Pigmented Nanoflagellates Research Articles

Response of Growth and Grazing Rate of Nanoflagellates on Synechococcus spp. to Experimental Nutrient Enrichment

As important bacterivores in planktonic food webs, mixotrophic nanoflagellates cancause mortality in marine Synechococcus spp. Our previous study found that the pigmented nanoflagellate (PNF) has a significant grazing impact on Synechococcus spp. In the current study, we applied the dilution approach to test the growth and grazing rates of nanoflagellates on Synechococcus spp. We then compared the differences between experimental nutrient additions and in situ conditions in the coastal waters of the East China Sea during the summer season from July to September. The growth rates of Synechococcus spp. in the ambient environment were between 0.54 and 0.62 day−1, which were slightly higher than the 0.56 and 0.66 day−1 with nutrient enrichment in summer. In contrast, our nutrient enrichment experiments produced a marked decline approximately from 21% to 58% in the nanoflagellate grazing rate on Synechococcus spp. The reason was that the mixotrophic PNFs directly used the added nutrients and reduced their supply of nutrients from prey during the incubation experiments.

Role of mixotrophic nanoflagellates in the Eastern Mediterranean microbial food web

In the oligotrophic, phosphorus (P)-limited Eastern Mediterranean Sea (EMS), grazing on heterotrophic bacteria (HB) by pigmented nanoflagellates (PNF) through mixotrophy is a significant source of bacterial mortality and is P-dependent. Heterotrophic nanoflagellates (HNF) are also important consumers of HB. However, there is still no conceptual framework describing the flows of carbon (C) and P through the EMS microbial food web that takes into account the mixotrophic behavior of PNF. In the present modelling study, we explore qualitatively the pathways of C- and P-flow through the HB-mixotrophic PNF-HNF link under varying availability of inorganic P. At low P availability, given the assumption that PNF can fulfill most of their P requirements through HB consumption when P-limited, model simulations show that PNF support bacterial production and P uptake by supplying HB with dissolved organic carbon (DOC), leading to a win-win stable scenario, with PNF holding a key role in P-transfer through the microbial food web. Conversely, increased P availability results in lower PNF grazing upon HB, amplifying the relative contribution of HNF to bacterial grazing. Based on our modelling study we propose a novel conceptual framework for bacteria-nanoflagellate interactions in the EMS, where PNF have the functional role of DOC primary producers and also share with HNF the functional role of bacterial consumers via mixotrophy. This alternative microbial loop operates in a Pdependent way. Our modelling study pinpoints the significance of the adaptive physiology of PNF for the functioning of the oligotrophic ecosystem of the EMS.

Seasonal and Spatial Variation of Pelagic Microbial Food Web Structure in a Semi-Enclosed Temperate Bay

Microbial food web (MFW) in the seawater encompasses the smallest organisms: viruses, autotrophic prokaryotes and heterotrophic prokaryotes (HP), nanoflagellates, eukaryotic phytoplankton and ciliates. For many years, scientists investigated the MFW structure differences in distinct water masses. However, the MFW structure seasonal variation in coastal areas remains poorly documented. In this study, we report on the seasonal and spatial variations of the MFW structure in the temperate Sanggou Bay in four successive seasons, from spring to winter. With a temperature increase from 1.90 to 24.20C, HP biomass increased from 3.77 to 135.77 μg C dm-3, almost covering the whole variation range for the global ocean. The autotrophic (AUTO) components, including Synechococcus (SYN), phototrophic picoeukaryotes (PEUK) and pigmented nanoflagellates (PNF), exhibited biomass variation ranges as large as previously reported. The MFW structure seasonal variation was driven by MFW relative biomasses (biomass ratios of MFW components to HP). With the increase of HP biomass, PNF and PEUK relative biomasses increased more rapidly than those of other groups while that of ciliates slightly decreased. The HETE:AUTO (biomass ratio of heterotroph to autotroph organisms) decreased with temperature, it was 1 in other seasons. Cluster analyses distinguished Inside Bay and Outside Bay on the basis of hydrological characteristics. Consistently, the two subdivisions of Sanggou Bay exhibited different MFW structures as well as distinct tintinnid communities. The main MFW structure difference between Inside and Outside Bay was the biomass ratios of AUTO components to HP. Our results showed that the variations of autotrophic component biomass ratios relative to HP were the main factor responsible for the MFW structure seasonal variation. The spatial difference in MFW structure as well as in tintinnid taxonomic composition between Inside and Outside Bay was linked to the semienclosed nature of the Bay that does not favour efficient mixing with outside Yellow Sea waters.

About Pigmented Nanoflagellates and the Importance of Mixotrophy in a Coastal Upwelling System

Mixotrophy, understood as food ingestion and photosynthesis occurring in the same organism, is a nutrition mode relatively common in marine protists. Among these, pigmented nanoflagellates 2-20 µm in size (PNF) are now known to be responsible for a significant part of consumption of bacteria in the open ocean. However, knowledge about the importance of the mixotrophic nutrition of these organisms in coastal upwelling systems, where autotrophy prevails, is very limited. Here we compile the limited available information about mixotrophy of PNF in coastal upwelling systems, focusing on the NW Iberian upwelling, to show that this type of nutrition is relevant in these productive systems and to urge for further studies. Several indirect approaches allow inferring that mixotrophy was significant for PNF in the NW Iberian upwelling, with heterotrophy supplying seventy-five percent of the total carbon requirements in this plankton group. This new insight has major implications for our view of marine food webs in coastal upwelling regions, and must be taken into account in the construction of more accurate biogeochemical models of the transfer of matter and energy in these marine areas.

Abiotic and biotic factors influencing nanoflagellate abundance and distribution in three different seasons in PRE, South China Sea

Spatial distribution characteristics of two nanoflagellate groups, together with physico-chemical and biological factors, were studied in three seasons in the Pearl River Estuary (PRE), South China Sea. Nanoflagellates were more abundant in warm periods than that in winter. The average abundance in the three observations (spring, summer and winter) was as follow: 1.28 ± 1.17, 0.88 ± 1.02 and 0.28 ± 0.23 × 103cellsml−1 of heterotrophic nanoflagellate (HNF), and 1.26 ± 0.85, 0.89 ± 0.77 and 0.65 ± 0.52 × 103cellsml−1 of pigmented nanoflagellate (PNF). In our three studied seasons, NF density was generally higher in the inner estuary and decreasing to the lowest in the outer estuary. Our results suggested that PNF classes were more sensitive than HNF groups to freshwater discharge. The proportion of PNF gradually increased from spring (49.7%) to winter (67.7%), with the river flow was accordingly decreasing. Moreover, spatial distribution pattern in three seasons showed the response of PNF populations to freshwater input was similar to phytoplankton assemblages in the PRE. Total bacterial and live bacterial abundance (measured by LIVE/DEAD kit) were associated with both two NF components, which implied that NF was a potential predator controlling the bulk abundance of bacteria and proportion of active cells. These results revealed the seasonal and spatial variations of NF abundance in diverse conditions in the PRE and how their response to different ecological processes.

Vertical distribution of pigmented and non-pigmented nanoflagellates in the East China Sea

Nanoflagellates can be separated into two groups according to their trophic mode, i.e. pigmented nanoflagellates (PNF) and heterotrophic nanoflagellates (HNF). However, a newly identified group, mixotrophic nanoflagellates (MNF), are pigmented and show the ability of prey on bacteria. To examine the vertical variations in PNF and HNF abundances, as well as their relationships and the nutritional strategies that they might use, two summer cruises were undertaken in the East China Sea in July 2011 (OR1 966) and July 2012 (OR1 1004). The results show that both HNF and PNF abundances decline with increasing water depth. Vertical variations of abundances are believed to be influenced by prey and light, for HNF and PNF respectively. Over a large part of the sampling area, the ratio of PNF to HNF abundances is about 1:1 in the disphotic and euphotic zones, but exceeds 1.5 in the nutrient-depleted environment along the margin of the continental shelf. The correlation between PNF abundance and bacteria/Synechococcus abundance is positive where PNF/HNF >1.5. However, there is no significant correlation between PNF/HNF abundance when PNF/HNF >1.5 and light/nutrients, indicating that vertical distributions are influenced mainly by prey (bacteria and Synechococcus) in the nutrient-depleted environment. This study assumes that PNF consists mostly of MNF. In the euphotic zone they receive energy from photosynthesis, which is stimulated by the available nutrients from grazing. Their abundance is thus higher than that of HNF. However, in the disphotic zone, both PNF and HNF satisfy their nutrient demands by grazing, and PNF/HNF is close to 1. In other words, mixotrophy might be the main trophic mode for PNF in the nutrient-depleted, oligotrophic environment. Meanwhile, in deeper water (300m), the much lower prey density means that MNF cannot satisfy the basic energy demands of metabolism and photosynthesis, and thus HNF abundance exceeds that of PNF.

Control of nanoflagellate abundance by microzooplankton and viruses in a coastal ecosystem of the subtropical western Pacific

Little is known about the factors controlling variations in nanoflagellate abundance in oceanic environments. In this study, combined size-fractionation with dilution experiments were performed to measure nanoflagellate growth rates, microzooplankton grazing, and virus-mediated mortality collected three times during the summer of 2013. Heterotrophic and pigmented nanoflagellate growth rates varied from 0.39 to 0.49d−1 and 0.39 to 0.52d−1, respectively. Moreover, PNF and HNF loss rates varied from 0.19 to 0.23d−1 and 0.17 to 0.25d−1, respectively. In these cases, a significant impact of microzooplankton grazing was detected, highlighting the important role of microzooplanktons in transferring carbon bound in nanoflagellates to higher trophic levels in this study area. Nanoflagellate net growth rate was not increased in the virus-diluted treatments, suggesting that there was no impact of viral lysis on the mortality of nanoflagellates during summer periods. Furthermore, these results showed a decrease of Synechococcus spp. and bacterial abundance with removal of viruses, and a subsequent decrease in nanoflagellate growth. This result implies that viral infection is an important mechanism in nutrient recycling under oligotrophic oceanic conditions in summer.

Effects of nitrogen and phosphorus on the abundance and cell size of planktonic nanoflagellate communities

AIM: We experimentally investigated the effects of nutrients (Nitrogen and Phosphorus) enrichment on the density, biomass, and cell size of pigmented and heterotrophic plankton nanoflagellates communities. METHODS: The experiment was done in mesocosms in a tropical reservoir during a 19-day period. Four different treatments were carried out: Control (non-nutrient addition - C), phosphorus additions (P), nitrogen addition (N) and phosphorus + nitrogen addition (N + P). Each treatment was performed in triplicate, sorted randomly, thus giving a total of 12 experimental carboys, which were placed transversely in the middle of the reservoir. RESULTS: In general, pigmented and heterotrophic nanoflagellates fractions responded to nutrient addition, increasing densities and biomass values at the fertilized treatments. Opposed to expected, enriched treatments resulted in a slight decrease in mean cell size of the pigmented fraction. Moreover, in nutrient-rich treatments, pigmented nanoflagellates had higher relative abundance than in the control. CONCLUSIONS: Our results indicate that: i) the density and biomass of nanoflagellates responded to the nutrient enrichment, mainly when N and P were added together; ii) the pigmented and heterotrophic fractions showed distinct time responses to fertilization; iii) the growth of nanoflagellate community seems to be co-limited by N and P; iv) the nutrient enrichment led to a greater pigmented than heterotrophic fraction contribution; and v) among the analyzed variables, nanoflagellate densities seem to be more sensitive to changes in nutrient availability than biomass or mean cell size.

Responses of microbial food web to increased allochthonous DOM in an oligotrophic subarctic lake

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 68:171-184 (2013) - DOI: https://doi.org/10.3354/ame01614 Responses of microbial food web to increased allochthonous DOM in an oligotrophic subarctic lake Laura Forsström1,3,*, Toni Roiha1,2, Milla Rautio1,2 1Department of Biological and Environmental Science, 40014 University of Jyväskylä, Finland 2Département des sciences fondamentales and Centre for Northern Studies (CEN), Université du Québec à Chicoutimi, Québec G7H 2B1, Canada 3Present address: Department of Environmental Sciences, 00014 University of Helsinki, Finland *Email: laura.forsstrom@helsinki.fi ABSTRACT: Climate-induced changes in catchment area vegetation and runoff alter the quality and quantity of carbon that enters lakes, with implications for food webs in recipient water bodies. The effect of dissolved organic matter (DOM) on the ratio between heterotrophic and autotrophic biomass and productivity was studied in a subarctic, clear water lake in northern Finland. In a mesocosm experiment, natural DOM from a subarctic bog and a boreal lake was added to the lake water, doubling the initial dissolved organic carbon (DOC) concentration. Optical indices suggested that the subarctic DOM addition was more bioavailable, which was in line with the greater increase in bacterial biomass and production observed in this treatment. Both DOM additions increased the abundance of heterotrophic nanoflagellates (HNF) and decreased primary productivity. They also led to lower ratios of primary to bacterial production, autotrophic to mixotrophic algae and pigmented nanoflagellates (PNF) to HNF relative to the control samples, indicating a shift from a primary production-based food web towards one based on bacterial production. A comparable increase in DOM in the natural environment would lead to a considerable decrease in the euphotic layer and loss of areas available for primary production, resulting in a shift towards a heterotrophic production based food web. KEY WORDS: Dissolved organic matter · Dissolved organic carbon · Subarctic · Microbial food web Full text in pdf format PreviousCite this article as: Forsström L, Roiha T, Rautio M (2013) Responses of microbial food web to increased allochthonous DOM in an oligotrophic subarctic lake. Aquat Microb Ecol 68:171-184. https://doi.org/10.3354/ame01614 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 68, No. 2. Online publication date: March 01, 2013 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2013 Inter-Research.

Possible mixotrophy of pigmented nanoflagellates: Microbial plankton biomass, primary production and phytoplankton growth in the NW Iberian upwelling in spring

Microbial plankton biomass, primary production (PP) and phytoplankton growth rates ( μ) were estimated along the NW Iberian margin during an upwelling relaxation event. Although the interaction between wind forcing and coastline singularities caused high spatial variability in PP (0.4–8.4 g C m −2 d −1), two domains (coastal and oceanic) could be distinguished regarding microbial plankton biomass and μ. At the coastal domain, with higher influence of upwelling, diatoms showed an important contribution (27 ± 17%) to total autotrophic biomass (AB). Nonetheless, AB was dominated by autotrophic nanoflagellates (ANF) at both realms, accounting for 62 ± 16% and 89 ± 6% of the integrated AB at the coastal and oceanic domain respectively. AB and heterotrophic biomass (HB) were significantly higher at the oceanic than at the coastal domain, with both biomasses covarying according to HB:AB = 0.33. Whereas the low phytoplankton carbon to chlorophyll a ratio (C ph:chl a = 38 ± 3) and the high μ = 0.54 ± 0.09 d −1 registered at the coastal stations suggest that phytoplankton was not nutrient limited at this domain, the values (C ph:chl a = 157 ± 8; μ = 0.17 ± 0.02 d −1) recorded at the oceanic domain point to severe nutrient limitation. However, the high Fv/Fm fluorescence ratios (0.56 ± 0.09) measured at the sea surface in the oceanic domain suggest that nutrient limitation did not occur. To reconcile these two apparently opposite views, it is suggested the occurrence of mixotrophic nutrition of ANF, with heterotrophic nutrition supplying about 75% of carbon requirements.

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.

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.

The impact of the Changjiang River plume extension on the nanoflagellate community in the East China Sea

Variation in the summer nanoflagellate community on the continental shelf ecosystem of East China Sea (ECS) is closely coupled with environmental variation due to extension of the Changjiang River plume. Spatial patterns of nanoflagellate abundance were studied in June and August 2003, June 2006 and July 2007 over the ECS shelf. The Changjiang River plume was smaller during the August 2003 and July 2007 cruises than during the rest other 2 cruises. Total nanoflagellates densities varied between 1 and 120 × 10 2 cells ml −1 with the highest abundances occurring within the Changjiang River plume during large plume periods. In the small plume periods, the range of nanoflagellates abundance was 3–33 × 10 2 cells ml −1 and the highest abundances were observed during these periods either within the Changjiang River plume or the Yellow Sea Coastal Water (YSCW). During large plume periods, nanoflagellate abundance closely related to changes in salinity and during the small period, abundance was most related to water temperature. The pigmented nanoflagellate community (PNF) within Changjiang River plume, especially in the <3 μm size class, appears to increase in response to terrestrial or anthropogenic inorganic nutrient loading in the discharge of fresh water from the Changjiang River. The PNF abundance pronounced increase caused the variation of nanoflagellate community of ECS in summer. We suggest that the discharge of fresh water from Changjiang River has significant ecological impacts on spatial variations in nanoflagellate community in the ECS.

Trophic coupling between Synechococcus and pigmented nanoflagellates in the coastal waters of Taiwan, western subtropical Pacific

In our attempt to characterize the interaction of trophic coupling between Synechococcus and pigmented nanoflagellates (PNFs), successive size-fraction experiments were performed at a coastal station on the northeast coast of Taiwan from June, 2005 to January, 2006. By estimating the growth rate and grazing rate of Synechococcus in the presence of nanoflagellates of different sizes, we truncated the food web by removing organisms with different body sizes ( 5 µm PNFs should be 1:1 to 2:1.

Diel patterns of grazing by pigmented nanoflagellates on Synechococcus spp. in the coastal ecosystem of subtropical western Pacific

Natural populations of planktonic Synechococcus spp. exhibit diel variations in abundance and frequency of dividing cells (FDC) during the warm seasons (>25°C) in coastal water of the western subtropical Pacific ocean. We hypothesized that differences in grazing rates during the day/dark cycle were a major cause of the observed diel variations in Synechococcus spp. abundance. We used fluorescently labeled particles (FLP) as tracers of feeding on Synechococcus spp. in June and August 2007. Our results showed that FDC of Synechococcus spp. were highest at dusk (50%) and lowest (<10%) between midnight and early morning. Synechococcus spp. had three abundance phases, accrual (I), peak (II), and diminished (III). Moreover, FDC values gradually increased in phase I, declined to the lowest values during phase II, and remained at low levels throughout phase III. Our results strongly indicated that pigmented nanoflagellate (PNF) grazing was the underlying biological factor regulating diel variations in Synechococcus spp. abundance. And, the rate of PNF ingestion peaked during the night on smaller non-dividing cells following a peak of Synechococcus spp. abundance in phase II. These findings provide evidence that diel variations in ingestion rates are affected by non-dividing cells of Synechococcus spp. and imply that the impacts of PNF grazing on Synechococcus spp. is based on food selectivity by size.

Pigmented Nanoflagellates Grazing on Synechococcus: Seasonal Variations and Effect of Flagellate Size in the Coastal Ecosystem of Subtropical Western Pacific

We investigated seasonal variation of grazing impact of the pigmented nanoflagellates (PNF) with different sizes upon Synechococcus in the subtropical western Pacific coastal waters using grazing experiments with fluorescently labeled Synechococcus (FLS). For total PNF, conspicuous seasonal variations of ingestion rates on Synechococcus were found, and a functional response was observed. To further investigate the impact of different size groups, we separated the PNF into four categories (<3, 3-5, 5-10, and >10 microm). Our results indicated that the smallest PNF (<3 microm PNF) did not ingest FLS and was considered autotrophic. PNF of 3-5 microm in size made up most of the PNF community; however, their ingestion on Synechococcus was too low (0.1-1.9 Syn PNF(-1) h(-1)) to support their growth, and they had to depend on other prey or photosynthesis to survive. The ingestion rate of the 3-5 microm group exhibited no significant seasonal variation; by contrast, the ingestion rates of 5-10 and >10 microm PNFs showed significant seasonal variation. During the warm season, 3-5 microm PNF were responsible for the grazing of 12% of Synechococcus production, 5-10 microm PNF for 48%, and >10 microm PNF for 2%. Taken together, our results demonstrate that the PNF of 3-10 microm consumed most Synechococcus during the warm season and exhibited a significant functional response to the increase in prey concentration.

Pigmented nanoflagellates in the coastal western subtropical Pacific are important grazers on Synechococcus populations

Although the key grazers on Synechococcus and other planktonic marine bacteria are generally thought to be nanoflagellates (both non-pigmented and pigmented) as well as ciliates, we previously found in our western subtropical Pacific coastal study site that ciliates exerted almost no grazing pressure. In this study, we used fluorescently labeled particles (FLP) as Synechococcus-sized mimics to examine the contribution of pigmented (may include autotrophic and mixotrophic spp.) versus non-pigmented (heterotrophic) nanoflagellate grazing to Synechococcus morality. During the warm season from June to September, >50% of the nanoflagellate population was pigmented (1.8-2 x 103 versus 1.2-1.6 x It)3 cells mL -1 ). Consumption, or clearance rates per pigmented nanoflagellate, varied considerably (0.50-46.90 nL cell -1 h -1 ), with the highest rates in June. Raw data showed pigmented nanoflagellate grazing to account for 2-94% (mean 43%) of Synechococcus production from May to October. Pigmented nanoflagellates consumed 12.5-fold more Synechococcus than did ciliates. This study provides the first evidence that pigmented nanoflagellates are key grazers of Synechococcus populations in subtropical western Pacific coastal waters.

Virioplankton and microbial communities in aquatic systems: a seasonal study in two lakes of differing trophy

SUMMARY1. The seasonal and vertical distribution of the abundance of virus‐like particles (VLPs), together with the abundances of other microbial organisms (bacteria, unpigmented and pigmented nanoflagellates and ciliates), were determined in an oligomesotrophic lake (Pavin, France) and in a eutrophic lake (Aydat, France) between March and December 2000.2. The abundance of the viral plankton and those of other microbial taxa were significantly higher in the more productive system. The same was for the virus‐to‐bacteria quotient (VBQ), which averaged seven in Lake Pavin and nine in Lake Aydat.3. The abundance of viruses increased during the period of thermal stratification in both lakes, with the highest values being recorded at the end of summer/early autumn in the epi‐ and the metalimnion. The seasonal pattern of abundance of viruses in both lakes in the surface layer was similar, indicating that the dynamics of viruses may be controlled by environmental factors such as light conditions.4. There was no correlation between the abundance of viruses and protists. We found correlations between viruses and heterotrophic bacteria in the whole water column in Lake Pavin, but only in the dark bottom waters in Lake Aydat.5. Overall, the empirical findings in this study lead us to speculate that the weaker correlation between bacteria and viruses in Lake Aydat than in Lake Pavin, as well as the higher VBQ in the former, is a consequence of the increasing relative abundance of non‐bacteriophage VLPs along the trophic gradient of aquatic systems.

Sequential resuspension of protists by accelerating tidal flow: Implications for community structure in the benthic boundary layer

We measured resuspension thresholds of protists and bacteria at a subtidal coastal site with in situ flumes and by sampling the benthic boundary layer during tidal accelerations. Heterotrophic nanoflagellates, oligotrich ciliates, the diatom Navicula distans, and bacteria resuspended in weak flow (friction velocity u*crit = 0.25–0.80 cm s-1), likely associated with a surficial fluff layer of sediment. Hypotrich ciliates, scuticociliates, and the diatoms N. transitans and Pleurosigma sp. resuspended in moderate flow (u*crit = 0.82–1.3 cm s-1), followed by pigmented nanoflagellates and diatoms of two Nitzschia spp. in strong flow (u*crit &gt;= 1.5 cm s-1). Hypotrichs and scuticociliates resuspended independent of sediment erosion thresholds, whereas most diatoms resuspended with bulk sediment. Differing thresholds may be due to cell size, specific gravity, behavior, or association with particles. As tidal currents accelerated to u* = 1.3 cm s-1, resuspension caused cell concentrations at 5 cm above bottom to increase by 2–16 times, varying among taxa. Community structure shifted accordingly, with total oligotrichs, hypotrichs, and scuticociliates changing from 75% to 96% of the ciliate community and the total diatom taxa listed above changing from 37% to 63% of the pennate cells. Sequential resuspension suggests that the species assemblage entering the water column during a resuspension event depends on the maximal bed shear stress, thus varying with the springneap cycle as well as atmospheric forcing and local hydrography. Flow‐induced fluctuations of community structure may influence microbial food‐web dynamics in the benthic boundary layer and sediment.