Heterotrophic Nanoflagellates Research Articles

Cascading effects of larval Crucian carp introduction on phytoplankton and microbial communities in a paddy field: top-down and bottom-up controls

Effects of fish predation propagate through aquatic food webs, where the classical grazing food chain and microbial loop are interwoven by trophic interactions. The overall impact on aquatic food webs is further complicated because fish may also exert bottom-up controls through nutrient regeneration. Yet, we still have limited information about cascading effects among fish, zooplankton, phytoplankton, and microbes. In this study, we performed a mesocosm experiment to evaluate effects of fish introduction on plankton communities. Six plots were set in factorial combination with fish introduction and rice straw plowing in a paddy field, and the experiment was continued for 4 weeks. Introduction of fish significantly increased chlorophyll a concentrations in smaller size fractions (<15 μm) and abundances of filamentous bacteria (>5 μm in length) and heterotrophic nanoflagellates in 3–15 μm fraction. Microbes in 0.8–3 μm fraction showed increasing but not significant trends in response to fish introduction. These results indicate cascading effects of fish predation operating via two pathways, one through grazing food chain and the other through microbial food web. Phytoplankton community compositions shifted in similar fashion in all plots until 1 week after fish introduction, and then diverged between plots with and without fish thereafter. Bottom-up effects of fish introduction were suggested by increases of total chlorophyll a and inedible phytoplankton species in response to fish introduction. This study provides an example of how fish predation regulates biomass and structure of phytoplankton and microbial communities.

Distribution patterns of heterotrophic flagellates and bacteria in acidic and neutral Karelian lakes

The abundance and taxonomic and trophic structures of the community of planktonic heterotrophic nanoflagellates, as well as the quantitative distribution of bacteria as their main object of food, have been studied in six lakes of Karelia with differing pH values, concentrations and compositions of organic substances, and degrees of anthropogenic load. Thirty-eight species and forms of colorless flagellates from seven orders and groups of uncertain taxonomic statuses have been recorded. Most of them belong to the trophic group of bacteriodetritophages. The minimal abundance and the lowest diversity of flagellates have been documented in clear- and brown-water lakes with low pH values, and maximal values have been observed in the polluted Kondopoga Bay in Lake Onega.

Phytoplankton allelochemical interactions change microbial food web dynamics

This study investigates the effect of filtrates from an allelopathic dinoflagellate, Alexandrium tamarense, on four microbial food webs that have been manipulated experimentally from natural seawater by modifying the availability of resources in the form of dissolved organic carbon with additions of peptone, and by altering the grazing pressure with size fractionation. Bacterial production was generally not affected by allelochemicals, but bacteria showed higher net growth in all food webs when allelochemicals were added, whereas heterotrophic nanoflagellates &gt; 7 μm and ciliates were constrained in all food webs. Allelochemicals had the largest negative effects on microbial communities with low grazing pressure. In food webs with high grazing pressure and additional resources, phytoplankton and small nanoflagellates were positively affected by the addition of allelochemicals, suggesting that those were interfering with trophic interactions in the microbial communities. By the lysis of organisms sensitive towards allelochemicals, resources are made available and grazing pressure on certain microorganisms is reduced. However, the intensity of these interactions is modulated by both the availability of resources and the biomass of grazers in the initial food web.

Occurrence and diversity of free-living protozoa on butterhead lettuce.

The occurrence and diversity of free-living protozoa (FLP) on butterhead lettuce (Lactuca sativa L.) was investigated using four different sampling techniques (washing, swabbing, homogenization, and excising). FLP were recovered from all leaf samples (n=64), and cultures were FLP-positive after 1 week. Identification of FLP was performed by light microscopy and sequencing of denaturing gradient gel electrophoresis (DGGE)-separated 18S rRNA gene fragments. Bodo saltans, Spumella (-like) spp. and Cercozoa were the most common heterotrophic nanoflagellates. Amoebae belonged mainly to the Vannellida and Tubulinida. Colpoda steinii and Cyclidium glaucoma were the most common ciliates. The total number of FLP on middle leaves estimated by the Most Probable Number method ranged from 9.3 × 10(2)MPN/g to 2.4 × 10(5)MPN/g leaf, with flagellates (92 MPN/g to 2.4 ×10(5)MPN/g) being more abundant than amoebae (<3 MPN/g to 9.3 × 10(3)MPN/g) and ciliates (<3 MPN/g to 9.3 × 10(2)MPN/g). Washing or rinsing leaves followed by spin-drying in a household salad spinner reduced the protozoan number with maximum one log unit. Our survey shows that FLP on lettuce leaves are a common and diverse but largely unexplored group of microorganisms.

Trophic interactions of pelagic crustaceans in Comau Fjord (Chile): their role in the food web structure

Grazing experiments were used to study the trophic interactions among plankton components in a stratified fjord of southern Chile. A strong pycnocline (7–10 m) separating the low-salinity layer from the deeper, high-salinity, marine layer, affected the composition, abundance and distribution of phytoplankton and zooplankton every season. Nanoflagellates averaging 59% were consistently the main components of the plankton biomass in both layers. The microphytoplankton contribution was always higher within the marine layer. Cladocerans and some small copepods showed a vertical gradient with greater abundances in the low-salinity layer, whereas medium-large copepods and euphausiids were more abundant below the pycnocline. Podon leuckarti averaged an ingestion rate of 3.1 µg C ind.−1 day−1, consisting mostly of particles from the nanoplankton size spectrum. Given significant clearance on ciliates and athecate dinoflagellates plus high ingestion rates of heterotrophic nanoflagellates it seems that this species selects for microzooplankton and that heterotrophic components are important in their diet. Calanus australis ingested 15 µg C ind.−1 day−1 on average, covering a wide spectrum of prey sizes (5–65 µm equivalent spherical diameter) and diversity, with active selection for ciliates. The average ingestion rate for Euphausia vallentini was 78.7 µg C ind.−1 day−1, including wide variety of prey sizes but no apparent selectivity. The potential grazing impact (11%) on primary productivity, together with high abundances, feeding selectivity on nanoplankton and production of unconsolidated fecal material indicate that cladocerans could have a double positive feedback effect on the microbial loop, thereby having an important impact by modifying the composition and structure of the planktonic community in Comau Fjord in spring-summer.

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 (&amp;gt;10 kg d−1); primary production (&amp;gt;300 g C m−2 y−1); chlorophyll a (&amp;gt;12 μg l−1) and phytoplankton biomass (&amp;gt;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.

Importance of bacterivory by pigmented and heterotrophic nanoflagellates during the warm season in a subtropical western Pacific coastal ecosystem

We investigated temporal variations in the effects of bacterivory by different sizes of heterotrophic nanoflagellates (HNF) and pigmented nanoflagellates (PNF) during a warm period (May to September) in oligotrophic coastal waters of the subtropical western Pacific. Short-term experiments with fluorescently labeled bacteria (FLB) demonstrated ingestion rates of 0.3 to 5.8 bacteria HNF ―1 h ―1 by HNF in the size range 3―6 pm—rates that were higher than observed for other sizes of HNF. Rates of ingestion by PNF ranged between 0.9 and 15.5 cells PNF- 1 h ―1 , and, as for HNF, were greatest for PNF in the 3―6 μm size group. Nanoflagellates of size < 6 μm removed about 98 % of the total amount of bacteria consumed. The 3―6 um PNF, 2―3 um HNF, and 3―6 um HNF were major consumers in the nanoflagellate community and were responsible for an average of 52, 28 and 16% of the total consumption of bacteria, respectively. The smallest PNF (2―3 um) consumed only about 2 % of the total and were considered to be primarily autotrophic. Despite ingestion rates in the range of those reported elsewhere, the low abundance of nanoflagellates observed resulted in relatively low grazing impacts (<10% of bacterial standing stock). We found a significant negative correlation between PO 4 concentrations and ingestion rates of the 3―6 μm PNF, suggesting that the PNF ingestion rate increased under nutrient-deficient conditions.

Airborne microeukaryote colonists in experimental water containers: diversity, succession, life histories and established food webs

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 62:139-152 (2011) - DOI: https://doi.org/10.3354/ame01463 Airborne microeukaryote colonists in experimental water containers: diversity, succession, life histories and established food webs Savvas Genitsaris1, Maria Moustaka-Gouni1,*, Konstantinos A. Kormas2 1Department of Botany, School of Biology, Aristotle University of Thessaloniki, 541 24 Thessaloniki, Greece 2Department of Ichthyology and Aquatic Environment, School of Agricultural Sciences, University of Thessaly, 384 46 Nea Ionia, Magnisia, Greece *Corresponding author. Email: mmustaka@bio.auth.gr ABSTRACT: Airborne freshwater and marine microeukaryotes in the city of Thessaloniki, situated in Thermaikos Bay, Greece, were examined as sources of colonization of experimental water containers during the period autumn 2007 to spring 2008. The microeukaryote composition of the plankton in the nearby aquatic systems (distance <1 km) was also examined. Airborne microeukaryotes were examined by morphology and 18S rRNA gene diversity. A total of 29 species of airborne microeukaryotes were identified, most of them commonly observed in aerobiological studies. Airborne organisms of only 8 taxa were also detected in the nearby aquatic systems. The algae Haematococcus lacustris, a Chlorella-like taxon, and Scenedesmus cf. obliquus, the heterotrophic nanoflagellates (HNF) Bodo sp., Cafeteria minuta and Rynchomonas nasuta, and the ciliate Pattersoniella vitiphila were present in all 3 seasons, indicating capabilities of successful dispersal and colonization under a wide range of meteorological conditions. Rapid colonization of the water containers by the microeukaryotes occurred at the beginning of the experiment, but the rate of colonization quickly stabilized. The initial phase of colonization was dominated by HNF; subsequently, members of the Chlorophyta were the dominant autotrophs. The heterotrophic and autotrophic colonists established similar food webs in all 3 seasons, with P. vitiphila being the common top predator. KEY WORDS: Airborne microeukaryotes · Colonization · Diversity · Food web · Haematococcus · Pattersoniella Full text in pdf format Supplementary material PreviousNextCite this article as: Genitsaris S, Moustaka-Gouni M, Kormas KA (2011) Airborne microeukaryote colonists in experimental water containers: diversity, succession, life histories and established food webs. Aquat Microb Ecol 62:139-152. https://doi.org/10.3354/ame01463 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 62, No. 2. Online publication date: January 19, 2011 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2011 Inter-Research.

Grazing of heterotrophic nanoflagellates on the eukaryotic picoautotroph Choricystis sp.

Autotrophic picoplankton (APP, <2 µm), composed of both prokaryotes and eukaryotes, play an important role in the production and transformation of organic carbon in freshwaters. Eukaryotic APP are commonly found in winter and spring, and heterotrophic nanoflagellates (HNF) are regarded as significant consumers of APP. Here, we analysed the grazing impact and the growth ability of the HNF culture derived from a boreal clearwater lake on the picoalga Choricystis sp. For comparison, we used HNF monocultures of Rhynchomonas sp. and Bodo saltans flagellates with dis- tinct feeding behaviours. In the grazing experiments, all HNF cultures ingested picoalgae and reached feeding rates between 0.08 and 0.81 cells HNF -1 h -1 . We observed large species-specific dif- ferences in grazing rate (0.97 to 1.96 d -1 ), growth rate (0.35 to 0.85 d -1 ) and final abundance of HNF (5.6 × 10 4 to 74.2 × 10 4 cells ml -1 ) in the growth experiments. Surprisingly, we found an increase in picoalgal growth in the presence of a grazing HNF population. Although HNF appear to be active herbivores capable of significantly reducing APP stock, flagellates were unable to control the picoal- gal population. Overall, this study demonstrates an unexpected role of HNF in microbial food webs by imposing a positive feedback on picoalgal growth through grazing.

The contribution of heterotrophic nanoflagellate grazing towards bacterial mortality in tropical waters: comparing estuaries and coastal ecosystems

Heterotrophic nanoflagellate (HNF) grazing depends on both temperature and trophic status of an ecosystem. As most microbes already function at their temperature optimum in tropical waters, we hypothesised that HNF grazing rates would be higher in more productive sites such as estuaries than in less productive areas such as coastal waters. We sampled two estuaries and three coastal sites along the Straits of Malacca and the South China Sea near the Malaysia Peninsula. Bacterial abundance ranged 0.9–6.3 × 106 cells mL–1, whereas HNF abundance ranged 1.8–10.1 ×103 cells mL–1. Bacterial production ranged 1.1–12.7 × 105 cells mL–1 h–1, whereas HNF grazing rates were an order of magnitude lower at 1.0–78.5 × 104 cells mL–1 h–1. Bacterial abundance, net bacterial production and HNF grazing rates were higher in estuaries than coastal waters but HNF abundance did not differ between the two areas. Across all stations, HNF grazing rates increased with bacterial production, and accounted for ~33% of bacterial production. Our results suggest that in the tropical waters studied, there was a bacterial production–grazing imbalance. Other loss factors such as viral lysis, sedimentation or the presence of benthic filter feeders could account for this imbalance.

Seasonal microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): I. Heterotrophic bacteria, picoplankton and nanoflagellates

Temporal dynamics of the microbial food web in the Barents Sea and adjacent water masses in the European Arctic are to a large extent unknown. Seasonal variation in stocks and production rates of heterotrophic bacteria and phototrophic and heterotrophic picoplankton and nanoflagellates was investigated in the upper 50 m of the high-latitude Kongsfjorden, Svalbard, during six field campaigns between March and December 2006. Heterotrophic bacteria, picoplankton and nanoflagellates contributed to ecosystem structure and function in all seasons. Activity within the microbial food web peaked during spring bloom in April, parallel to low abundances of mesozooplankton. In the nutrient-limited post-bloom scenario, an efficient microbial loop, fuelled by dissolved organic carbon from abundant mesozooplankton feeding on phytoplankton and protozooplankton, facilitated maximum integrated primary production rates. A tight microbial food web consisting of heterotrophic bacteria and phototrophic and heterotrophic picoplankton and nanoflagellates was found in the stratified water masses encountered in July and September. Microbial stocks and rates were low but persistent under winter conditions. Seasonal comparisons of microbial biomass and production revealed that structure and function of the microbial food web were fundamentally different during the spring bloom when compared with other seasons. While the microbial food web was in a regenerative mode most of the time, during the spring bloom, a microbial transfer mode represented a trophic link for organic carbon in time and space. The microbial food web’s ability to fill different functional roles in periods dominated by new and regenerated production may enhance the ecological flexibility of pelagic ecosystems in the present era of climate change.

Seasonal variations in abundance, growth and mortality of heterotrophic bacteria in Kagoshima Bay

Seasonal changes in abundance, growth and mortality of heterotrophic bacteria were investigated monthly from collections and dilution experiments in Kagoshima Bay, the southernmost of Japan. Bacteria occurred abundantly with considerable variation in the surface layers where chlorophyll a concentrations were high, whereas seasonal variations were obscure below 100 m. Especially, bacteria showed a decline of cell density toward summer when heterotrophic nano-flagellates increased their abundance. Seasonal and vertical variations in bacterial cell number during the study period were positively correlated with those of water temperature and pico-sized chlorophyll a concentration. Maximum growth and mortality rates showed positive correlations with water temperature but no positive relationships to size-fractionated chlorophyll a. Net increase rate (i.e. in situ rate if abundance changes) was negatively correlated with cell density of heterotrophic nano-flagellates. It is concluded that bacterial abundance is controlled by the resultant (i.e. net growth rate) of the balance between maximum growth and predatory mortality by heterotrophic nano-flagelllates which are both dependent on ambient temperature.

Microbial dynamics in an oligotrophic bay of the western subtropical Pacific: Impact of short-term heavy freshwater runoff and upwelling

This two-year study investigates the possible factors that determine spatial and temporal dynamics of picoplankton (heterotrophic bacteria, autotrophic picoplankton—Synechococcus spp., Prochlorococcus, and picoeukaryotes) and nanoflagellate abundance in the subtropical Ilan Bay, Taiwan, where the inner bay is affected by freshwater run-off from the Lanyang River and the eastern outer bay by the Kuroshio Current. In the inner bay, there was more rain and freshwater discharge in 2005 than in 2004 during the warm season (>24° C, June–September). The abundance of bacteria, Synechococcus spp., Prochlorococcus, and picoeukaryotes and the percentage contributions of pigmented nanoflagellate (PNF %) were two- to eight-fold greater during this period (July in 2005) than for other sampling periods. Relatively low abundance of heterotrophic nanoflagellates (HNF) in the presence of abundant picoplankton prey suggests that top-down control determined HNF abundance in the Ilan Bay, Taiwan.

Disentangling the effect of viruses and nanoflagellates on prokaryotes in bathypelagic waters of the Mediterranean Sea

Bathypelagic ecosystems depend on prokaryotic heterotrophic biomass fuelled by vertical particulate organic matter (POM) fluxes, but very little information is available on the interactions among viruses, prokaryotes and nanoflagellates in deep waters. We simultaneously investigated the relative importance of the viral and heterotrophic nanoflagellate (HNF) grazing-induced prokaryotic mortality in bathypelagic waters by means of dilution experiments performed on samples collected at 1500 m depth from the Atlantic Ocean to the Eastern Mediterranean Sea. Prokaryotic abundance (range: 1.4 to 8.9 × 104 cells ml–1), although different from one station to another, was on average not significantly different among biogeographic regions. The potential predators of prokaryotes (viruses, HNF and microzooplankton) followed a similar spatial pattern. Viruses were responsible for an important fraction of prokaryotic mortality (on average 13.4% d–1). Dilution experiments carried out to estimate the potential predation of HNF suggested a high effect on prokaryotic abundance. However, since the latter experiments also include the effect of viruses on prokaryotes, when this factor was disentangled from the overall mortality, the potential rates of HNF predation on prokaryotes (on average 49.5%) were ca. 4 times higher than the effect due to viral infections. Conversely to patterns of distribution, the relative importance of virus-mediated mortality vs. HNF predation changed significantly among different regions. Results should be treated with caution due to the intrinsic difficulty in reproducing experimentally natural deep-sea conditions, but they permit disentangling of the relative effect of viruses and HNF on prokaryotes and compare the potential predatory control in different biogeographic regions.

Checking the pulse of Lake Ontario's microbial-planktonic communities: A trophic transfer hypothesis

The structure and function of the microbial food web of Lake Ontario was assessed at 15 stations distributed across 4 transects during the spring and summer of 2003. This was the first major binational study of Lake Ontario since the Lake Ontario Trophic Transfer initiative of 1990. The microbial loop (bacteria, autotrophic picoplankton, heterotrophic nanoflagellates (HNF) and ciliates) and phytoplankton, were enumerated microscopically in addition to measurements of chlorophyll a, size fractionated primary productivity (14C) and bacterial growth (3H). HNF dominated the total biomass in spring (≈300 mg m−3) and summer (≈1250 mg m−3). The size of the organic carbon pool increased from ≈90 mg C m−3 in spring to ≈270 mg C m−3 with HNF contributing 36% of the total organic carbon in the spring and 52% in the summer; however the net balance of the organic carbon pool shifted from autotrophic in the spring to heterotrophic in the summer. The available evidence suggests that HNF are a poor quality food resource for zooplankton and it is likely that the carbon sequestered by HNF is not available to higher trophic levels resulting in dietary stress for planktivores. The implications of high HNF for both organic carbon cycling and maintaining healthy fisheries needs further research. Independent observations show that oligotrophic conditions prevail as evidenced by low phosphorus, low chlorophyll a, low plankton and high water clarity. Such conditions have been generally regarded as the gold standard for managing healthy lakes. Lake Ontario is oligotrophic and healthy from a water quality perspective, but from a food web dynamics point of view, Lake Ontario appears to be unhealthy due to the dominance of HNF, low zooplankton and poor quality of food available to higher trophic levels. We hypothesize that the lake's poor health is attributable to inefficient energy transfer from lower to higher trophic levels. The traditional understanding of trophic state based mainly on water quality criteria needs to be broadened by the inclusion of food web and fisheries based metrics.

Bathymetric, latitudinal and vertical distribution of protozooplankton in a cold-temperate shelf (southern Patagonian waters) during winter

Although there have been many ecological field studies on the key components of planktonic food webs, there are still gaps in knowledge on some groups, environments and seasons. This is the first report on the spatial distribution of the density and biomass of almost all the taxonomic groups and size fractions of protozooplankton across a cold-temperate shelf during winter. Twenty-eight stations (two or three depths) were sampled on four cross-shelf transects in Patagonian waters (south-western Atlantic; 47–558S, 60–698W) during September 2006. Loricate ciliates, radiolarians and foraminiferans showed the lowest densities, and were distributed mainly in coastal, slope or the whole shelf waters, respectively. The density and biomass of aloricate ciliates and heterotrophic nanoflagellates and dinoflagellates were low and homogeneous both vertically and across the shelf south of 518S, but peaked in the upper 40 m in offshore waters at 478S. Microplanktonic aloricate ciliates, which represented 53% of the total protozooplankton biomass, reached values as high as 16 mg C L on the last transect. Consequently, both protozooplankton biomass and its ratio to chlorophyll a concentration were significantly higher in the northern offshore waters. These trends were linked to higher subsurface temperature and chlorophyll a concentration, and lower copepod nauplii biomass. Our results probably reflect changes in both the availability of food resources and predators and the physical structure of the water column, which are a consequence of the different environmental conditions that coexist over the large latitudinal and longitudinal gradients covered during late winter.

The Effect of River Water Circulation on the Distribution and Functioning of Reservoir Microbial Communities as Determined by a Relative Distance Approach

The effect of river water quality, its inflow rate, and temperature on planktonic food web composition and activities were studied in the eutrophic Sau Reservoir (Catalonia, NE Spain). We analyzed 8 longitudinal transects conducted between July 1996 and April 1999 covering a wide range of variability in both seasonal and spatial circulation patterns. To compare objectively the biological longitudinal gradients under seasonally fluctuating water levels and different types of water circulation patterns, we applied a model based on the relative distance of a sampling station from the river inflow. Even under different hydrological scenarios, the model was able to characterize epilimnetic food chain successions and locations of peaks of bacteria, heterotrophic nanoflagellates, ciliates, phytoplankton, and zooplankton along the longitudinal gradient. The amplitude of microbial peaks was directly related to the proportion of nutrient and organic carbon rich river water that mixed into the reservoir epilimnion. Enhanced abundances and activities of microbes were detected in spring and summer periods, mainly during events of river water overflow when a large proportion of the river was directly mixed into the epilimnion. Thus, the relative input of river water is suggested to be a useful predictor of the amplitude of the development of the epilimnetic microbial food webs in highly loaded canyon-shaped reservoirs. These results may have important implications in the context of global change in Mediterranean regions, where expected reductions in runoff may profoundly affect river water circulation patterns in reservoirs and hence organic carbon cycling in these ecosystems.

Element content of Ochromonas danica: a replicated chemostat study controlling the growth rate and temperature

Ecological stoichiometry focuses on the balance between multiple nutrient elements in resources and in consumers of those resources. The major consumers of bacteria in aquatic food webs are heterotrophic and mixotrophic nanoflagellates. Despite the importance of this consumer-resource interaction to understanding nutrient dynamics in the aquatic food web, few data are available addressing the element stoichiometry of flagellate consumers. Ochromonas danica, a mixotrophic bacterivore, was used as a model organism to study the relationships among temperature, growth rate and element stoichiometry. Ochromonas danica was grown in chemostats at dilution rates ranging between 0.03 and 0.10 h(-1) and temperatures ranging between 15 and 28 °C. Cells accumulated elements as interactive functions of temperature and growth rate, with the highest element concentrations corresponding to cells grown at a low temperature and high growth rates. The highest concentrations of elements were associated with small cells. Temperature and growth rate affected the element stoichiometry (as C:N, C:P and N:P) of O. danica in a complex manner, but the growth rate had a greater effect on ratios than did temperature.

Heterotrophic microplankton in the lower Hudson River Estuary: potential importance of naked, planktonic amebas for bacterivory and carbon flux

The present study is the first to simultaneously document the contributions of bacteria, heterotrophic flagellates, ciliates, and naked, planktonic amebas to the carbon (C) budget of an estu- arine water column, and is also the first study of protistan bacterivory in the lower Hudson River Estu- ary (HRE). Observations were collected at a single near-shore location between June 2006 and May 2009. Bacterial counts and biomass varied approximately 1 order of magnitude on different dates, but were comparable to previous studies of the HRE and other estuaries. Of the 3 heterotrophic protist groups enumerated, heterotrophic nanoflagellates were the least variable and generally had the highest biomass (on average equaling 38% of the bacterial biomass). Counts and biomasses of cili- ates and amebas were highly variable, ranging over at least 3 orders of magnitude between sampling dates. Much of the variability in ameba abundance was consistent with previous observations of sea- sonality. Ciliate biomass averaged 8%, and ameba biomass averaged 15% of the bacterial biomass. Thus, at this location, the importance of amebas as micropredators may be comparable to that of the ciliates, a group generally receiving greater research attention. Ameba ingestion rates could not be measured directly but 3 indirect approaches for calculating ingestion rates produced mean values ranging from 1.2 to 2.5 ng C d -1 ng -1 ameba biomass. Each approach demonstrated that ameba C consumption at the study location was highly variable, but was at times high relative to the bacterial standing stock. Taken together, these data suggest that amebas may be more common and of greater importance in estuarine C-fluxes than generally appreciated.

How well can the fatty acid content of lake seston be predicted from its taxonomic composition?

Abstract1. Results from the few field studies that have tried to relate seston taxonomic and fatty acid (FA) composition suggest that phytoplankton composition only partially explains seston FA composition. However, in these studies, the heterotrophic components of seston (i.e. bacteria and heterotrophic protists) have not been accounted for.2. The general premise of this article was that including the contribution of heterotrophs to seston biomass can improve understanding of the variability in seston FA composition. This was tested for an oligotrophic clearwater lake, in which the taxonomic and FA compositions of seston, fractionated into three size classes, were monitored every 2 weeks over a growth season. The relationship between seston taxonomic and FA composition was studied using canonical correlation analyses.3. Because of their relative richness in branched FA and lack of highly unsaturated FAs (HUFA) compared to autotrophs and other protists, the contribution of bacteria to seston biomass was shown to explain an important part of the differences in FA composition between the different seston size classes. Phytoplankton seasonal succession also affected the FA composition of seston but only for size classes that were dominated by autotrophs.4. The results also indicated that heterotrophic protists such as ciliates and heterotrophic nanoflagellates might substantially influence the seston FA, and especially, HUFA, composition.5. The per cent of variability in seston FA composition that was explained by its taxonomic composition was still relatively low, even when taking account of heterotrophs. Hence, other possible influences, such as phytoplankton species composition, physiological state and the contribution of terrestrial detritus, need investigation.