Nanoflagellate Grazing Research Articles

Vertical variations of bacterial growth, mortality loss to nanoflagellates, and viruses in the subtropical northwestern Pacific Ocean

Nanoflagellate grazing and viral lysis are the two main causes of mortality losses of marine bacterioplankton. Deciphering the mortality losses across the water column helps us understand their ecological and biogeochemical consequences. In this study, we implemented the two-point modified dilution method consisting of treatment of undiluted and 25% nanoflagellates and/or virus density at the surface (3 m), deep chlorophyll maximum (DCM), and the mesopelagic zone (300 m and 500 m deep) in the subtropical northwestern Pacific Ocean in summer. We found that the bacterial per capita growth (2.3 ± 0.6 d−1) and production (12.4 ± 6.9 μgC L−1 d−1) were significantly higher at the DCM layer than at the mesopelagic zone, possibly because of tight bacteria-phytoplankton coupling and trophic interactions between bacteria, nanoflagellates, and viruses. Further, we found that ∼70% of the bacterial mortality loss can be attributed to nanoflagellate grazing in the DCM layer, while most of the mortality loss in the surface and the mesopelagic zone can be attributed to viral lysis. We argue that while bacterial production is more efficiently transferred to higher trophic levels at the DCM layer, it is predominately recycled in the viral loop on the surface and the mesopelagic zone. Our results reveal the vertical variation of bacterial growth, production, mortality loss to nanoflagellate grazing, and viral lysis, from which we could deduct their depth-dependent impacts on carbon flux in the water column. Our study facilitates the understanding of the impacts of nanoflagellates and viruses on bacterioplankton and the bacteria-mediated biogeochemical cycling.

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.

Is the Dilution Technique Underestimating the Picophytoplankton Growth Measurements?

In oceanic communities, picophytoplankton often dominates phytoplankton biomass and productivity. Diel variations in picophytoplankton abundance and growth have been well documented. In the current study, we used flow cytometry to assess the short-term variations (3 h) of the abundance of the most dominant picophytoplankton, Synechococcus spp. and picoeukaryotes, in the coastal regions of northeastern Taiwan. To explore the change in growth and mortality rate in the daytime and over 24 h incubation, we performed a two-point modified dilution experiment for measuring growth, viral lysis, and nanoflagellate grazing rate. In this study, the growth rates of picoeukaryotes were 0.21 and 0.06 h−1, and those of Synechococcus spp. were 0.15 and 0.06 h−1 for daytime and 24 h incubation, respectively, and the values were higher at significant levels in the daytime than those for 24 h incubation. These growth rate values of picoeukaryote and Synechococcus spp. after incubation for 24 h were approximately underestimated at 71% and 55%, respectively. This finding suggests that estimates based on 24 h sampling may not accurately reflect the true growth rate of these populations on ecologically relevant timescales.

Different impact of nanoflagellate grazing and viral lysis on Synechococcus spp. and picoeukaryotic mortality in coastal waters

Flow cytometry was used to follow the diel variations in abundance of Synechococcus spp. and picoeukaryotes in subtropical western Pacific coastal waters in June 2017. A modified dilution method was also used to estimate diel changes in the effect of nanoflagellate and viruses on Synechococcus spp. and picoeukaryotes. Obvious diel patterns were found in changes in abundance in both Synechococcus spp. and picoeukaryotes, the highest values recorded at night. Synechococcus spp. and picoeukaryotic growth decreased significantly in more highly diluted samples incubated during the daytime, suggesting that picophytoplankton production is partially dependent on grazing- and virus-mediated nutrient cycling during the daytime. At nighttime, nanoflagellate grazing and viral lysis were responsible for Synechococcus spp. mortality at rates of 0.02 h−1 and 0.04 h−1, respectively, and nanoflagellate grazing was responsible for all picoeukaryotic mortality. The most important finding of this study was that nanoflagellate grazing exerts great control over picoeukaryotes at night in these subtropical western Pacific coastal waters.

Contribution of Viral Lysis and Nanoflagellate Grazing to Bacterial Mortality at Surface Waters and Deeper Depths in the Coastal Ecosystem of Subtropical Western Pacific

Since nanoflagellate grazing and viral lysis differ in their impact on the microbial food web, it is important to assess their relative importance on bacterial mortality fractions. This study investigated the abundance of microbial assemblages and the loss of bacteria due to viral lysis and nanoflagellate grazing at two different depths (3 and 135 m) in the coastal waters of subtropical western Pacific during summer (July, August, and September 2012). The bacterial growth rates in the surface waters varied from 0.07 to 0.09 h−1, a range higher than those found in the deep waters (0.013 to 0.016 h−1). The grazing rates of nanoflagellates on bacteria ranged from 0.051 to 0.058 h−1 and from 0 to 0.004 h−1 in the surface and deep waters, respectively. During the study period, nanoflagellate grazing was responsible for most of bacterial mortality (81–87 %) relative to viral lysis in the surface waters, while viral lysis was responsible for 67–75 % of the total bacterial mortality in the deep waters.

The effect of grazing and viral lysis on the diel variations of Synechococcus spp. abundance in the East China Sea

Diel variations in the nanoflagellate grazing and viral-mediated mortality of Synechococcus spp. were simultaneously estimated using a dilution and size-fractionation approach in the inner (I-1 and I-2) and outer regions (O-1, O2 and O-3) of the Changjiang River plume in the East China Sea during summer 2014. Synechococcus spp. abundance generally tended to increase during the dark period, followed by a plateau until midnight at all sampling stations. Overall, gross growth rate of Synechococcus spp. ranged from 0.069 h−1 to 0.122 h−1 during the growth phase. Microzooplankton, nanoflagellate grazing, and viral lysis had no effect on the Synechococcus spp. abundance during this phase. Moreover, nanoflagellate grazing was the largest cause of Synechococcus spp. mortality during the loss phase at nighttime. Compared to the predators, viruses exerted only a minor impact on mortality at St. I-1, where we detected some of the effect that this community had on Synechococcus spp.. Little is known about the impact of nanoflagellates and viruses on the short-term dynamics of Synechococcus spp. in the East China Sea. Therefore, this study's characterization of the relative importance of nanoflagellates and viruses may help provide a better understanding of trophic structures and the energy flow within the microbial loop.

Viral lysis and nanoflagellate grazing on prokaryotes: effects of short-term warming in a coastal subtropical marine system

Although temperature is a key parameter in the control of the activity and growth of all microorganisms, little is known about how temperature might simultaneously affect viral lysis and nanoflagellate grazing on prokaryotes. This study investigated the effect of rising temperatures on nanoflagellate grazing, viral lysis, and prokaryotic growth in coastal waters of the western subtropical Pacific Ocean during winter ( 25°C) in 2013. To do this, water samples were incubated for 24 h at ambient and experimentally increased temperatures (averages 2–3°C higher than in situ values) and analyzed for prokaryotic gross growth, grazing, and viral lysis using a modified dilution method. The experimental warming conditions were found to cause an increase in prokaryotic gross growth rates in the winter and summer season, respectively. Although the experimental warming conditions also caused an increase in the average rate at which prokaryotes were being grazed, they also caused a general decrease in viral lysis during summer. These findings suggest that short-term warming influenced the function of the microbial food web by increasing the rates of prokaryotic production and flux in prokaryotic carbon to predators.

Viral and grazer regulation of prokaryotic growth efficiency in temperate freshwater pelagic environments.

In aquatic systems, limited data exists on the impact of mortality forces such as viral lysis and flagellate grazing when seeking to explain factors regulating prokaryotic metabolism. We explored the relative influence of top-down factors (viral lysis and heterotrophic nanoflagellate grazing) on prokaryotic mortality and their subsequent impact on their community metabolism in the euphotic zone of 21 temperate freshwater lakes located in the French Massif Central. Prokaryotic growth efficiency (PGE, index of prokaryotic community metabolism) determined from prokaryotic production and respiration measurements varied from 5 to 74% across the lakes. Viral and potential grazer-induced mortality of prokaryotes had contrasting impact on PGE. Potential flagellate grazing was found to enhance PGE whereas viral lysis had antagonistic impacts on PGE. The average PGE value in the grazing and viral lysis dominated lake water samples was 35.4% (±15.2%) and 17.2% (±8.1%), respectively. Selective viral lysis or flagellate grazing on prokaryotes together with the nature of contrasted substrates released through mortality processes can perhaps explain for the observed variation and differences in PGE among the studied lakes. The influences of such specific top-down processes on PGE can have strong implications on the carbon and nutrient fluxes in freshwater pelagic environments.

Estimates of bacterioplankton and Synechococcus spp. mortality from nanoflagellate grazing and viral lysis in the subtropical Danshui River estuary

To better understand picoplankton dynamics in the surface waters of upriver the Danshui River and its estuary, we assessed nanoflagellate-induced and virus-induced mortality of bacteria and Synechococcus spp. during different seasons (October, 2012 and January, April and July, 2013) using a modified dilution technique. Bacteria and viruses were significantly higher in abundance upriver than at the estuary. The distribution of Synechococcus spp. did not follow this spatial pattern. Abundance of Synechococcus spp. was relatively low during the whole sampling period in the upriver region. Furthermore, bacterial mortality resulting from nanoflagellate grazing were generally higher than those resulting from viral lysis in the upriver region, while Synechococcus spp. losses appeared to be mainly due to viral lysis upriver and in the estuary. Our dilution experiments suggested that nanoflagellates largely depend on bacteria as an important energy source there.

The relative importance of viral lysis and nanoflagellate grazing for prokaryote mortality in temperate lakes

Summary We explored the factors that potentially affect viral abundance, viral life strategies (lytic versus lysogenic cycle) and the relative importance of viral lysis and heterotrophic nanoflagellate grazing for prokaryote mortality in the euphotic zone of 19 freshwater lakes in a single geographical region (the French Massif Central). Flow cytometry indicated that viral abundance ranged from 0.5 to 26.3 × 107 viruses mL−1, exceeding prokaryote abundance by c. 16‐fold on average. Multiple regressions revealed that prokaryotic abundance was the best predictor of viral abundance (r2 = 0.74), suggesting that the prokaryotes were the major hosts for viral proliferation in these lakes. Viral life strategies varied with month of sampling (June, July, August 2010). Lytic infection was the prominent mode of viral infection in June and August, whereas lysogenic infection was more evident in July in the majority of sampled lakes. This temporal shift in viral strategy was controlled by the availability of phosphate and chlorophyll concentration via host cell growth, with high concentrations favouring lytic infection and low concentrations favouring lysogenic infection. Such antagonistic interactions between viral life strategies support the hypothesis that lysogeny may represent a survival strategy for viruses in harsh nutrient/host conditions. The dominance of viruses over flagellates, or vice versa, as prokaryotic mortality factors differed between the lakes. By taking into account both forms of mortality, our study indicates that viruses and flagellates are likely to act additively in their effects on prokaryotes by regulating the proportion of the most active members of the prokaryotic community. Such a combined effect of viruses and flagellates may strongly influence the planktonic food web and whole‐ecosystem functioning.

Variable Viral and Grazer Control of Prokaryotic Growth Efficiency in Temperate Freshwater Lakes (French Massif Central)

The effects of viral lysis and heterotrophic nanoflagellate grazing (top down forces) on prokaryotic mortality and their subsequent impact on their metabolism were estimated in the upper euphotic and deeper aphotic depth of 11 freshwater lakes located in the French Massif Central. The standing stocks of viruses (VA) and heterotrophic nanoflagellate (HNF) varied significantly (p < 0.05) with sampled depth. VA was substantially (twofold on an average) and significantly higher (p < 0.03) at the aphotic compared to euphotic depth, whereas the reverse was true (p < 0.02) for HNF. Among the prokaryote subgroup, high nucleic acid content prokaryotes explained for significant variability in the total VA and served as principle host target for viral proliferation. Like standing stocks, flagellate grazing and viral infection rates also followed similar patterns. In the investigated lakes, the mechanism for regulating prokaryotic production varied with sampled depth from grazing control in the euphotic to control due to viral lysis in the aphotic. We also tested the hypothesis of top down control on prokaryotic growth efficiency (PGE, which we used as an index of prokaryotic physiological and energetic status at the community level) at both depths. Overall, among the studied lakes, PGE varied widely (4-51%) with significantly (p < 0.05) lower values in the aphotic (mean = 18 ± 4%) than euphotic depth (mean = 32 ± 9%). Contrasting observations on the top down control of PGE between sampled depths were observed. The presence of grazers was found to stimulate PGE at the euphotic, whereas viruses through their lytic infection had a strong negative impact on PGE at the aphotic depth. Such observed differences in PGE and the mechanism controlling prokaryotic production with depth could eventually have strong implication on carbon and nutrient flux patterns in the studied lakes.

Contribution of viral lysis and nanoflagellate grazing to bacterial mortality in the inner and outer regions of the Changjiang River plume during summer

AN-YI TSAI1*, GWO-CHING GONG1,2,3, ROBERT W. SANDERS4 AND JUN-KAI HUANG1 1 INSTITUTE OF MARINE ENVIRONMENTAL CHEMISTRY AND ECOLOGY, NATIONAL TAIWAN OCEAN UNIVERSITY, KEELUNG 202-24, TAIWAN, REPUBLIC OF CHINA, 2 CENTER OF EXCELLENCE FOR THE OCEANS, NATIONAL TAIWAN OCEAN UNIVERSITY, TAIWAN, REPUBLIC OF CHINA, NATIONAL APPLIED RESEARCH LABORATORIES, TAIWAN OCEAN RESEARCH INSTITUTE, TAIWAN, REPUBLIC OF CHINA AND DEPARTMENT OF BIOLOGY, TEMPLE UNIVERSITY, PHILADELPHIA, PA 19122, USA

Temporal variation of picoplankton in the spring bloom of Yellow Sea, China

Temporal variation of Synechococcus, picoeukaryote and heterotrophic bacteria abundance and depth integrated biomass during three spring blooms in 2007 and 2009 were investigated in the Yellow Sea, China. Synechococcus and picoeukaryote responded differently to different types and course of spring blooms. During the diatom blooms of 2007 and Bloom B20 in 2009, Synechococcus and picoeukaryote abundances decreased sharply during the bloom period. However, during a mixed dinoflagellate and diatom bloom of Bloom B23 in 2009, Synechococcus and picoeukaryote increased in abundance and biomass along the bloom. During all three spring blooms, heterotrophic bacteria biomass had a similar increasing trend. Ciliate and heterotrophic nanoflagellate grazing could be responsible for Synechococcus and picoeukaryote abundance and biomass decrease during the spring blooms.

Seasonal variations of virus- and nanoflagellate-mediated mortality of heterotrophic bacteria in the coastal ecosystem of subtropical western Pacific

Abstract. Since viral lysis and nanoflagellate grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both bacterial mortality factors. In this study, an adapted version of the modified dilution method was applied to simultaneously estimate the impact of both virus and nanoflagellate grazing on the mortality of heterotrophic bacteria. A series of experiments was conducted monthly from April to December 2011 and April to October 2012. The growth rates of bacteria we measured ranged from 0.078 h−1 (April 2011) to 0.42 h−1 (September 2011), indicating that temperature can be important in controlling the seasonal variations of bacterial growth. Furthermore, it appeared that seasonal changes in nanoflagellate grazing and viral lysis could account for 34% to 68% and 13% to 138% of the daily removal of bacterial production, respectively. We suggest that nanoflagellate grazing might play a key role in controlling bacterial biomass and might exceed the impact of viral lysis during the summer period (July to August) because of the higher abundance of nanoflagellates at that time. Viral lysis, on the other hand, was identified as the main cause of bacterial mortality between September and December. Based on these findings in this study, the seasonal variations in bacterial abundance we observed can be explained by a scenario in which both growth rates and loss rates (grazing &amp;amp;plus; viral lysis) influence the dynamics of the bacteria community.

Viral and nanoflagellate control of bacterial production in the East China Sea summer 2011

This study investigated spatial patterns in bacterial, viral, nanoflagellate abundance and the loss of bacterial production due to viral lysis and nanoflagellate grazing across the surface water of East China Sea (ECS) during the summer (1–12 July) of 2011. Abundance of bacteria ranged from 1.3 × 105 to 12.4 × 105 cells mL−1, with the highest value found in surface coastal waters, coinciding with a peak in Chl a concentrations. Spatial variations in bacterial growth rates ranged from 0.029 to 0.071 h−1, the highest growth rate near the river mouth of the Changjiang River. Nanoflagellate grazing was responsible for most of the bacterial mortality, accounting for 59% of total mortality within the plume (salinity <31) and 66% outside the plume (salinity >31) on average. Variation in viral lysis was associated with environmental gradients and bacterial abundance. We found a balance budget between bacterial losses due to both bacterivory and viral infection and production, which suggests strong top–down control of bacteria in the ECS during the summer.

Viral lysis and nanoflagellate grazing as factors controlling diel variations of Synechococcus spp. summer abundance in coastal waters of Taiwan

Diel variations in the protozoan grazing and viral-mediated mortality of Synechococ- cus spp. were simultaneously estimated using a modified dilution method in the coastal waters off Taiwan during 4 cruises in summer 2011. During the study period, the abundance of Synechococ- cus spp. showed a clear diel cycle and ranged from 1.2 × 10 4 cells ml �1 at 08:00 h to 9.8 × 10 4 cells ml �1 at midnight. The frequency of dividing cell (FDC) values were highest between afternoon and dusk and decreased to <10% between 20:00 and 08:00 h. Synechococcus spp. specific growth rates varied during the experimental period from 0.025 to 0.033 h �1 and 0.050 to 0.085 h �1 at day- time and nighttime, respectively. Rates of Synechococcus spp. mortality due to nanoflagellate grazing averaged 0.031 h �1 during daytime and 0.041 h �1 at night. The estimated mortality due to viral lysis was between 0.011 and 0.019 h �1 during daytime and between 0.026 and 0.065 h �1 at night. Nanoflagellate grazing was the dominant cause of Synechococcus spp. mortality during daytime; however, mortality from viral lysis and nanoflagellate grazing was more balanced at night.

Relative Contribution of Viral Lysis and Grazing to Bacterial Mortality in Tropical Coastal Waters of Peninsular Malaysia

We investigated the relative importance of viral lysis and heterotrophic nanoflagellate (HNF) grazing in bacterial mortality as part of a study to understand the microbial loop functioning in tropical waters of Straits of Malacca and South China Sea above the Sunda Shelf. Bacterial abundance ranged from 0.8 × 106 to 2.9 × 106 cells ml-1 and bacterial production ranged from 0.7 × 105 to 4.2 × 105 cells ml-1 hr-1. HNF and viral abundance ranged from 0.6 × 103 to 10.1 × 103 cells ml-1 and 0.3 × 107 to 1.4 × 107 virus particles ml-1, respectively. HNF grazing rates were from 1.1 × 104 to 16.8 × 104 cells ml-1 hr-1, whereas viral lysis rates ranged from 0.7 × 104 to 3.9 × 104 cells ml-1 hr-1. There was predator-prey coupling across the stations with both viral lysis and HNF grazing rates significantly correlated with bacterial production. HNF grazing accounted for 26.1% ± 14.6 of bacterial production, whereas viral lysis was 9.0% ± 3.5 of bacterial production. Bacterial mortality by both HNF and viruses averaged 35.5% ± 13.9, and was outpaced by bacterial production. Other loss factors, such as grazing by ciliates, sedimentation, benthic filter feeders, autolysis, or predatory bacteria, could be important.

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 variation in viral-induced mortality of bacterioplankton in the water column of a large mesotrophic lake (Lake Biwa, Japan)

Viruses are ubiquitous and abundant in aquatic systems, yet knowledge of virus-bacteria interactions in thermally stratified water columns of large lakes is limited. We explored the possible factors that affect viral abundance, infection rate, and the relative importance of viral lysis to heterotrophic nanoflagellate (HNF) grazing as a mortality factor of bacterioplankton in the upper (euphotic zone, 5 m) and deeper (aphotic zone, 50 m) layers of the large mesotrophic Lake Biwa,Japan. Data obtained for a full seasonal cycle indicated that bacterial abundance was the best predictor (r2 = 0.85) of viral abundance (range 1.0 × 1010 to 4.1 × 1010 viruses l-1), yielding an average virus:bacteria ratio of 8.2 ± 1.3 (SD). Variation in the frequency of visibly infected cells (range 1.8 to 4.1%) was largely accounted for by the linear combination of bacterial production and HNF abundance (r2 = 0.81, p < 0.001, n = 24). The percentage of daily bacterial production destroyed by viruses was estimated to be high (52.7 ± 16.2%) in the upper layer during the stratification period, which was on average 3.0-fold greater than the percentage of bacterial production consumed by HNF in that layer. In contrast, the corresponding value in the deeper layer was moderate (13.6 ± 5.2%), being 0.6- fold lower than the percentage of bacterial production consumed by HNF. Our data suggest that carbon and nutrient flux patterns controlled by viruses and HNF vary with depth in thermally stratified water columns of Lake Biwa.

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.