Tests of autotrophic picoplankton as early indicators of nutrient enrichment in an ultra-oligotrophic lake

1. The potential of picocyanobacteria as early indicators of changes in nutrient loading and trophic status was examined in an ultra-oligotrophic lake.
2. The study consisted of in situ, short-term, nutrient-addition bioassays and surveys of picocyanobacterial variables and other indices of trophic state at six sites selected to include a range of localized nutrient loadings to the lake from its catchment. The bioassays and surveys were conducted six times throughout 1 year.
3. Experimental additions of small amounts of ammonium-N and phosphate-P did not stimulate picocyanobacterial growth, and phosphate additions often reduced picocyanobacterial growth rates.
4. A correlation analysis using pooled data from all sites and sampling dates showed that the abundance of aggregated picocyanobacteria was strongly correlated with nutrient concentration and ratios. Other variables, including the concentration of single-cell picocyanobacteria, chlorophyll a and primary production, were poorly correlated with nutrients.
5. The results show that picocyanobacteria in oligotrophic lakes are sensitive to extremely small changes in nutrient availability and that they can respond in complex ways. The mechanisms by which they respond to such changes require further study before they can be used as bio-indicators of nutrient enrichment.     

Effects of UVB radiation on freshwater autotrophic and heterotrophic picoplankton in a subalpine lake

The effects of UVB radiation on the activity of heterotrophic (HPP) and autotrophic (APP) picoplankton (0.2-2 m) and of autotrophic assemblages >2 m has been measured and compared. Under natural UVB irradiance in a large, deep, oligotrophic subalpine lake (Lago Maggiore, 4555N) with mean dissolved organic carbon (DOC) concentrations of 1 mg Cl^-1, the microorganisms of the two size fractions were not significantly photoinhibited in their autotrophic and heterotrophic activities. The vertical attenuation coefficient (Kd) for irradiance at 305 nm ranged from 1.45 to 1.67 during spring and summer. The mixing layer extended to a greater depth than the layer affected by UVB radiation (z1% < zmix), thus enabling the microorganisms living there to photoadapt. As the assimilation numbers of APP and nanoplankton were higher at 2 m depth than near the surface, we suspected that the influence of longer wavelength (UVA, photosynthetically active radiation) could be stronger than UVB in affecting the photosynthetic efficiency of natural populations. The artificial increase in UVB irradiance had a higher detrimental effect on HPP due to their smaller size, less protection and indirect effects through autotrophic cell inhibition. Picocyanobacteria were percentually more affected by UVB than nanoplankton during April due to the presence of diatoms, which are more resistant than other algal groups to solar UVB irradiance. Furthermore, picocyanobacteria had lower assimilation numbers with respect to larger phytoplankton in the quartz tubes during stratification.      

Dynamics of autotrophic picoplankton in Lake Constance

The vertical distribution, biomass concentrations and growthrates of autotrophic picoplankton (APP) were investigated duringthe growing season (March-October) in Lake Constance in differentdepths. Cell numbers determined by epifluorescence microscopyvaried between 1.0 x 10³ and 1.6 times; 10⁵ cells ml^–1depending on season and water depth. Highest concentrationswere recorded above the thermodine in late summer. Numerically,APP consisted almost exclusively of chroococcoid cyanobactena.During lake stratification several peaks of biomass concentrationsoccurred in epilimsietic waters at intervals of 6–8 weeks.In-situ experiments using a dilution technique and dialysisbags revealed that during summer APP population dynamics wereprimarily driven by combined changes of their growth and grazingrates, whereas temperature was less important. Gross growthrates varied between 0.006 and 0.051 h^–1, grazing ratesbetween 0.002 and 0.053 h^–1. On average APP productionwas completely removed by grazing within the microbial community.Ciliates, heterotrophic nanoflagellates and rotifers have beenidentified as the major consumers of APP cells. APP biomassis small compared to larger phytoplankton, ranging from ito5% of total phytoplankton biovolume. Due to its high gross growthrates, which are on the same level as those of free-living pelagicbacteria, APP contributes slightly more to overall primary productionwith maximum percentages of {small tilde}15% in late summer.     

Spatio-temporal niche separation of planktonic Betaproteobacteria in an oligo-mesotrophic lake

We investigated the diversity of planktonic Betaproteobacteria and the seasonal population changes of betaproteobacterial taxa in an oligo-mesotrophic lake (Piburger See, Austria). Focus was put on the vertical distribution of the investigated populations and on differences between their respective cell fractions with apparent amino acid incorporation. On average, 66% of betaproteobacterial cells and 73% of their diversity could be attributed to four clades within three lineages that were further analysed by fluorescence in situ hybridization. The numbers of bacteria from the R-BT subclade of the beta I lineage and from the PnecB subgroup of the beta II lineage were rather constant throughout the water column. In contrast, members of another subgroup of beta II (PnecC) and bacteria related to Methylophilus (beta IV) were particularly numerous in the oxygen-depleted zone. In general, only moderate seasonal changes in abundance were observed in the upper water layers, whereas there was a clear relationship between decreasing oxygen levels and the rise of bacteria from the PnecC and beta IV clades in deeper strata. On average, almost 80% of beta I bacteria, but < 15% of cells from the beta IV clade, showed amino acid incorporation. Our results suggest that the studied populations occupy distinct vertical and ecophysiological niches in Piburger See.     

Meteorological drivers of the dynamics of autotrophic picoplankton

1. The tiny non‐motile autotrophic picoplankton (APP; size range 0.2–2 μm) occur in all types of aquatic habitats and are comprised of prokaryotic as well as eukaryotic taxa. In the Boreal Zone, the majority of lakes have high concentrations of coloured humic substances that can adversely affect lake light climate and cause steep summertime stratification resulting in epilimnetic nutrient depletion. APP are more effective in nutrient and light acquisition than larger phytoplankton and thus should be competitive in humic lakes. 2. Most lacustrine APP studies have been based on short sampling periods, and thus, interannual variation and its drivers are still unclear. We studied APP in the small, boreal, humic Lake Valkea‐Kotinen during five open‐water periods in 2002–06 to determine interannual variation and the importance of meteorological drivers for APP dynamics. 3. Total APP showed a bimodal annual pattern, but the timing and vertical location of the two maxima varied during the study. In general, APP thrived in warm water and the most important abiotic factor controlling APP was stability of the water column (N_s). On average, 82% of APP were found in the epilimnion or metalimnion during summertime stratification. 4. There was niche separation of APP and larger phytoplankton in the lake because, with only one exception, APP maxima occurred separately from the maxima of larger phytoplankton. 5. Two groups, solitary eukaryotic APP and colonial picocyanobacteria (Merismopedia warmingiana), responded differently to the abiotic factors. Solitary APP preferred high water colour and low pH, both of which occurred after heavy rain, whereas colonial APP did not fare well when water colour was high. Our findings suggest that when future climate change‐related processes increase incoming allocthonous organic matter load from the catchment, solitary eukaryotic APP will be favoured.     

Production of new organic carbon and its distribution between autotrophic picoplankton, bacteria, extracellular organic carbon and phytoplankton in an upland lake

• 1 Picoplankton community production (0.2–2μm) was investigated over 3 months, June-September 1991, in Llyn Padarn, a mesotrophic upland lake in north Wales.
• 2 The picoplankton was differentiated into autotrophic algae (<1–3μm) and heterotrophic bacteria (<0.2–1 μm) using differential filtration through a 1 μm pore size Nuclepore filter.
• 3 Efficient separation of these distinct metabolic constituents of picoplankton was obtained. A good correlation (r= 0.81, P < 0.001) was found between physical separation of bacterial and picoalgal cells from fluorescence microscopy and the distribution of heterotrophic metabolic activity between different cell size fractions measured by uptake of ¹⁴C-glucose.
• 4 Picoplankton community production was differentiated into the ‘absolute’ autotrophic production by picoalgae, corrected for overestimation due to retention of bacteria with the picoalgae, and the heterotrophic component, bacterial uptake of ‘extracellular organic carbon’ (EOC), derived from the entire phytoplankton community.
• 5 The heterotrophic contribution to picoplankton community production ranged from 88 to 1%, mean value 55% of total. Autotrophic picoplankton production was dominant in June and July, but in August and September heterotrophic uptake of EOC was the major input to picoplankton community production.
• 6 During the 3 months, the mean contributions to plankton production were autotrophic picoplankton 10.3%, heterotrophic bacterial uptake of EOC 9.7%, EOC in lake water 11.6% and phytoplankton (>3μm) 68.3%.
• 7 Bacteria accounted for about half the picopfankton community production via uptake of EOC. Thus although autotrophic picoplankton were ubiquitous, it is likely that their contribution via primary production to the carbon balance of planktonic environments has been overestimated in previous studies.

     

Seasonal changes in the abundance of autotrophic picoplankton and some environmental factors in hypereutrophic Furuike Pond

Seasonal change in the abundance of autotrophic picoplankton (APP) was investigated once or twice a week in relation to some environmental variables in a hypereutrophic pond, from July 1999 to June 2000. Cell density of APP ranged between 0.3×10⁵ and 10.1×10⁵ cells ml^–1, overlapping the lower range of APP abundances given in the literature for hypereutrophic systems. The pattern of seasonal change in concentration of dissolved inorganic phosphorus (0.3–20.3 mol P l^–1) was similar to that of cell density of APP, suggesting that phosphorus limitation on APP abundance. By contrast, nitrogen limitation seemed unlikely since the pattern of seasonal change in concentration of dissolved inorganic nitrogen was different from that of APP cell density. We could not find any coupled oscillations between APP abundance and heterotrophic nanoflagellates, or between that of APP and ciliates. The dominant ciliate taxa, based on their cell densities, were Cinetochilum margaritaceum, Cyclidium glaucoma, Halteria grandinella, Strobilidium sp. and Urotricha spp. The relative contribution of the <2 m fraction to total chlorophyll concentration was seasonally high (up to 16.2%), indicating seasonal importance of APP abundance as food for heterotrophs.     

Sources of the lipids in the bottom sediments of an English oligo-mesotrophic lake

SUMMARY. 1. The organic geochemistry of Coniston Water, a low productivity lake, has been investigated.
2. The relative inputs of the various known sources to different compound classes suggest terrestrial higher plants as the major source of sedimentary organic matter.
3. Early lipid diagenesis, proceeding in the bottom sediments as a result of microbial activity, produces relatively rapid changes, with shorter chain and unsaturated compounds being preferentially degraded.
4. Free and bound (acid liberated) lipids exhibit significant differences in composition, related to their respective sources and stability towards degradation.
5. The lipid composition of an aquatic higher plant, growing in the margins of the lake, has been determined; comparison with the sedimentary lipids reveal it not to be a significant source.     

Seasonal photosynthetic activity of autotrophic picoplankton in Lake Kinneret, Israel

Autotrophic picoplankton populations in Lake Kinneret are composedof picocyanobacteria and picoeukaryotes. Overall, the ratesof photosynthetic carbon fixed by autotrophic picoplankton duringthis study were low (0.01–1.5 mg Cm^–3 h^–1).The highest chlorophyll photosynthetic activity of the <3µm cell-size fraction was found in spring, when picoeukaryotespredominated and in addition small nanoplankton passed throughthe filters. The maximum cell-specific photosynthetic rate ofcarbon fixation by picocyanobacteria and picoeukaryotes was2.5 and 63 fg C cell^–1 h^–1, respectively. The highestspecific carbon fixation rate of autotrophic picoplankton was11 µg C µg^–1 Chl h^–1 The proportionalcontribution of autotrophic picoplankton to total photosynthesisusually increased with depth. Picocyanobacteria collected fromthe dark, anaerobic hypolimnion were viable and capable of activephotosynthesis when incubated at water depths within the euphoticzone. Maximum rates of photosynthesis (P_max) for picocyanobacteriaranged from 5.4 to 31.4 fg C cell^–1 h^–1 with thehighest values in hypolimnetic samples exposed to irradiance.Photosynthetic efficiency (     

A five-year study of autotrophic winter picoplankton in Lake Balaton,Hungary

Picoeukaryotes dominate the phytoplankton of Lake Balaton—the largest shallow lake in Central Europe—in the winter period. We examined the annual dynamics of picoplankton abundance and composition in the lake in order to establish if the picoeukaryotes merely survive the harsher winter conditions or they are able to grow in the ice-covered lake when the entire phytoplankton is limited by low light and temperature. Lake Balaton has an annual temperature range of 1–29°C, and it is usually frozen between December and February for 30–60 days. In the spring-autumn period phycocyanin and phycoerythrin rich Cyanobacteria are the dominant picoplankters, and picoeukaryotes are negligible. Our five-year study shows the presence of three types of picophytoplankton assemblages in Lake Balaton: (1) Phycoerythrin-rich Cyanobacteria—the dominant summer picoplankters in the mesotrophic lake area; (2) Phycocyanin-rich Cyanobacteria—the most abundant summer picoplankters in the eutrophic lake area and; (3) Picoeukaryotes—the dominant winter picoplankters in the whole lake. The observed winter abundance of picoeukaryotes was high (up to 3 × 10⁵ cells ml⁻¹), their highest biomass (520 μg l⁻¹) exceeded the maximum summer biomass of picocyanobacteria (500 μg l⁻¹). Our results indicate that the winter predominance of picoeukaryotes is a regular phenomenon in Lake Balaton, irrespective of the absence or presence of the ice cover. Picoeukaryotes are able to grow at as low as 1–2°C water temperature, while the total phytoplankton biomass show the lowest annual values in the winter period. In agreement with earlier findings, the contribution of picocyanobacteria to the total phytoplankton biomass in Lake Balaton is inversely related to the total phytoplankton biomass, whereas no such relationship was observable in the case of picoeukaryotes.     

Effects of UV radiation on grazing by two marine heterotrophic nanoflagellates on autotrophic picoplankton

Phytoplankton <4-3 µm in diameter, or autotrophic picoplankton,can constitute the majority of the biomass and productivityof photosynthetic organisms in marine and freshwater systems.Indirect evidence has indicated that mortality of autotrophicpicoplankton occurs principally at night in the open ocean,but continuously in coastal water. The predominant view of thefate of autotrophic picoplankton production in the ocean isthat they are consumed by heterotrophic nanoflagellates. A possiblemechanism to explain these observations is that grazing of heterotrophicnanoflagellates on autotrophic picoplankton is inhibited byultraviolet radiation (W), at least in clear open-ocean environments.A series of laboratory experiments was conducted to examinethe effects of UV radiation on the grazing impact of two heterotrophicnanoflagellates on Synechococcus spp., a commonly occurringgenus of autotrophic picoplankton. The two nanoflagellates usedwere Paraphysomonas bandaieensis and Paraphysomonas imperforata.For both nanoflagellates, there was an inverse relationshipbetween the grazing mortality of Synechococcus and UV irradiance.The grazing mortality of Synechococcus was reduced less withP.imperforata than with P.bandaiensis. In some experiments,the effect of UV on the grazing impact of the nanoflagellatepopulations was caused in part by UV-related reductions in nanoflagellatesurvival. However, UV reduced the grazing impact of nanoflagellatesprimarily by reducing the rates of consumption of Synechococcusby individual nanoflagellates, to a degree directly relatedto UV irradiance. The results suggat that UV radiation may bean important selection factor in clear open-ocean water, andthat in order to predict the effect of increasing UV radiationon marine microbial plankton communities, we must consider interactionsbetween trophic levels as well as effects on single trophiclevels.     

Siliceous protozoan plates in lake sediments

The silicified plates of certain testate amoebae have the necessary characteristics to be useful paleoindicators. These plates are usually abundant and well preserved in sedimentary deposits, where they can be studied using the same preparation techniques developed for other siliceous microfossils. Individual plates can usually be identified to the generic and, in some cases, to the species level. A preliminary survey of plates recorded in the surficial (top centimetre) sediments of 38 Adirondack Mountain (NY) lakes suggests that these testaceans may reflect catchment features (e.g. bog development) more closely than lakewater characteristics. The siliceous, species-specific scales of heliozoans are also shown to preserve in sedimentary profiles.     

Potential effects of abiotic factors on the abundance of autotrophic picoplankton in four boreal lakes

The seasonality of autotrophic picoplankton (APP) in four boreallakes of varying trophic state and light availability was studied.Red-fluorescing cyanobacteria (CyAPP) dominated the APP in threelakes, but in the fourth, most humic lake, Valkea Kotinen, eukaryoticpicoalgal cells (EuAPP) predominated. The most productive, leasthumic, and stratified lake (Ormajärvi) was the only onein which low numbers of orange-fluorescing cyanobacteria werepresent in winter (7 x 10² cells ml^-1) and early spring (1 x10² cells ml^-1) under the ice cover. The numbers of CyAPP didnot correspond to the trophic status of the lakes. The highestaverage and maximal numbers of CyAPP were recorded in shallowand moderately productive Jylisjärvi (6.2 x 10⁵ cells ml^-1)while the lowest number (8 x 10⁴ cells ml^-1) was recorded inthe deepest lake, Pääjärvi, with fairly similarwater quality. Light availability and water temperature seemedto be more important abiotic factors in regulating CyAPP andEuAPP density than nutrient concentrations. In the deepest lakes,light and temperature correlated very strongly with densityof CyAPP in spring, while in the shallowest lake this correlationonly occurred in autumn. Although light and temperature wereoccasionally significantly correlated with EuAPP cell density,no common trend throughout all the seasons (as seen for prokaryoticAPP) was found.     

Measurements of sediment toxicity of autotrophic and heterotrophic picoplankton by epifluorescence microscopy

Sediment toxicity from Toronto (Ontario) and Toledo (Ohio) harbours to autotrophic and heterotrophic picoplankton was evaluated simultaneously using epifluorescent microscopy. The approach is a simple, fast and effective combination of autofluorescence and fluorescence probes - 1-anilino-8-naphthalene sulfonic acid. The procedure is ideally suited for use with sediment slurries since it can differentiate fluorescent biotic material against a background of abiotic sediment particles and detritus.     

Diel abundance and productivity patterns of autotrophic picoplankton in the lower Chesapeake Bay

Diet studies associating phototrophic picoplankton abundanceand productivity to tidal flow and pycnocline formation wereconducted during summer and winter. Higher picoplankton concentrationsand productivity were associated with waters above the pycnoclinein summer, with picoplankton abundance highest during ebb tidewhen phycocyanin enriched Synechococcus sp. were dominant. Inwinter, greater concentrations and productivity occurred inbottom waters and during flood tide, when phycoerythrin enrichedSynechococcus sp. were dominant.     

Photosynthetic efficiency and seasonality of autotrophic picoplankton in Lago Maggiore after its recovery

1. Lago Maggiore is a deep subalpine lake that has regained its previous oligotrophic state during the last decade after going through a process of cultural eutrophication and subsequent restoration. Autotrophic picoplankton (APP) have been studied in Lago Maggiore since 1992, with the primary production of the fraction being measured in 1994 and 1995. In 1998, we began to study the population structure and photosynthetic characteristics of APP after the restoration of the lake. We also compared the seasonal dynamics of APP and nanoplankton biomass and production. In this paper, we show the trend of annual and interannual dynamics of APP from 1993 to 1998. 2. Since 1993 and 1994, APP have almost tripled in numbers (mean values: 1993=44 × 10³ cell mL^–1 1998=123 × 10³ cell mL^–1) and in production. On average, APP produced 16 gC m^–2 year^–1 in 1994 and 41.2 gC m^–2 year^–1 in 1998, accounting for 10 and 20%, respectively, of total phototrophic production. 3. Although nanoplankton dominated in terms of biomass and production in the first 6 months of the year, APP were able to compete successfully with them in periods of P limitation. In September 1998, APP carbon fixation actually surpassed that of nanoplankton, reaching 13 mgC m^–3 h^–1 at 3 m. In a comparison of the daily chlorophyll‐specific photosynthesis rates of nanoplankton and APP, the latter proved to be more efficient in fixing carbon, particularly in summer and autumn. 4. While the spatial distribution of APP abundance and production followed the isotherms of 18 and 20 °C, and was strictly related to water stratification and thermocline appearance, the APP efficiency [mgC (mg chl)^–1Em^–2] had its maximum at the bottom of the euphotic zone, with irradiance in the range 0.5–18 μE m^–2 s^–1. 5. The existence of a significant regression between APP abundance and carbon fixation (P < 0.001, r²=0.92, d.f.=17; APP_prod=10.6 × 10³ cell mL^–1 + 15) enabled us to infer APP production from cell numbers, and supports the view that picocyanobacteria production is closely dependent on its standing crop. 6. Lago Maggiore is a good example of how APP in a stabilised oligotrophic system can, under favourable conditions, increase in numbers and effectively exploit the few available resources. We can conclude that the APP population reacts to the reduction in P‐levels, but only after several years of stable low nutrient conditions.     

Rapid establishment of clonal isolates of freshwater autotrophic picoplankton by single-cell and single-colony sorting

We describe single-cell and single-colony sorting protocols which allowed for rapid establishment of a diverse culture collection of clonal autotrophic picoplankton (APP) isolates originating from oligotrophic and oligo-mesotrophic subalpine lakes. Overall sort recoveries, expressed as the percentage of sorted microwells exhibiting APP growth, ranged from 5% to 17% depending on the type of APP, but the growth success varied greatly (from 0% to 68%) depending on the origin of the sorted sample. We applied two direct sequencing and two denaturing gradient gel electrophoresis (DGGE) protocols to identify and characterize the genetic purity of 21 of our picocyanobacteria cultures, namely, direct sequencing of the 16S rRNA gene and cpcBA-IGS region, and DGGE analyses involving a 194-bp fragment of the internal transcribed spacer (ITS) and a ca. 500-bp fragment of the phycocyanin (PC) operon (cpcBA-IGS, novel protocol described herein). Of those 21 picocyanobacteria cultures obtained by single-cell/single-colony sorting and subsequently characterized genetically/screened for genetic purity, only one culture was composed of multiple picocyanobacterial strains.     

Using frequency of dividing cells in estimating autotrophic picoplankton growth and productivity in the Chesapeake Bay

In situ incubations of natural autotrophic picoplankton populations during a 15 month study were used to test the frequency of dividing cells proceduresin estimating phototrophic picoplankton growth rates. These rates were estimated using dilution experiments and compared to the average frequency of dividing cells over the same time interval. The regression equation of µ = 2.85 × 10^–3 (FDC) + 0.022 was calculated to relate autotrophic picoplankton growth rate and the frequency of dividing cells in this study. The resulting relationship was compared to ¹⁴C-bicarbonate derived growth rates. Productivity estimates using frequency of dividing cells correlated closely to sodium ¹⁴C-bicarbonate results and indicated a range of productivity by autotrophic picoplankton of 55.6% the total phytoplankton primary productivity in July to a January rate of 2.3%. Annual autotrophic picoplankton abundance varied seasonally in the lower Chesapeake Bay ranging from 7.26 × 10⁶ cells 1^–1 in winter to 9.28 × 10⁸ cells 1^–1 during late summer.     

The hypolimnetic protozoan plankton of a eutrophic lake

The seasonal distribution of benthic species in the water column above and below the thermocline in a small eutrophic lake is described. During summer stratification populations of Spirostomum spp, Loxodes spp., Plagiopyla and Deltopylum become established in the plankton on or below the oxycline/thermocline. At shallow sites no migration occurred and populations of the migratory species in the benthos were sparse, with the exception of Plagiopyla which occurred in high densities in the sediment. Two distinct planktonic populations are established during stratification: an epilimnetic community of obligate planktonic ciliates and a hypolimnetic community of benthic migrants.