic cascading. In addition, similar grazing rates on Synechococcus and picophytoeukaryotes can be well related to the biomass of HNFs but not to that of total grazers. Growth and grazing rates of Synechococcus and picophytoeukaryotes mostly fell to a 1:1 relationship, suggesting grazing was the major loss pathway for picophytoplankton. The trophic linkages of micrograzers and their preys in coastal waters do not appear to differ substantially from those observed in open oceans.

Community structure and trophic role of ciliates and heterotrophic nanoflagellates in Rhone River diluted mesoscale structures (NW Mediterranean Sea)

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 57:263-277 (2009) - DOI: https://doi.org/10.3354/ame01339 AME Special: 'Rassoulzadegan at Villefranche-sur-Mer: 3 decades of aquatic microbial ecology' Community structure and trophic role of ciliates and heterotrophic nanoflagellates in Rhone River diluted mesoscale structures (NW Mediterranean Sea) Urania Christaki1,*, Claude Courties2, Fabien Joux3, Wade H. Jeffrey4, Jacques Neveux3, Jean-Jacques Naudin3 1Univ Lille Nord de France, ULCO, LOG, CNRS, UMR 8187, 32 Avenue Foch, 62930 Wimereux, France 2UMPC Univ Paris 06, FRE 3247, MBCE, CNRS, Observatoire Océanologique, 66651 Banyuls-sur-mer, France 3UMPC Univ Paris 06, FRE 3247, MBCE, LOBBCNRS, UMR 7621, Observatoire Océanologique, 66651 Banyuls-sur-mer, France 4Center for Environmental Diagnostics and Bioremediation, 11000 University Parkway, University of West Florida, Pensacola, Florida 32514, USA *Email: urania.christaki@univ-littoral.fr ABSTRACT: Studies of the Rhone River plume, and in particular its diluted mesoscale structures, have shown over recent years that low salinity water (LSW) is characterised by both high primary and high secondary productivity. Here we compare the structure and grazing activity of the hetorotrophic community (nanoflagellates, ciliates) in the LSW and the surrounding marine water (MW) of the northwestern Mediterranean Sea. Furthermore, we report on the temporal variability of these characteristics all along a Lagrangian track in the LSW. In May 2006, salinity of the LSW ranged from 31.41 to 38.25 down to a depth of 35 m, and surface chlorophyll a concentration reached 15.5 µg l–1. The ciliate community structure was similar in both the LSW and the MW, characterised by the dominance of the mixotrophic ciliates Laboea strobila and Myrionecta rubra. The ciliate biomass was approximately 6 times greater in the LSW, compared to the MW. This difference was due to the mixotrophic ciliates, which were about 4- to 5-fold more important in terms of abundance and biomass in the LSW, while heterotrophic ciliates only increased by a maximum of 2-fold. M. rubra was the most abundant species reaching 4600 cells l–1, while L. strobila was dominant in terms of biomass, accounting for 47% of total biomass in the LSW. Ciliates, in particular mixotrophs, were very active in the LSW where they grazed on phytoplankton at rates near their maximum clearance capacities. Mixotrophic and heterotrophic ciliates in LSW consumed about 50 and 10% of the nanophytoplankton stock per day, respectively. The mixotrophic ciliate L. strobila was found to be an efficient grazer of nanophytoplankton. In contrast to the ciliates, the biomass of heterotrophic nanoflagellates was similar in both the LSW and the MW; thus, we suggest that the transfer of bacterial biomass in the LSW towards higher trophic levels was moderate. KEY WORDS: Bacterivory · Grazing · Mixotroph · Laboea strobila · Picoeucaryotes · Rhone River plume Full text in pdf format PreviousNextCite this article as: Christaki U, Courties C, Joux F, Jeffrey WH, Neveux J, Naudin J (2009) Community structure and trophic role of ciliates and heterotrophic nanoflagellates in Rhone River diluted mesoscale structures (NW Mediterranean Sea). Aquat Microb Ecol 57:263-277. https://doi.org/10.3354/ame01339 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 57, No. 3. Online publication date: November 24, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.

Predation impact of ciliated and flagellated protozoa during a summer bloom of brown sulfur bacteria in a meromictic coastal lake

Anaerobic phagotrophic protozoa may play an important role in the carbon flux of chemically stratified environments, especially when phototrophic sulfur bacteria account for a high proportion of the primary production. To test this assumption, we investigated the vertical and temporal distribution of microbial heterotrophs and of autotrophic picoplankton throughout the water column of the meromictic coastal lake Faro (Sicily, Italy), in the summer of 2004, coinciding with a bloom of brown-colored green sulfur bacteria. We also assessed the grazing impact of ciliated and flagellated protozoa within the sulfur bacteria plate using a modification of the fluorescently labeled bacteria uptake approach, attempting to minimize the biases intrinsic to the technique and to preserve the in situ anoxic conditions. Significant correlations were observed between ciliate biomass and bacteriochlorophyll e concentration, and between heterotrophic nanoflagellate biomass and chlorophyll a concentration in the water column. The major predators of anaerobic picoplankton were pleuronematine ciliates and cryptomonad flagellates, with clearances of 26.6 and 9.5 nL per cell h(-1), respectively, and a cumulative impact on the picoplankton gross growth rate ranging between 36% and 72%. We concluded that protozoan grazing channels a large proportion of anaerobic picoplankton production to higher trophic levels without restraining photosynthetic bacteria productivity.

Response of heterotrophic bacteria, autotrophic picoplankton and heterotrophic nanoflagellates to re-oligotrophication

We investigated the response of the microbial components of the pelagic food web to re-oligotrophication of large, deep Lake Constance where total phosphorus concentrations during mixing decreased from a maximum of 2.81 μmol L−1 in 1979 via 1.87 μmol L−1 in 1987 to 0.26 μmol L−1 in 2007. Measurements of heterotrophic bacteria, autotrophic picoplankton (APP) and heterotrophic nanoflagellates (HNF) in 2006 and 2007 were compared to values from 1987 to 1997. We hypothesized that the biomass and seasonal variability of all groups will decrease under more oligotrophic conditions due to reduced resource availability, particularly for APP and HNF but less for the competitively stronger bacteria. Average bacterial biomass between spring and autumn was unrelated to phosphorus, whereas the ratio of bacterial biomass to chlorophyll a concentration increased with decreasing trophy due to declining chlorophyll concentrations. In contrast, a unimodal relationship was found between APP and phosphorus with low biomass at low and high phosphorus concentrations and maximum biomass in between. Average HNF biomass decreased strongly by a factor of 10–30 with decreasing trophy, and chlorophyll-specific HNF biomass was unimodally related to phosphorus. The relative seasonal biomass variability did not change for any group during re-oligotrophication. To conclude, HNF responded much more strongly and bacteria less so than chlorophyll concentrations to oligotrophication, whereas APP exhibited a more complex pattern.

Heterotrophic nanoflagellate and ciliate bacterivorous activity and growth in the northeast Atlantic Ocean: a seasonal mesoscale study

The grazing effect of heterotrophic nanoflagellates (HNF) and ciliates on bacterial pro- duction (BP), as well as their growth rates, was studied in winter, spring and autumn 2001 during the French research project Programme Ocean Multidisciplinaire Meso-Echelle (POMME) in the north- east Atlantic Ocean (38 to 45° N, 16 to 22° W). The variability of different parameters studied appears to be largely controlled by the seasonal and latitudinal gradients of primary production rather than the strong eddy activity at the mesoscale level in the area. Heterotrophic microbial abundance, bio- mass and protistan grazing varied temporally, presenting highest values during the phytoplankton bloom, during the spring period and following the northward propagation of the bloom. HNF biomass integrated over the upper 100 m was highest in spring (270 to 850 mg C m -2 ). Ciliate integrated bio- mass was generally ≤160 mg C m -2 except in a Tintinnus sp. bloom in a northern anticyclonic eddy (A1) in spring when it reached 637 mg C m -2 . HNF and ciliate growth rates varied from 0.2 to 0.7 d -1 and 0.2 to 1.4 d -1 , respectively. The fraction of BP consumed by ciliates was generally <10% except in the anticyclonic eddy A1 in spring during a tintinnid bloom when it reached 37% of BP. In conclu- sion, our data revealed that HNF can remove a large fraction of bacterial production in the northeast Atlantic Ocean (83 ± 27%, average of all sampling sites and seasons). Ciliates transferred less carbon to higher trophic levels than did HNF; however, episodic high occurrence of large bacterivorous cili- ates, primarily tintinnids, increased the role of theses organisms as C-links in the microbial food web.

The role of ciliates within the microbial food web in the eutrophicated part of Kaštela Bay (Middle Adriatic Sea)

Interactions among phytoplankton, bacterioplankton, heterotrophic nanoflagellates (HNF), ciliated protozoa and copepod nauplii were studied in the eutrophicated part of Kas?tela Bay from May 1998 to November 1999. Special emphasis was placed on relationships between size categories of nonloricate ciliates (NLC) and other microbial food web components. Biomasses of phytoplankton and bacteria were primarily influenced by abiotic parameters. Temperature indirectly controlled variation in HNF biomass through the changes in biomass of bacteria and the smaller phytoplankton fraction. Besides HNF, bacterial biomass was affected by the NLC 3 (Cell Length 3 , CL 104 µm 3 (CL >40 µm) had a strong mutual correlation and they seemed to be controlling the microphytoplankton fraction. During the colder part of the year, HNF abundance was regulated by ciliate grazing. The high impact of ciliates in summer 1998 could have been influenced by the taxonomic composition of the phytoplankton community changing, as well as the higher eutrophication level in the study area. Predation by copepod nauplii on ciliates and carbon transfer to higher trophic levels appear to be of relative importance only in the period when they are most abundant. This paper outlines the dominant relationships within the microbial food web and suggests that a significant amount of bacterial production, phytoplankton and HNF biomass could be transferred to higher trophic levels through the microbial food web.

Growth and production of free-living heterotrophic nanoflagellates in a eutrophic lake - Lake Donghu, Wuhan, China

In situ growth of heterotrophic nanoflagellates (HNF) in Lake Donghu, a eutrophic shallow lake in mainland China, was studied from January 1999 to March 2000 using a modified Weisse protocol. The study results indicated that the growth rates of HNF showed pronounced seasonal variation (-0.37-1.25 d(-1)), reaching the maximum during spring to early summer. When the water temperature was higher than 25.5 degreesC, HNF growth was inversely proportional to water temperature. There was an effect by bacterial abundance and autotrophic picoplankton on HNF growth that depended on location. HNF biomass was the highest in late spring, and the HNF production ranged from -2.25 to 35.45 mg l(-1) d(-1) with mean of 3.17 mg l(-1) d(-1). When considered in the context of biomass and production data for zooplankton in Lake Donghu, it was evident that HNF contributed significantly to the total zooplankton production in Lake Donghu. These in situ studies indicate that temperature and food supply are the major determinants of HNF abundance and productivity.

Seasonal Changes in Nano/Micro-Zooplankton Herbivory and Heterotrophic Nano-Flagellates Bacterivory off Cape Esan, Southwestern Hokkaido, Japan

Seasonal changes in nano/micro-zooplankton grazing on pico-, nano- and micro-size phytoplankton and heterotrophic nano-flagellates (HNF) feeding on heterotrophic bacteria were quantified by the dilution technique in the surface layer off Cape Esan, southwestern Hokkaido, Japan. Pico- and nano-size phytoplankton were major components throughout the year except in spring when a diatom bloom was observed. Although there was little seasonal variation in bacteria and HNF biomass throughout the year, the micro-zooplankton biomass varied appreciably with a peak in spring. Nano/micro-zooplankton grazing or feeding on pico-size chl-a and bacteria were well balanced throughout the year. However, nano-size and micro-size chl-a growth were much greater than grazing in summer. Nano/micro-zooplankton ingestion of phytoplankton was greater than their ingestion of bacteria almost throughout the year, which suggests phytoplankton are more important as food sources of nano/micro-zooplankton in microbial food webs off Cape Esan than bacteria off Cape Esan.

The microbial communities and primary productivity of cryoconite holes in an Arctic glacier (Svalbard 79°N)

The microbial communities and photosynthetic capacity of cryoconite holes on the Midre Lovenbreen Glacier at 79°N in Spitzbergen (Svalbard Archipelago) were investigated in July/August 2000 and July 2001. The constituents of the microbial assemblages were more abundant in material on the cryoconite bottoms than in the overlying water. Bacterial concentrations ranged from 1.00 to 4.502104 ml-1 in the water and from 4.67 to 7.072104 ml-1 in the bottom material; virus-like particles (VLP) ranged from 3.97 to 12.702104 ml-1 in the water and from 27.5 to 37.592104 ml-1 on the bottom. VLP: bacteria ratios ranged between 0.24 and 8.11, with highest ratios in the bottom assemblages. Heterotrophic nanoflagellate (HNAN) abundances were significantly lower than those of the autotrophic nanoflagellates (PNAN). Moreover, HNAN biomass was lower than bacterial biomass, indicating that the HNAN were exploiting other energy sources as well as bacteria, for example, VLP and dissolved organic carbon. The bottom material was dominated by cyanobacteria (mostly Phormidium sp.), while both the water and the bottom layer contained a small number of chlorophyte species (Chlorella sp., Cylindromonas sp. and Chlamydomonas nivalis). Ciliates were very sparse, only occurring on the bottom. On occasions, the glacier surface carried meltwater with well-developed biofilms, in which ciliates (Monodinium, Strombidium and Halteria) occurred. All of these species are found in nearby lakes. One to three rotifers were noted in the biofilm samples and in samples from three of the cryoconite holes. The assemblages of the cryoconite holes were comparable to the truncated food webs seen in Antarctic lakes, but were even more simplified and sparse in terms of biomass. Photosynthesis in the meltwater on the glacier surface ranged between 0.60 and 8.33 µg C l-1 h-1. Within the cryoconites, photosynthetic rates were usually highest on the bottom (0.63-156.99 µg C l-1 h-1), while in the overlying water, rates ranged between 0.34 and 10.56 µg C l-1 h-1. Given the density of cryoconite holes (circa 6% of the glacier surface, or 12 holes m-2), there was significant carbon fixation and nutrient cycling occurring on the glacier, associated with cryoconite communities. (Less)

Seasonal succession and taxon-specific bacterial grazing rates of heterotrophic nanoflagellates in Lake Constance

We investigated the taxonomic composition of the heterotrophic nanoflagellate (HNF) assemblage and its taxon-specific bacterial grazing rates in Lake Constance (Germany) over the course of 1 yr. Bacterial grazing rates were measured using natural fluorescently labelled bacteria (FLB) and compared to bacterial production estimated by the uptake of 14 C-leucine incorporation. Glutaraldehyde-fixed, DAPI-stained flagellates were counted using epifluorescence microscopy. Based on annual averages, small species such as Spumella sp. (2 to 6 µm) were the most numerous HNF and the dominant bacterivores. Larger flagellates such as Kathablepharis sp. contributed sig- nificantly to total HNF biomass, in particular during spring, but were relatively unimportant as bac- terial grazers. The HNF community structure changed during the transition from the phytoplankton spring bloom to the clearwater phase, with small flagellates such as heterokonts, kinetoplastids and choanoflagellates becoming increasingly abundant. The flagellate community composition was more diverse during summer and autumn than in spring. Per capita ingestion rates ranged from 0 to 31 bac- teria HNF -1 h -1 and changed seasonally up to 10-fold within a given taxon. Mixotrophic species con- tributed little to total bacterivory. We provide evidence that the relative significance of bacterial ingestion by a given flagellate taxon may change seasonally. Based upon our experimental results, we discuss potential shortcomings inherent in the FLB technique.

The production and potential loss mechanisms of bacterial biomass in the southern Gulf of Riga

Bacterial biomass and production were measured in the water column and sediment of the southern Gulf of Riga. The potential loss of bacteria in the water column by lysis, grazing and sedimentation was estimated. A generally higher biomass and production of bacteria (135–195 mg C m −3 and 53–80 mg C m −3 day −1) were measured during the midsummer when compared to the spring (56–123 mg C m −3 and 7–26 mg C m −3 day −1) and late summer (51–98 mg C m −3 and 4–16 mg C m −3 day −1) periods. Also heterotrophic nanoflagellate biomass (13–25 mg C m −3) and virus numbers (5–16×10 10 l −1) had a maximum during midsummer. The average benthic bacterial production was highest in spring (1132 mg C m −2 day −1) when compared to the other seasons (706–806 mg C m −2 day −1). Benthic bacterial production exceeded the bacterial productivity in the water column above (404–1750 mg C m −2 day −1) in spring and late summer but not in midsummer. Throughout the productive season grazing was estimated to consume 6–50% and viral lysis 55–>100% of the total bacterial production. Loss of bacteria by sedimentation was less than 2% of the bacterial production in the water column.