Protistan Grazing Research Articles

Impact of protist grazing on a key bacterial group for biogeochemical cycling in Baltic Sea pelagic oxic/anoxic interfaces

Barrier zones between oxic and anoxic water masses (redoxclines) host highly active prokaryotic communities with important roles in biogeochemical cycling. In Baltic Sea pelagic redoxclines, Epsilonproteobacteria of the genus Sulfurimonas (subgroup GD17) have been shown to dominate chemoautotrophic denitrification. However, little is known on the loss processes affecting this prokaryotic group. In the present study, the protist grazing impact on the Sulfurimonas subgroup GD17 was determined for suboxic and oxygen/hydrogen sulphide interface depths of Baltic Sea redoxclines, using predator exclusion assays and bacterial amendment with the cultured representative 'Sulfurimonas gotlandica' strain GD1. Additionally, the principal bacterivores were identified by RNA-Stable Isotope Probing (RNA-SIP). The natural Sulfurimonas subgroup GD17 population grew strongly under oxygen/hydrogen sulphide interface conditions (doubling time: 1-1.5 days), but protist grazing could consume the complete new cell production per day. In suboxic samples, little or no growth of Sulfurimonas subgroup GD17 was observed. RNA-SIP identified five active grazers, belonging to typical redoxcline ciliates (Oligohymenophorea, Prostomatea) and globally widespread marine flagellate groups (MAST-4, Chrysophyta, Cercozoa). Overall, we demonstrate for the first time that protist grazing can control the growth, and potentially the vertical distribution, of a chemolithoautotrophic key-player of oxic/anoxic interfaces.

Effect of East Asian aerosol enrichment on microbial community composition in the South China Sea

East Asian aerosol has been regarded as an increasingly important nutrient source in the Northwest Pacific region due to intensifying human activities in the last decades. However, its effects on bacterial communities, an important component of the marine microbial food web and biogeochemical cycles, have been little studied. In our study, two types of aerosol enrichment microcosm experiments were conducted in the oligotrophic South China Sea by adding the aerosol initially or daily. Our results showed that the potential stimulus of bacteria by aerosol enrichment was likely masked by a more intense grazing pressure in these treatments. Distinct responses of various bacterial phylogenetic groups, including Actinobacteria, Cyanobacteria, Alphaproteobacteria and Gammaproteobacteria, were revealed by denaturing gradient gel electrophoresis analysis. High aerosol addition resulted in a decrease in autotrophic picocyanobacteria abundance, but no significant composition change was observed. Such responses of microbial abundance and composition could be directly triggered by the input of aerosol-derived organic or inorganic components and further shaped by the changes in the phytoplankton community structure or differentiated mortality loss caused by enhanced protist grazing, which may have implications for oceanic carbon dynamics under future scenarios of increasing atmospheric deposition.

Phytoplankton Cell Size: Intra- and Interspecific Effects of Warming and Grazing

Decreasing body size has been suggested as the third universal biological response to global warming after latitudinal/altitudinal range shifts and shifts in phenology. Size shifts in a community can be the composite result of intraspecific size shifts and of shifts between differently sized species. Metabolic explanations for the size shifts dominate in the literature but top down effects, i.e. intensified size-selective consumption at higher temperatures, have been proposed as alternative explanation. Therefore, we performed phytoplankton experiments with a factorial combination of warming and consumer type (protist feeding mainly on small algae vs. copepods mainly feeding on large algae). Natural phytoplankton was exposed to 3 (1st experiment) or 4 (2nd experiment) temperature levels and 3 (1st experiment: nano-, microzooplankton, copepods) or 2 (2nd experiment: microzooplankton, copepods) types of consumers. Size shifts of individual phytoplankton species and community mean size were analyzed. Both, mean cell size of most of the individual species and mean community cell size decreased with temperature under all grazing regimes. Grazing by copepods caused an additional reduction in cell size. Our results reject the hypothesis, that intensified size selective consumption at higher temperature would be the dominant explanation of decreasing body size. In this case, the size reduction would have taken place only in the copepod treatments but not in the treatments with protist grazing (nano- and microzooplankton).

The Newly Described Heterotrophic Dinoflagellate Gyrodinium moestrupii, an Effective Protistan Grazer of Toxic Dinoflagellates

Few protistan grazers feed on toxic dinoflagellates, and low grazing pressure on toxic dinoflagellates allows these dinoflagellates to form red-tide patches. We explored the feeding ecology of the newly described heterotrophic dinoflagellate Gyrodinium moestrupii when it fed on toxic strains of Alexandrium minutum, Alexandrium tamarense, and Karenia brevis and on nontoxic strains of A. tamarense, Prorocentrum minimum, and Scrippsiella trochoidea. Specific growth rates of G. moestrupii feeding on each of these dinoflagellates either increased continuously or became saturated with increasing mean prey concentration. The maximum specific growth rate of G. moestrupii feeding on toxic A. minutum (1.60/d) was higher than that when feeding on nontoxic S. trochoidea (1.50/d) or P. minimum (1.07/d). In addition, the maximum growth rate of G. moestrupii feeding on the toxic strain of A. tamarense (0.68/d) was similar to that when feeding on the nontoxic strain of A. tamarense (0.71/d). Furthermore, the maximum ingestion rate of G. moestrupii on A. minutum (2.6 ng C/grazer/d) was comparable to that of S. trochoidea (3.0 ng C/grazer/d). Additionally, the maximum ingestion rate of G. moestrupii on the toxic strain of A. tamarense (2.1 ng C/grazer/d) was higher than that when feeding on the nontoxic strain of A. tamarense (1.3 ng C/grazer/d). Thus, feeding by G. moestrupii is not suppressed by toxic dinoflagellate prey, suggesting that it is an effective protistan grazer of toxic dinoflagellates.

Protist grazing and viral lysis as prokaryotic mortality factors at Baltic Sea oxic−anoxic interfaces

Interface zones between oxic and anoxic water masses (pelagic redoxclines) host highly active prokaryotic communities, mediating important biogeo- chemical transformations. However, for marine pelagic redoxclines almost no knowledge exists on the magni- tude of the loss processes affecting these prokaryotic communities. We assessed the importance of protist grazing and viral lysis as prokaryotic mortality factors for 2 central Baltic Sea redoxclines using a combination of microscopy and experimental community manipula- tion techniques. Our results demonstrate that protist grazing dominated prokaryotic mortality at suboxic (<30 µmol l !1 oxygen) and oxygen!hydrogen sulphide interface depths, with 50 to 100% of prokaryotic stand- ing stocks grazed daily, compared to 2 to 20% of virally infected cells. Grazing was mediated by 2 different pro- tist associations, viz. dinoflagellates and Strombidium- like ciliates in the suboxic zone, and larger ciliates (cf. Mesodinium, Metacystis spp., cf. Coleps and un - identified morphotypes) at the oxygen!hydrogen sul- phide interface. In contrast, heterotrophic nanoflagel- lates (HNF) played a minor role, generally grazing <5% daily of prokaryotic standing stocks. Thus, these redox zones show major differences in microbial food web structure when compared to surface waters, with ciliates and dinoflagellates constituting the major bacterivores instead of HNF. At sulphidic depths, grazing was below the detection limit, and the frequency of virally infected cells decreased, leaving the identity of the major pro - karyotic mortality factor at these depths unresolved.

Temporal dynamics of carbon flow through the microbial plankton community in a coastal upwelling system off northern Baja California, Mexico

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 461:31-46 (2012) - DOI: https://doi.org/10.3354/meps09782 Temporal dynamics of carbon flow through the microbial plankton community in a coastal upwelling system off northern Baja California, Mexico Lorena Linacre1, 2,*, Michael R. Landry3, Ramón Cajal-Medrano4, J. Rubén Lara-Lara5, J. Martín Hernández-Ayón6, Rosa R. Mouriño-Pérez2, Ernesto García-Mendoza5, Carmen Bazán-Guzmán5 1Programa de Doctorado en Oceanografía Costera, Facultad de Ciencias Marinas/Instituto de Investigaciones Oceanológicas, 4Facultad de Ciencias Marinas, and 6Instituto de Investigaciones Oceanológicas; Universidad Autónoma de Baja California (UABC), Ensenada, Baja California 22860, Mexico 2Departamento de Microbiología, División de Biología Experimental y Aplicada, and 5Departamento de Oceanografía Biológica, División de Oceanología; Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Ensenada, Baja California 22860, Mexico 3Scripps Institution of Oceanography, University of California at San Diego, La Jolla, California 92093-0227, USA *Email: lorenalinacre@gmail.com ABSTRACT: We investigated the temporal dynamics of carbon flow through the microbial food web of a coastal upwelling system (ENSENADA station) off northern Baja California during 6 cruises (September 2007 to November 2008). Carbon biomass assessments for major autotrophic size groups (pico- to micro-sized cells) and their microzooplankton grazers were based on analyses using flow cytometry, HPLC pigments and epifluorescence microscopy. Taxon-specific phytoplankton growth and microzooplankton grazing rates were determined from 24 h in situ incubations in the euphotic zone using an abbreviated 3-treatment dilution technique. Carbon biomass and instantaneous growth and grazing rate determinations were used to estimate daily rates of taxon-specific production and losses due to microzooplankton grazing. Overall, microbial biomass showed a close balance between autotrophic and heterotrophic components, except during a period of very strong upwelling (April 2008), which favored large phytoplankters and high primary production. Throughout a wide range of environmental conditions, the community primary production (PP) attributed both to small (mostly picophytoplankton and prasinophytes) and large (mostly diatoms and autotrophic dinoflagellates) autotrophs was significantly grazed (78 ± 9% of PP) by small (<20 µm) and large (>20 µm) ciliates and flagellates (including mixotrophic dinoflagellates), respectively, showing complementary temporal shifts in protistan grazer types that matched the dominant phytoplankton. While large diatoms were strongly consumed by large ciliates during the 2 most productive periods (September 2007 and April 2008), pico- and nano-sized phytoplankton were grazed most by nanoflagellates and small ciliates from November 2007 to January 2008. Consequently, biogenic carbon production in this ecosystem is transferred through a multivorous food web. KEY WORDS: Biogenic carbon flow · Multivorous food web · Phytoplankton growth rate · Phytoplankton grazing rate · Primary production Full text in pdf format PreviousNextCite this article as: Linacre L, Landry MR, Cajal-Medrano R, Lara-Lara JR and others (2012) Temporal dynamics of carbon flow through the microbial plankton community in a coastal upwelling system off northern Baja California, Mexico. Mar Ecol Prog Ser 461:31-46. https://doi.org/10.3354/meps09782 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 461. Online publication date: August 08, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Drivers of protistan grazing pressure: seasonal signals of plankton community composition and environmental conditions

Rates of heterotrophic protist grazing and phytoplankton growth were measured weekly to bi-weekly in Narragansett Bay, Rhode Island, USA, from January 2010 to February 2011. In situ sensor data and species composition were collected concomitantly to link patterns in plankton dynamics with ancillary environmental and biological processes. Annual average phyto- plankton growth rates were 0.69 ± 0.58 d �1 , and heterotrophic protist grazing rates were 0.79 ± 0.61 d �1 . Phytoplankton growth rates were at times negative in both winter and spring. Nutrient limitation was only detected during summer, and negative growth rates in winter did not result from nutrient limitation. On an annual average, grazing removed 96% (20 to 200%) of primary production, with peaks in both phytoplankton growth and heterotrophic protist grazing rates dur- ing summer. There was no relationship between protistan herbivory rates and initial chlorophyll a concentration. Dominant grazer taxa changed seasonally. Heterotrophic dinoflagellates domi - nated in summer and were associated with significantly higher than average grazing rates (>1 d �1 ). Seasonal changes in grazing rates were most significantly characterized by seasonal changes in both temperature and plankton community composition. The relative effects of temperature and species composition could not be distinguished statistically. The magnitude of protistan grazing and subsequent effects on trophic transfer and primary production rates as well as phytoplankton community composition may be better understood and parameterized when grazing pressure is evaluated in relation to species composition and environmental conditions rather than bulk measures of biomass.

Grazing impact on the cyanobacterium Microcystis aeruginosa by the heterotrophic flagellate Collodictyon triciliatum in an experimental pond

We estimated the grazing impact of the heterotrophic flagellate Collodictyon triciliatum on the harmful, bloom-forming cyanobacterium Microcystis aeruginosa in an experimental pond during a Microcystis bloom from summer to winter in 2010. For these experiments, we calculated the grazing rates from the digestion rate of C. triciliatum and its food vacuole contents. During the study period, M. aeruginosa exhibited one bloom event with a maximum density of 1.1 × 105 cells ml−1. The cell density of C. triciliatum fluctuated from below the detection limit to 291 cells ml−1. The number of M. aeruginosa cells ingested by C. triciliatum food vacuoles ranged between 0.4 and 10.8 cells flagellate−1, and the digestion rate of C. triciliatum at 25 °C was 0.73 % cell contents min−1. The grazing rate of C. triciliatum on the M. aeruginosa prey was 0.2–6.9 cells flagellate−1 h−1, and its grazing impact was 0.0–25.3 % standing stock day−1. The functional response of C. triciliatum to the M. aeruginosa prey followed the Michaelis–Menten model of significance (r 2 = 0.873, p < 0.001) in our experimental systems, in which the prey concentration varied from 1.0 × 104 to 2.1 × 106 cells ml−1. The maximum grazing rate was 6.2 prey cells grazer−1 h−1, and the half-saturation constant was 1.2 × 105 cells ml−1. We present evidence that C. triciliatum grazing explained the remarkable decrease in M. aeruginosa cell density in the pond. The present study is the first demonstration of the high potential of protistan grazing on M. aeruginosa to reduce cyanobacterial blooms.

The influence of coastal waters on distributions of heterotrophic protists in the northern East China Sea, and the impact of protist grazing on phytoplankton

Spatial and temporal variations in the abundance and biomass of heterotrophic protists and of their grazing impact were investigated during five cruises between July 2006 and February 2009 in the continental shelf waters of the northern East China Sea (ECS). Strongly patchy distributions were observed on all cruises, generally with a higher biomass in the western areas affected by the Changjiang River discharge. An opposite pattern was observed in February when the Kuroshio onshore transport is greatest, with a greater biomass in the eastern area. Small heterotrophic dinoflagellates (,20 mm) were most abundant numerically, whereas ciliates contributed the most to the biomass, accounting for 28–58% of the total heterotrophic protist biomass. Small heterotrophic dinoflagellates were more strongly correlated with phytoplankton biomass than were other types of protists. The total protist biomass was often most strongly related to amounts of particulate organic carbon of non-phytoplankton origin, suggesting that their abundance distribution often depended on prey other than phytoplankton, such as heterotrophic bacteria. Heterotrophic protists consumed 30.1–91.5% of Chl a production (mean 68.2%), with grazing rates increasing with the phytoplankton biomass. The results suggest that heterotrophic protists were the major consumers of primary production, and that their grazing is one of the most important losses affecting the phytoplankton biomass in the northern ECS.

Warming induces shifts in microzooplankton phenology and reduces time-lags between phytoplankton and protozoan production

Indoor mesocosm experiments were conducted to test for potential climate change effects on the spring succession of Baltic Sea plankton. Two different temperature (Δ0 °C and Δ6 °C) and three light scenarios (62, 57 and 49 % of the natural surface light intensity on sunny days), mimicking increasing cloudiness as predicted for warmer winters in the Baltic Sea region, were simulated. By combining experimental and modeling approaches, we were able to test for a potential dietary mismatch between phytoplankton and zooplankton. Two general predator–prey models, one representing the community as a tri-trophic food chain and one as a 5-guild food web were applied to test for the consequences of different temperature sensitivities of heterotrophic components of the plankton. During the experiments, we observed reduced time-lags between the peaks of phytoplankton and protozoan biomass in response to warming. Microzooplankton peak biomass was reached by 2.5 day °C−1 earlier and occurred almost synchronously with biomass peaks of phytoplankton in the warm mesocosms (Δ6 °C). The peak magnitudes of microzooplankton biomass remained unaffected by temperature, and growth rates of microzooplankton were higher at Δ6 °C (μ∆0 °C = 0.12 day−1 and μ∆6 °C = 0.25 day−1). Furthermore, warming induced a shift in microzooplankton phenology leading to a faster species turnover and a shorter window of microzooplankton occurrence. Moderate differences in the light levels had no significant effect on the time-lags between autotrophic and heterotrophic biomass and on the timing, biomass maxima and growth rate of microzooplankton biomass. Both models predicted reduced time-lags between the biomass peaks of phytoplankton and its predators (both microzooplankton and copepods) with warming. The reduction of time-lags increased with increasing Q10 values of copepods and protozoans in the tritrophic food chain. Indirect trophic effects modified this pattern in the 5-guild food web. Our study shows that instead of a mismatch, warming might lead to a stronger match between protist grazers and their prey altering in turn the transfer of matter and energy toward higher trophic levels.

Dynamics of phytoplankton community structure in the South China Sea in response to the East Asian aerosol input

Abstract. Recent studies have demonstrated atmospheric deposition as an important source of bioreactive compounds to the ocean. The South China Sea (SCS), where aerosol loading is among the highest in the world, however, is poorly studied, particularly on the in situ response of phytoplankton community structures to atmospheric deposition. By conducting a series of microcosm bioassays at different hydrographical locations and simulating different aerosol event scales, we observed both positive and negative responses to the input of East Asian (EA) aerosol with high nitrogen (N) and trace metal contents, in terms of biomass, composition and physiological characteristics of phytoplankton communities. High levels of aerosol loading relieved phytoplankton nitrogen and trace metal limitations in SCS, and thus increased total phytoplankton biomass, enhanced their physiological indicators (e.g. photosynthetic efficiency) and shifted phytoplankton assemblages from being dominated by picoplankton to microphytoplanton, especially diatoms. However, under low levels of aerosol loading, the composition shift and biomass accumulation were not apparent, suggesting that the stimulation effects might be counterbalanced by enhanced grazing mortality indicated by increased abundance of protist grazers. Trace metal toxicity of the aerosols might also be the reason for the reduction of picocyanobacteria when amended with high EA aerosols. The magnitude and duration of the deposition event, as well as the hydrographical and trophic conditions of receiving waters are also important factors when predicting the influence of an aerosol deposition event. Our results demonstrated different responses of phytoplankton and microbial food web dynamics to different scales of atmospheric input events in SCS and highlighted the need for achieving an accurate comprehension of atmospheric nutrient on the biogeochemical cycles of the oceans.

Seasonal Depth-Related Gradients in Virioplankton: Lytic Activity and Comparison with Protistan Grazing Potential in Lake Pavin (France)

This study presents an original depth-related survey of virioplankton lytic activity in relation to prokaryotic production and potential protistan bacterivory in the deep (Z(max) = 92 m) meromictic volcanic Lake Pavin (Massif Central, France). The sampling strategy was designed to be representative of the physico-chemical gradients of the water column of the lake, and of the seasonal variability as well, i.e. 12 different depths sampled in triplicates from April to December 2005. In the space, viral lytic activity estimated from the frequency of visibly infected prokaryotic cells and from burst size over the study period generally decreased with depth. This was viewed as a paradox compared to the abundances of viruses and prokaryotes and to the prokaryotic production which increased with depth. The seasonal variability in viral lytic activity was correlated with prokaryotic variables (abundance and production) in the deepest waters, i.e. from the hypolimnion downwards. Compared to previous studies known from the mixolimnion, we conclude that the deep waters in Lake Pavin represent an exclusive environment for heterotrophic prokaryotes whose seasonal activity offers an optimal and unique resource for thriving viral communities, some of which may be typical, endemic to the ambient dark, cold and stable deep water masses. Overall, the main findings in the present study get well around a previous statement that the ecology of the deepest waters of Lake Pavin is essentially driven by the dark viral loop (dissolved organic matter-prokaryotes-viruses) processes, which can sequester organic matters and nutrients for a long-lived turnover time. This is in agreement with recent demonstrations from marine systems that meso- and bathypelagic waters are optimal environments for viral survival and proliferation.

Grazing rates of protists in wetlands under contrasting light conditions due to floating plants

We examined the effect of light attenuation, due to floating plants, on the community structure of the main phagotrophic protists and their grazing rates in a wetland in the Lower Parana Basin. Ingestion experiments (winter and summer) were conducted at 2 sites in the same shallow lake that had contrasting light scenarios: open waters (light) and under profuse macro- phyte coverage (dark: light attenuation ~97%). We compared the rates at which protists ingested 3 types of tracer prey: fluorescently labelled heterotrophic bacteria (FLB), picocyanobacteria (FLC) and picoeukaryotic algae (FLA). Light influenced both the structure of the microbial com- munities and the protistan grazing rates. Heterotrophic flagellates (HF) were more abundant under the macrophytes, whereas mixotrophic algae (cryptophytes) and autotrophic and heterotro- phic picoplankton populations attained higher abundances in open waters. Specific grazing rates (SGRs) of mixotrophs on heterotrophic bacteria (HB) were higher in the light (7.9 to 15.5 prey cells grazer �1 h �1 ), than in darkness (0.1 to 5.1 prey cells grazer �1 h �1 ); the same trend was observed on picocyanobacteria (Pcy) (1.1 and 0.2 prey cells grazer �1 h �1 , light and dark). SGRs of HF were 1.0 to 7.3 cells grazer �1 h �1 (on HB) and 0.01 to 1.8 prey cells grazer �1 h �1 (on Pcy), with highest values in summer and no pattern in relation to light. SGRs of ciliates were higher in summer and in dark- ness. Clearance rates (CR) on Pcy were higher than on HB, for both HF and mixotrophic algae. In winter, cryptophytes contributed up to 93% of the microbial grazing in the light, whereas HF were more important in darkness; in summer, bacterivory was dominated by heterotrophs in both light scenarios. Our experimental results highlight the importance of light conditions in structuring bacterial grazing by protists.

Rapid successions affect microbial N‐acetyl‐glucosamine uptake patterns during a lacustrine spring phytoplankton bloom

The vernal successions of phytoplankton, heterotrophic nanoflagellates (HNF) and viruses in temperate lakes result in alternating dominance of top-down and bottom-up factors on the bacterial community. This may lead to asynchronous blooms of bacteria with different life strategies and affect the channelling of particular components of the dissolved organic matter (DOM) through microbial food webs. We followed the dynamics of several bacterial populations and of other components of the microbial food web throughout the spring phytoplankton bloom period in a pre-alpine lake, and we assessed bacterial uptake patterns of two constituents of the labile DOM pool (N-acetyl-glucosamine [NAG] and leucine). There was a clear genotypic shift within the bacterial assemblage, from fast growing Cytophaga-Flavobacteria (CF) affiliated with Fluviicola and from Betaproteobacteria (BET) of the Limnohabitans cluster to more grazing resistant AcI Actinobacteria (ACT) and to filamentous morphotypes. This was paralleled by successive blooms of viruses and HNF. We also noted the transient rise of other CF (related to Cyclobacteriaceae and Sphingobacteriaceae) that are not detected by fluorescence in situ hybridization with the general CF probe. Both, the average uptake rates of leucine and the fractions of leucine incorporating bacteria were approximately five to sixfold higher than of NAG. However, the composition of the NAG-active community was much more prone to genotypic successions, in particular of bacteria with different life strategies: While 'opportunistically' growing BET and CF dominated NAG uptake in the initial period ruled by bottom-up factors, ACT constituted the major fraction of NAG active cells during the subsequent phase of high predation pressure. This indicates that some ACT could profit from a substrate that might in parts have originated from the grazing of protists on their bacterial competitors.

Microbial community structure of a slow sand filter schmutzdecke: a phylogenetic snapshot based on rRNA sequence analysis

Slow sand filters (SSF) are widely used to treat water for potable use. The process is dependent on the activities of complex microbial communities in the biofilm (schmutzdecke) layer. In this study, we generated a comprehensive view of three-domain microbiological complexity within a model SSF. DNA was analysed using a high-density microarray (PhyloChip) and rRNA library analysis. The Eukaryotic community was dominated by Cercozoa (Ebridd-type protists); these are likely to be involved in predation of other organisms in the schmutzdecke layer, thus proving opportunity for successional development. Ciliate protozoa, green microalgae, stramenopiles, amoeboid protozoa and fungi in the Phylum Ascomycota and the deep-branching Chytridiomycota were also detected. The Archaeal community was dominated by Euryarchaeota, and most were Halobacteriales. These organisms may contribute to filter function through removal of dissolved organic carbon, a primary treatment goal of these filters. Given that the Eukaryotic and Archaeal communities were examined using clone libraries, the expected richness of taxa present is expected to be greater than detected here and dependent on sampling effort (library size). The bacterial community was rich in taxa (21 Phyla) but was not dominated by any phylogenetic group. The successful function of SSF's relies on interactions between taxa (e.g. grazing of Cercozoa protists on bacteria), and removal of dissolved organic matter from the influent. Understanding the taxa and functions in these systems will aid in monitoring and managing SSF for optimal water quality.

Species-specific patterns in the vulnerability of ­carbon-starved bacteria to protist grazing

Many heterotrophic bacteria possess adaptations for prolonged survival under carbon and energy limitation, generally involving a reduction in cell size and an increased resistance to environmen- tal stress factors. In order to reveal whether carbon- starved bacteria also become less vulnerable to protist grazing, we compared the growth of a bac- terivorous nanoflagellate, Cafeteria roenbergensis, on different physiological states of 3 bacterial strains with well-studied starvation responses (Vibrio vulnifi- cus, Photobacterium angustum and Sphingopyxis alaskensis). Protists achieved high growth rates on all 3 bacterial strains when they were provided in a non-starved state. However, for carbon-starved bac- teria, pronounced differences in the re sponse of the flagellates were observed. P. angustum provided sim- ilar protist growth for an equal biomass of non- starved and starved cultures, indicating no change in food quality or grazing resistance for carbon-starved cells, despite smaller cell size. In contrast, starved V. vulnificus did not support protist growth, even result- ing in a strong decrease in flagellate numbers at most concentrations tested; and starved S. alaskensis provided only reduced growth rates. Our results demonstrate that (1) feeding on bacteria of smaller cell size does not necessarily impose energy con- straints on a flagellate grazer, and (2) a pronounced species-specific variability exists in the susceptibility of carbon-starved bacteria to protist grazing.

The Giant Cafeteria roenbergensis Virus That Infects a Widespread Marine Phagocytic Protist Is a New Member of the Fourth Domain of Life

BackgroundA recent work has provided strong arguments in favor of a fourth domain of Life composed of nucleo-cytoplasmic large DNA viruses (NCLDVs). This hypothesis was supported by phylogenetic and phyletic analyses based on a common set of proteins conserved in Eukarya, Archaea, Bacteria, and viruses, and implicated in the functions of information storage and processing. Recently, the genome of a new NCLDV, Cafeteria roenbergensis virus (CroV), was released. The present work aimed to determine if CroV supports the fourth domain of Life hypothesis.MethodsA consensus phylogenetic tree of NCLDVs including CroV was generated from a concatenated alignment of four universal proteins of NCLDVs. Some features of the gene complement of CroV and its distribution along the genome were further analyzed. Phylogenetic and phyletic analyses were performed using the previously identified common set of informational genes present in Eukarya, Archaea, Bacteria, and NCLDVs, including CroV.FindingsPhylogenetic reconstructions indicated that CroV is clearly related to the Mimiviridae family. The comparison between the gene repertoires of CroV and Mimivirus showed similarities regarding the gene contents and genome organization. In addition, the phyletic clustering based on the comparison of informational gene repertoire between Eukarya, Archaea, Bacteria, and NCLDVs unambiguously classified CroV with other NCLDVs and clearly included it in a fourth domain of Life. Taken together, these data suggest that Mimiviridae, including CroV, may have inherited a common gene content probably acquired from a common Mimiviridae ancestor.ConclusionsThis further analysis of the gene repertoire of CroV consolidated the fourth domain of Life hypothesis and contributed to outline a functional pan-genome for giant viruses infecting phagocytic protistan grazers.

Grazing impact of heterotrophic dinoflagellates and ciliates on common red-tide euglenophyte Eutreptiella gymnastica in Masan Bay, Korea

The euglenophyte Eutreptiella gymnastica is a common red tide causative species. However, there have been no studies on the grazing impact of heterotrophic protists on this species. To investigate the grazing impact of heterotrophic protists on E. gymnastica, we measured daily the abundances of E. gymnastica and co-occurring potential heterotrophic protistan grazers in Masan Bay, Korea, in August 2004 when an E. gymnastica red tide occurred. In addition, we tested whether the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pfiesteria piscicida, Polykrikos kofoidii, Protoperidinium bipes, and Stoeckeria algicida and the naked ciliates Strobilidium sp. (30–40 μm in cell length) and Strombidinopsis sp. (70–100 μm in cell length) were able to feed on E. gymnastica. We also measured their growth and ingestion rates on E. gymnastica as a function of prey concentration. Finally, we calculated the grazing coefficients by combining field data on the abundance of the heterotrophic dinoflagellate and ciliate grazers and co-occurring E. gymnastica with laboratory data on ingestion rates obtained in this study. The maximum abundance of E. gymnastica in Masan Bay in August, 2004 was 7575 cells ml −1, while those of Gyrodinium spp., P. kofoidii, P. bipes, the naked ciliates (≤50 μm in cell length), and naked ciliates (>50 μm in cell length) were 50, 9, 58, 32, and 3 cells ml −1, respectively. The maximum growth rate of G. dominans on E. gymnastica (1.13 d −1) was higher than that of O. marina (0.81 d −1) or P. bipes (0.77 d −1). However, E. gymnastica did not support positive growth of P. kofoidii, Strobilidium sp., and Strombidinopsis sp. (−0.04 ∼ −2.8 d −1). The maximum ingestion rates of G. dominans, P. kofoidii, P. bipes, O. marina, and Strobilidium sp. on E. gymnastica (2.1–2.7 ng C predator −1 d −1) were similar, but they were much lower than that of Strombidinopsis sp. (156 ng C predator −1 d −1). The calculated grazing coefficients for P. bipes, small heterotrophic Gyrodinium spp. (25–35 μm in cell length), naked ciliates (≤50 μm in cell length), P. kofoidii, and naked ciliates (>50 μm in cell length) on E. gymnastica were up to 0.77, 0.61, 0.22, 0.07 and 0.03 d −1, respectively (i.e., up to 54%, 46%, 20%, 7%, and 3% of E. gymnastica populations were removed by the population of each of these heterotrophic protistan grazers in 1 d, respectively). The results of the present study suggest that P. bipes, small heterotrophic Gyrodinium spp., and naked ciliates (≤50 μm in cell length) sometimes have considerable potential grazing impact on the populations of E. gymnastica.

Variability in Protist Grazing and Growth on Different Marine Synechococcus Isolates

Grazing mortality of the marine phytoplankton Synechococcus is dominated by planktonic protists, yet rates of consumption and factors regulating grazer-Synechococcus interactions are poorly understood. One aspect of predator-prey interactions for which little is known are the mechanisms by which Synechococcus avoids or resists predation and, in turn, how this relates to the ability of Synechococcus to support growth of protist grazer populations. Grazing experiments conducted with the raptorial dinoflagellate Oxyrrhis marina and phylogenetically diverse Synechococcus isolates (strains WH8102, CC9605, CC9311, and CC9902) revealed marked differences in grazing rates-specifically that WH8102 was grazed at significantly lower rates than all other isolates. Additional experiments using the heterotrophic nanoflagellate Goniomonas pacifica and the filter-feeding tintinnid ciliate Eutintinnis sp. revealed that this pattern in grazing susceptibility among the isolates transcended feeding guilds and grazer taxon. Synechococcus cell size, elemental ratios, and motility were not able to explain differences in grazing rates, indicating that other features play a primary role in grazing resistance. Growth of heterotrophic protists was poorly coupled to prey ingestion and was influenced by the strain of Synechococcus being consumed. Although Synechococcus was generally a poor-quality food source, it tended to support higher growth and survival of G. pacifica and O. marina relative to Eutintinnis sp., indicating that suitability of Synechococcus varies among grazer taxa and may be a more suitable food source for the smaller protist grazers. This work has developed tractable model systems for further studies of grazer-Synechococcus interactions in marine microbial food webs.

Decoupled phytoplankton growth and microzooplankton grazing in the deep euphotic zone of the eastern equatorial Pacific

We conducted dilution depth-profile experiments in the eastern equatorial Pacific (EEP) to define regional characteristics of phytoplankton growth and microzooplankton grazing and to test the hypothesis that the process rates decouple in the deep euphotic zone where growth is negligible. We used an abbreviated 2-treatment dilution protocol to produce daily profiles at 8 depths of phytoplankton growth, microzooplankton grazing and cellular changes in chlorophyll a (chl a) content from surface waters to the 0.1% light depth. Experiments were conducted at 16 stations from 2°N to 4°S at 110°W and from 110° to 140°W along the equator. Results were surprisingly robust and coherent over this broad spatial area and showed a euphotic zone essentially divided into 3 equal depth intervals. Mean (±SD) growth rates (0.83 ± 0.16 d–1) exceeded grazing rates (0.42 ± 0.15 d–1) in the light-saturated upper third of the water column. Growth, and to a lesser extent grazing, declined with light in the middle third. Effective cell growth was negligible (0.02 ± 0.21 d–1) in the lower third (1 to 0.1% of surface irradiance), with grazing (0.18 ± 0.17 d–1) exceeding growth in this layer. The deep euphotic zone accounted for 25.4 ± 8.4% of the total euphotic zone chl a, 0.5 ± 7.8% of depth-integrated phytoplankton growth and 12.7 ± 7.2% of depth-integrated microzooplankton grazing on phytoplankton. The decoupling of growth and grazing processes under low light conditions at the base of the euphotic zone substantially affected our estimates of microzooplankton consumption of phytoplankton, which ranged from 51% of daily chlorophyll growth for experiments conducted in the upper euphotic zone to 69% for the depth-integrated euphotic zone. In addition, the excess of grazing over growth processes in the deepest stratum, which is typically overlooked in experimental studies, suggests that protistan grazers may have a much larger role in biogeochemical transformations of export fluxes than previously appreciated.