Picoplankton Populations Research Articles

Abundance and microbial diversity from surface to deep water layers over the Rio Grande Rise, South Atlantic

Marine microbes control the flux of matter and energy essential for life in the oceans. Until now, the distribution and diversity of planktonic microorganisms above Fe-Mn crusts have received relatively little attention. Future deep-sea mining is predicted to affect microbial diversity. Here, we studied the ecology of picoplankton among pelagic zones of a Fe-Mn deposit region, at Rio Grande Rise, Southwestern Atlantic Ocean. We investigated microbial community composition using high-throughput sequencing of 16S rRNA genes and their abundance estimated by flow cytometry.The picoplankton populations were more abundant in epi- and mesopelagic waters, corresponding to the Tropical and South Atlantic Central Water masses. Bacterial groups related to heterotrophy, such as Oceanospirillales (Gammapoteobacteria), SAR11 (Alphapoteobacteria), Flavobacteriales (Bacteroida), and Rhodobacterales (Alphapoteobacteria), were the main representatives of the pelagic microbial community. Additionally, we detected abundant assemblages belonging to acetate-oxidizing manganese reducers, i.e., Alteromonas. No differences were observed in microbial community diversity among pelagic zones and water masses. These results provide the first insights into the picoplankton abundance, taxonomy and diversity, and ecological processes in the Rio Grande Rise of the Atlantic Ocean. This may also support draft regulations for deep-sea mining in the region.

Microbial food web dynamics in the oceanic Gulf of Mexico

Abstract Phytoplankton growth and microzooplankton grazing rates were measured in repeated profiles of dilution experiments incubated in situ on a drift array in order to assess microbial production and food web characteristics in the oligotrophic bluefin tuna spawning habitat of the Gulf of Mexico (May peak spawning seasons, 2017–2018). Grazing often exceeded growth with the processes more balanced overall in the surface mixed layer, but biomass accumulated in the mid-euphotic zone. Community production estimates (260–500 mg C m−2 day−1) were low compared to similar open-ocean studies in the Pacific Ocean. Prochlorococcus was a consistent major contributor (113–204 mg C m−2 day−1) to productivity, while diatoms and dinoflagellates (2–10 and 4–13 mg C m−2 day−1, respectively) were consistently low. Prymnesiophytes, the most dynamic component (34–134 mg C m−2 day−1), co-dominated in 2017 experiments. Unexpected imbalances in grazing relative to production were observed for all picoplankton populations (Prochlorococcus, Synechococcus and heterotrophic bacteria), suggesting a trophic cascade in the absence of mesozooplankton predation on large microzooplankton. Study sites with abundant larval tuna had the shallowest deep chlorophyll maxima and significant net positive phytoplankton growth below the mixed layer.

Occurrence of enhanced herbivory in the microbial food web of a tropical estuary during southwest monsoon

Tropical monsoonal estuaries have a significant role in global fishery as they act as rich feeding and breeding grounds for many commercial species. Food web dynamic studies in these coastal ecosystems are mainly focused on the classic food chain which assumes that microbial food webs contribute more towards the recycling of organic carbon and nutrients rather than production. But recently it is observed that phyto-microbial food web which is largely dependent on pico-autotrophs can contribute significantly towards the higher trophic level. Based on these observations we have analysed phyto-microbial pathway of food web in a highly productive tropical monsoonal estuary. We assessed the grazer-prey interaction in the microbial food web of Cochin estuary during spring intermonsoon and southwest monsoon seasons. Our results show that during spring intermonsoon the food web is dependent more on heterotrophic bacteria, but in southwest monsoon the dependency of grazers switches over to autotrophic picoplankton. We also analysed the growth rate, grazing rate and carbon contribution of autotrophic picoplankton experimentally during both seasons and found that autotrophic picoplankton show higher growth rate during southwest monsoon. The higher grazing rate and clearance rate of autotrophic picoplankton recorded during southwest monsoon indicate the efficient consumption of autotrophic picoplankton crop during the season. We could also observe a change in autotrophic picoplankton community structure between the seasons with the dominance of picocyanobacteria during spring intermonsoon and that of picoeukaryotes during southwest monsoon. Thus we infer that high freshwater influx into the estuary during monsoon results in high turbid and low saline (mesohaline to oligohaline) environment which increases the growth rate of low light adapted autotrophic picoplankton populations and in turn enhances herbivory (phyto-microbial pathway) in the microbial food web. Apart from protists, autotrophic picoplankton were found to be widely consumed by many larval forms of mesozooplankton, gastropods, bivalves, rotifers and large sized filter feeders either as aggregates or as unicellular forms. Thus monsoon enhanced herbivory can have a significant role in the growth and survival of larval forms recruited into the estuary during monsoon season and in turn the fishery.

Size dependence of metabolism within marine picoplankton populations

AbstractCell size is broadly applied as a convenient parameterization of ecosystem models and is widely applicable to constrain the activities of organisms spanning large size ranges. However, the size structure of the majority of the marine picoplankton assemblage is narrow and beneath the lower size limit of the empirical allometric relationships established so far (typically >1 μm). We applied a fine‐resolution (0.05 μm increments) size fractionation method to estimate the size dependence of metabolic activities of picoplankton populations in the 0.10–1.00 μm size interval within the surface North Pacific Subtropical Gyre microbial assemblage. Group‐specific carbon retained on each filter was quantified by flow cytometric conversion of light scatter to cellular carbon quotas. Median carbon quotas were 25.7, 22.6, and 5.9 fg C cell−1 for populations of the picocyanobacterium Prochlorococcus, high‐scatter heterotrophs, and low‐scatter heterotrophs, respectively. Carbon‐specific rates of primary production as a function of cell size, using the 14C method, and phosphate transport, using 33P radiotracers, resulted in negative power scalings (b) within populations of Prochlorococcus and heterotrophs of b = −1.3 and b = −1.1, respectively. These findings are in contrast to the positive empirical power scaling comprising the broader and larger prokaryote category (b = 0.7) and point to within‐population variability in cell physiology and metabolism for these important microbial groups.

New to total primary production ratio (f-ratio) in the Bay of Bengal using isotopic composition of suspended particulate organic carbon and nitrogen

Variations in the content and isotopic compositions of carbon and nitrogen in the suspended particulate matter were analysed to identify sources of nutrients and compute f-ratios (ratio of new production to primary production) in the Bay of Bengal (BoB) during the post-monsoon period (November-December 2013). Remote sensing data during our observation period indicated that the study region encompassed meso-scale cyclonic (CE) and anticyclonic eddies (ACE) and was influenced by tropical cyclone Madi between 6 and 12 December 2013. Relatively higher concentrations of nitrate were observed in the CE and in the cyclone-influenced region compared to that in the ACE. Variable concentrations of nutrients controlled the size distributions of phytoplankton. Overall, the picoplankton population dominated, but the contributions of micro- and nano-plankton populations were significantly higher in the CE region because of higher concentration of nutrients. The δ13Csus, C:N molar ratios and C: Chl-a ratios suggested that suspended matter is mainly contributed by in situ processes during the study period. The δ15Nsus of suspended matter suggested that vertical supply of nutrients brought from subsurface water supported new production in the CE and cyclone-influenced regions, whereas regenerated nutrients supported phytoplankton production in the ACE. The f-ratios computed using δ15Nsus and depth of nitracline were high in the CE (0.41 ± 0.14) and cyclone-influenced region (0.54 ± 0.12) and lower in the ACE (0.27 ± 0.06). This study revealed that the meso-scale CEs are more important in enhancing exportable production in the BoB. As more than 30 CEs form annually in the BoB, their impact on basin-wide new production and export flux to the deeper ocean must be evaluated using both observations and high-resolution models.

Influence of the northern Yellow Sea Cold Water Mass on picoplankton distribution around the Zhangzi Island, northern Yellow Sea

Picoplankton distribution around the Zhangzi Island (northern Yellow Sea) was investigated by monthly observation from July 2009 to June 2010. Three picoplankton populations were discriminated by flow cytometry, namely Synechococcus, picoeukaryotes and heterotrophic prokaryotes. In summer (from July to September), the edge of the northern Yellow Sea Cold Water Mass (NYSCWM) resulting from water column stratification was observed. In the NYSCWM, picoplankton (including Synechococcus, picoeukaryotes and heterotrophic prokaryotes) distributed synchronically with extremely high abundance in the thermocline (20 m) in July and August (especially in August), whereas in the bottom zone of the NYSCWM (below 30 m), picoplankton abundance was quite low. Synechococcus, picoeukaryotes and heterotrophic prokaryotes showed similar response to the NYSCWM, indicating they had similar regulating mechanism under the influence of NYSCWM. Whereas in the non-NYSCWM, Synechococcus, picoeukaryotes and heterotrophic prokaryotes exhibited different distribution patterns, suggesting they had different controlling mechanisms. Statistical analysis indicated that temperature, nutrients (NO3– and PO43–) and ciliate were important factors in regulating picoplankton distribution. The results in this study suggested that the physical event NYSCWM, had strong influence on picoplankton distribution around the Zhangzi Island in the northern Yellow Sea.

Influence of agriculture and aquaculture activities on the response of autotrophic picoplankton in Laguna Macapule, Gulf of California (Mexico)

IntroductionThe lagoon is a component of coastal zones, whose populations of autotrophic picoplankton (APP) remain largely unstudied. These lagoons display high-nutrient productivity and additionally may also be subjected to anthropogenic activities. This study selected Laguna Macapule, located on the eastern shore in the mid-region of the Gulf of California, due to the fact that a drainage network servicing the surrounding agricultural region (>230,000 hectares under cultivation) directs irrigation runoff, shrimp farm effluents, and urban wastewater containing large quantities of nutrients to be discharged into this lagoon. We propose to identify the APP’s response to various types of environmental and anthropogenic influence in this highly impacted coastal lagoon. MethodsTwo sites (separated by 2.7 km) were monitored from December 2007 to December 2008. One, located at the entrance to Laguna Macapule (oceanic influence) and the other a discharge canal (eutrophic conditions) inside the lagoon at El Tortugón.ResultsAPP was the numerically dominant phytoplankton fraction (15 × 106 to 620 × 106 cells L−1) with coccoidal cyanobacteria as the dominant fraction throughout the year. Peak levels were reached in spring-early autumn and they were the second largest contributor to biomass. Abundance of APP cells corresponds to the lagoon’s eutrophic status. Maximum numbers and a higher average of APP were recorded at the El Tortugón channel during the warm season (months with SST higher than 24 °C). The general positive relationship of the APP’s annual cycle at both sites as well as a negative relationship with heterotrophic nanoflagellates (HNF) abundance, supports the idea that natural forcing, in particular sea surface temperature (SST) is the predominant influences on APP’s seasonal variability.ConclusionsDistinguishable significant differences in APP abundances and nutrients were recognizable between the two sites. The interplay of these variables contributed to lower densities of APP in winter and high densities in spring-early autumn. N:P = ~4 suggests that spring-early autumn abundance of the APP autotrophic component was sustained by urea from shrimp farm discharge water. Thus, a total nutrient-based approach is likely the most suitable tool for establishing nitrogen limitation of biological production in Laguna Macapule and similarly impacted ecosystems around the world.

Distribution pattern of picoplankton carbon biomass linked to mesoscale dynamics in the southern gulf of Mexico during winter conditions

In order to characterize the carbon biomass spatial distribution of autotrophic and heterotrophic picoplankton populations linked to mesoscale dynamics, an investigation over an extensive open-ocean region of the southern Gulf of Mexico (GM) was conducted. Seawater samples from the mixed layer were collected during wintertime (February–March 2013). Picoplankton populations were counted and sorted using flow cytometry analyses. Carbon biomass was assessed based on in situ cell abundances and conversion factors from the literature. Approximately 46% of the total picoplankton biomass was composed of three autotrophic populations (Prochlorococcus, Synechococcus, and pico-eukaryotes), while 54% consisted of heterotrophic bacteria populations. Prochlorococcus spp. was the most abundant pico-primary producer (>80%), and accounted for more than 60% of the total pico-autotrophic biomass. The distribution patterns of picoplankton biomass were strongly associated with the mesoscale dynamics that modulated the hydrographic conditions of the surface mixed layer. The main features of the carbon distribution pattern were: (1) the deepening of picoplankton biomass to layers closer to the nitracline base in anticyclonic eddies; (2) the shoaling of picoplankton biomass in cyclonic eddies, constraining the autoprokaryote biomasses to the upper layers, as well as accumulating the pico-eukaryote biomass in the cold core of the eddies; and (3) the increase of heterotrophic bacteria biomass in frontal regions between counter-paired anticyclonic and cyclonic eddies. Factors related to nutrient preferences and light conditions may as well have contributed to the distribution pattern of the microbial populations. The findings reveal the great influence of the mesoscale dynamics on the distribution of picoplankton populations within the mixed layer. Moreover, the significance of microbial components (especially Prochlorococcus) in the southern GM during winter conditions was revealed, indicating that they may play an important role in the pelagic food web, and that they may have a substantial impact on the carbon cycle in oligotrophic regions.

Microbial food web components, bulk metabolism, and single-cell physiology of piconeuston in surface microlayers of high-altitude lakes

Sharp boundaries in the physical environment are usually associated with abrupt shifts in organism abundance, activity, and diversity. Aquatic surface microlayers (SML) form a steep gradient between two contrasted environments, the atmosphere and surface waters, where they regulate the gas exchange between both environments. They usually harbor an abundant and active microbial life: the neuston. Few ecosystems are subjected to such a high UVR regime as high altitude lakes during summer. Here, we measured bulk estimates of heterotrophic activity, community structure and single-cell physiological properties by flow cytometry in 19 high-altitude remote Pyrenean lakes and compared the biological processes in the SML with those in the underlying surface waters. Phototrophic picoplankton (PPP) populations, were generally present in high abundances and in those lakes containing PPP populations with phycoerythrin (PE), total PPP abundance was higher at the SML. Heterotrophic nanoflagellates (HNF) were also more abundant in the SML. Bacteria in the SML had lower leucine incorporation rates, lower percentages of “live” cells, and higher numbers of highly-respiring cells, likely resulting in a lower growth efficiency. No simple and direct linear relationships could be found between microbial abundances or activities and environmental variables, but factor analysis revealed that, despite their physical proximity, microbial life in SML and underlying waters was governed by different and independent processes. Overall, we demonstrate that piconeuston in high altitude lakes has specific features different from those of the picoplankton, and that they are highly affected by potential stressful environmental factors, such as high UVR radiation.

BiotecVisions 2013, January

BiotecVisions 2013, January

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.

First studies of pico- and nanoplankton populations by a laser scanning flow cytometer

The innovative laser scanning flow cytometer (LSFC) CLASS, developed at ENEA Research Center of Frascati, merges the ability to perform very fast analyses on a statistically significant number of particles to the capability of characterizing, from a morphological and optical point of view, cells even if they are not fluorescent. In this work two cultured populations of picoplankton and nanoplankton have been analyzed by means of CLASS to evaluate the future applicability of this system as a tool for the early detection of algal blooms. The analyses were performed on a red pigmented Synechococcus sp. and C hlamydomonas reinhardtii by measuring simultaneously the side scattering, scattering in the wide angular range 5–100°, fluorescence in two spectral bands and depolarization of the laser light by every single cell contained in the sample. The results highlighted the ability of CLASS to characterize cell populations in which standard flow cytometers encounter difficulties and in which the use of traditional time-consuming image analyses is often required, such as those characterized by different morphologies in a small size with little or no fluorescence. In particular the system presented was demonstrated to be suitable for the study of composition and changes in a phytoplankton population.

Pelagic food web patterns: do they modulate virus and nanoflagellate effects on picoplankton during the phytoplankton spring bloom?

As agents of mortality, viruses and nanoflagellates impact on picoplankton populations. We examined the differences in interactions between these compartments in two French Atlantic bays. Microbes, considered here as central actors of the planktonic food web, were first monitored seasonally in Arcachon (2005) and Marennes-Oléron (2006) bays. Their dynamics were evaluated to categorize trophic periods using the models of Legendre and Rassoulzadegan as a reference framework. Microbial interactions were then compared through 48 h batch culture experiments performed during the phytoplankton spring bloom, identified as herbivorous in Marennes and multivorous in Arcachon. Marennes was spatially homogeneous compared with Arcachon. The former was potentially more productive, featuring a large number of heterotrophic pathways, while autotrophic mechanisms dominated in Arcachon. A link was found between viruses and phytoplankton in Marennes, suggesting a role of virus in the regulation of autotroph biomass. Moreover, the virus-bacteria relation was weaker in Marennes, with a bacterial lysis potential of 2.6% compared with 39% in Arcachon. The batch experiments (based on size-fractionation and viral enrichment) revealed different microbial interactions that corresponded to the spring-bloom trophic interactions in each bay. In Arcachon, where there is a multivorous web, flagellate predation and viral lysis acted in an opposite way on picophytoplankton. When together they both reduced viral production. Conversely, in Marennes (herbivorous web), flagellates and viruses together increased viral production. Differences in the composition of the bacterial community composition explained the combined flagellate-virus effects on viral production in the two bays.

Macrophyte influence on the structure and productivity of photosynthetic picoplankton in wetlands

We used multiple approaches to analyze photosynthetic picoplankton (PPP) structure and production in a wetland in the Lower Parana Basin (Argentina). A seasonal field survey was combined with an experimental study to analyze PPP variability under different light conditions. Flow cytometry analyses showed differences in PPP structure among the aquatic environments. Three main picoplankton populations were distinguished: phycocyanin-rich picocyanobacteria (Pcy), picoeukaryote (Peuk) and anaerobic anoxygenic photosynthetic bacteria (AnAnPB). The experimental study revealed important changes in PPP structure in relation to the light conditions imposed by floating plants: enclosures exposed to light showed a higher proportion of Peuk and Pcy over AnAnPB; in mesocosms covered by plants, just as in the highly vegetated relict lakes (ROLs), AnAnPB were dominant. Total picophytoplankton abundances varied from 1.7 × 10 4 to 4.6 × 10 5 cells mL -1 in the shallow lakes, and were lower (0.69 × 10 4 to 2.5 × 10 5 cells mL -1 ) in the ROLs. Annual variations in temperature and hydrological conditions influenced the PPP abundances, observing maximum values during the warm dry phase. The photosynthetic rates per unit area of PPP (P APPP ) and algae >3 μm (P A>3 μm ) were measured in the aquatic environments in winter and summer: P A PPP (1.5 to 100 mg C m -2 h -1 ) was lower than P A >3 μm and was directly affected by light, which was limiting under the dense floating plant cover.

Toxicity of copper in natural marine picoplankton populations

Standard microalgae toxicity tests should be able to establish responses in real ecosystems. Natural marine picoplankton samples collected during the months of March, June, August, October 2007 and January 2008, where exposed to 72 h copper toxicity tests. Results analysed by flow cytometry distinguished two groups, with different cytometric characteristics that can match with two of Synechococcus populations. EC(50) values for these two populations resulted low, ranging from 0.62 to 26.28 microg L(-1), this converts copper in a very powerful contaminant and Synechococcus in one of the most sensitive groups of phytoplankton. Differences in EC(50) values for a same population can be related to the month of collection including different initial cellular densities and oceanographic parameters that can affect the picoplankton's tolerance and distribution.

Grazing and virus-induced mortality of microbial populations before and during the onset of annual hypoxia in Lake Erie

Lake Erie is the most productive of the North American Great Lakes and experiences annual periods of hypolimnetic hypoxia with unknown consequences for the microbial food web. We established the abundances and mortality rates of microbes in Lake Erie during thermal stratification and determined how they varied with changes in bottom-water dissolved oxygen concentrations. The microbial plankton community (heterotrophic bacteria, Cyanobacteria, eukaryotic phytoplankton, nanozooplankton, microzooplankton) was quantified in surface and bottom waters along with mea- surements of herbivory and bacterivory rates on eukaryotic and prokaryotic picoplankton and rates of viral lysis of bacteria. High rates of grazing mortality of prokaryotic picoplankton (1.4 ± 0.6 d -1 ) and eukaryotic algae (0.66 ± 0.27 d -1 ) and significant correlations between microzooplankton abundances and all picoplankton populations quantified demonstrated the strong impacts of grazing on Lake Erie picoplanktonic communities. Microbial herbivory accounted for half of total phytoplankton mortality per day. Bacterivory and viral lysis turned over 85% of the heterotrophic bacterial community each day. During the onset of hypolimnetic hypoxia, abundances of ciliates and rotifers decreased signifi- cantly and herbivory was undetectable. Concurrently, bacterivory persisted at rates equal to those found in shallower oxygenated waters, and abundances of heterotrophic nanoflagellates did not change significantly. These results suggest that, during hypoxia events in Lake Erie, herbivory by microzooplankton is disrupted, but bacterivory by heterotrophic nanoflagellates persists. Finally, rates of viral lysis of heterotrophic bacteria were higher in the hypolimnion than in surface waters, suggesting that increased viral lysis may enhance regeneration of organic matter in bottom waters during hypoxic events.

Comparative study of picoplankton biomass and community structure in different provinces from subarctic to subtropical oceans

Picoplankton biomass and community structure in the subtropical and subarctic Pacific Oceans were investigated during November 2003, April-August 2005 and July-August 2005. The sampling covered the subarctic K2 station, the Western North Pacific subtropical gyre (WNPG1 and 2 stations) and the Eastern North Pacific subtropical area (ENP1, 2, 3 and 4 stations). Distinct differences in community structure and autotrophic and heterotrophic picoplankton biomass were observed among the above provinces. In subtropical areas, the picoplankton community comprised Prochlorococcus, Synechococcus, picoeukaryotes and heterotrophic bacteria. While in the subarctic area (K2 station), Prochlorococcus were absent. Prochlorococcus were numerically dominant in the subtropical oceans, their abundance tended to decrease with increasing nutrient levels, which is the opposite of the other picoplankton populations. Although the aerobic anoxygenic phototrophic heterotrophic bacteria (AAPB), accounted for only a small proportion of total heterotrophic bacterial abundance, their potential contribution to carbon export may be important due to their larger cell size and higher cell turnover rates compared with other heterotrophic bacteria. Biomass contribution of the AAPB increased distinctly along the oligotrophic to relatively eutrophic gradient. Vertically, AAPB generally followed the phytoplankton except in the subtropical WNPG. Spatial variability of biomass in the autotrophic picoplankton was distinctly larger than that in the heterotrophic bacteria. Changes in the picoplankton community were more closely associated with latitude while nutrient availability was more important for differences in picoplankton biomass. The biomass of autotrophic picoplankton in the upper mixed layer, and also the depth attenuation, were higher in eutrophic relative to oligotrophic waters. Picoplankton seemed to be an important source of new organic carbon for higher trophic level organisms and for detritus production, especially in the oligotrophic subtropical gyre.

Nanoplanktonic assemblages in the upwelling area off Concepción (∼36°S), central Chile: Abundance, biomass, and grazing potential during the annual cycle

The structure and functioning of nanoplanktonic assemblages in coastal upwelling areas have usually been overlooked in explorations of the productivity of these areas. As part of a multidisciplinary, time-series station in the coastal area off Concepción, seasonal variations (upwelling and non-upwelling) in the abundance and biomass of these assemblages were investigated. Hydrographic measurements and biological samples were taken monthly over a 2-year period (18 August 2004–28 July 2006). Nanoflagellates dominated the total integrated abundance (3–317 × 10 9 cells m −2; 0–80 m). Diatoms and dinoflagellates usually contributed to a lesser degree (<20%) but sporadically made important contributions to the total integrated nanoplankton biomass (0.02–10.6 g C m −2). Most of the nanoplankton was concentrated in surface waters (<30 m) during all the samplings and no seasonal differences in abundance or biomass were found in this layer, although the mean values and dispersions around them were highest during the upwelling period along with maximum integrated (0–80 m) chlorophyll- a values, as total or in the <20 μm fraction. Changes in nanoplankton abundance were significantly but weakly ( r < 0.4) correlated with changes in the hydrographic variables; the highest correlation values were positive for temperature and oxygen, factors that varied with depth and date. The potential grazing rates of heterotrophic nano-predators (flagellates and dinoflagellates) on prokaryotic prey, estimated with a generic model, ranged from 3 to 242 bacterioplankton predator −1 h −1 and from 0.1 to 14 cyanobacteria predator −1 h −1. Our results imply a small impact of seasonal hydrographic variability on the abundance and biomass of nanoplanktonic assemblages and suggest that grazing by nanoheterotrophs might control the prokaryotic picoplankton populations in the upwelling area off Concepción.

Fate of heterotrophic microbes in pelagic habitats: focus on populations.

Major biogeochemical processes in the water columns of lakes and oceans are related to the activities of heterotrophic microbes, e.g., the mineralization of organic carbon from photosynthesis and allochthonous influx or its transport to the higher trophic levels. During the last 15 years, cultivation-independent molecular techniques have substantially contributed to our understanding of the diversity of the microbial communities in different aquatic systems. In parallel, the complexity of aquatic habitats at a microscale has inspired research on the ecophysiological properties of uncultured microorganisms that thrive in a continuum of dissolved to particulate organic matter. One possibility to link these two aspects is to adopt a"Gleasonian" perspective, i.e., to study aquatic microbial assemblages in situ at the population level rather than looking at microbial community structure, diversity, or function as a whole. This review compiles current knowledge about the role and fate of different populations of heterotrophic picoplankton in marine and inland waters. Specifically, we focus on a growing suite of techniques that link the analysis of bacterial identity with growth, morphology, and various physiological activities at the level of single cells. An overview is given of the potential and limitations of methodological approaches, and factors that might control the population sizes of different microbes in pelagic habitats are discussed.

Molecular identification of picoplankton populations in contrasting waters of the Arabian Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 39:145-157 (2005) - doi:10.3354/ame039145 Molecular identification of picoplankton populationsin contrasting waters of the Arabian Sea Bernhard M. Fuchs1,*, Dagmar Woebken1, Mikhail V. Zubkov2, Peter Burkill2, Rudolf Amann1 1Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany2Southampton Oceanography Centre, Waterfront Campus, European Way, Southampton SO14 3ZH, UK *Email: bfuchs@mpi-bremen.de ABSTRACT: The composition of picoplankton in the southern oligotrophic, northern mesotrophic waters and deep oxygen minimum zone (OMZ) of the Arabian Sea was determined by 16S ribosomal RNA gene cloning and fluorescence in situ hybridisation (FISH). It was hypothesised that the composition of the heterotrophic picoplankton would be different in these contrasting waters. To reduce the total diversity, cells were sorted by flow cytometry according to their scatter and DNA content before PCR amplification. The 16S rRNA clone libraries resulting from flow-sorted populations were different and often dominated by a small number of clades. Libraries from the Prochlorococcus-dominated southerly waters were dominated by sequences related to uncultured clusters of SAR11, SAR86 and Actinobacteria (HGC I). From surface waters of the Synechococcus-dominated northern part of the Arabian Sea, mostly sequences related to the uncultured gammaproteobacterial group ‘Svalbard’ and HGC I were retrieved. The clone libraries from the OMZ were also dominated by sequences falling in the clades SAR11 and SAR406, but included sequences related to those of sulfate-reducing (Desulfosarcina, Desulfofrigus) and sulfide-oxidising bacteria (endosymbionts of Riftia and Calyptogena). With a recently developed more sensitive FISH protocol approximately 60% of all DAPI stained cells could be identified by general probes as Bacteria, Cren- or Euryarchaeota in both provinces of the Arabian Sea; 40% remained undetected. On this level and on that of the major phylogenetic groups like Alpha- and Gammaproteobacteria only minor differences were detected by FISH. However, the composition of heterotrophic picoplankton clearly differed for the proteobacterial subgroups SAR86, SAR11 and SAR116. These were more abundant in the oligotrophic waters throughout the water column than in the mesotrophic surface waters and the OMZ. This supports our original hypothesis that the contrasting waters in the Arabian Sea harbor different heterotrophic picoplankton communities. In the future, FISH with a larger set of probes for more narrow phylogenetic groups will enable us to quantify these differences in more detail. KEY WORDS: Phylogenetic composition · Arabian Sea · Oxygen minimum zone · OMZ · Fluorescence in situ hybridisation · Catalyzed reporter deposition (CARD)-FISH · Oligonucleotide probes · Flow cytometry Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 39, No. 2. Online publication date: May 30, 2005 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2005 Inter-Research.