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ABSTRACTPicocyanobacteria in lakes generally occur as single cells (single-celled picocyanobacteria; SPcy) or colonies (colonial picocyanobacteria; CPcy), and the latter form has been considered an adaptation to grazing pressure. In addition to direct effects of grazing, grazers may also have important indirect effects on picocyanobacteria, such as those from nutrient regeneration and trophic cascades. Interactions between picocyanobacteria and their grazers in lakes can thus be complex and difficult to predict. To evaluate the influence of various grazers on SPcy and CPcy in Lake Biwa, Japan, we followed seasonal changes in their abundances and potential grazers at 2-week intervals over 2 years. The data collected were analyzed using empirical dynamic modeling (EDM), a model-free, nonlinear time-series method. We found that heterotrophic nanoflagellates (HNF), rotifers (Keratella, Polyarthra, and Trichocerca), cladocerans, and copepods played important and differing roles in controlling the abundances of SPcy and CPcy. Notably, HNF had an apparent positive influence on SPcy abundance, despite being considered major consumers of SPcy. This result suggested that the enhancement of SPcy growth due to nutrient regeneration by HNF might exceed losses from mortality due to grazing by HNF. EDM also suggested that colony formation by picocyanobacteria may be unidirectional, with SPcy tending to form CPcy. Our findings show that the seasonal dynamics of SPcy and CPcy in Lake Biwa are influenced by a variety of grazers, which may play differing ecological roles in the aquatic food web.

We developed 10 novel simple sequence repeat markers from expressed sequence tags for the honey locust seed beetle, Megabruchidius dorsalisFåhraeus 1839 (Coleoptera: Bruchidae, using de novo transcriptome analysis based on next-generation sequencing. In a M. dorsalisHarataima (Kanagawa-pref.) population, the number of alleles per locus ranged from 2 to 6, with an average of 4.0. Observed and expected heterozygosities ranged from 0.13 to 0.72 and 0.17 to 0.72, respectively. We initially developed 11 novel markers, but one was eliminated because it showed significant linkage disequilibrium with another locus after Bonferroni correction. To check the applicability of the remaining 10 markers, we used them to analyze two additional geographic populations (Yashima, Akita-pref., and Kameoka, Kyoto-pref.). Mean numbers of alleles per locus in the Yashima and Kameoka populations were 2.6 and 2.9, respectively, with corresponding mean observed heterozygosities of 0.36 and 0.52. These results based on the two additional populations confirm that our developed markers worked efficiently. The simple sequence repeat markers developed from expressed sequence tags in the present study should, therefore, be useful for explicating the population genetic structure of M. dorsalisand for future paternal analyses.

The mechanisms for the coexistence of multiple species occupying the same ecological niche are often puzzling. Predator effects on competitively superior species is one possible mechanism. In this study, we tested whether the presence of size-selective predators (fishes) acts as a mechanism for the coexistence of two species of case-bearing caddisfly larvae, Perissoneura paradoxa and Psilotreta kisoensis (Odontoceridae, Trichoptera). The larvae of these two species have similar ecological and life history traits except their body size, and they have been found to coexist only in habitats shared with predatory fishes. Experiments on intra and interspecific competition revealed that the larger Pe. paradoxa always outcompeted the smaller Ps. kisoensis in the absence of predatory fishes, suggesting that Pe. paradoxa performed intra-guild predation on Ps. kisoensis. We also conducted experiments to examine how strongly each of these species responded in terms of case repair with/without a predator chemical cue after their cases were partly dismantled. Perissoneura paradoxa exhibited a stronger case repair response in the presence of a predator chemical cue than that exhibited by Ps. kisoensis, suggesting that Pe. paradoxa is more vulnerable to fish predation, probably because their body size is in the preferred prey range of fishes. We suggest that the presence of predators works in the favor of smaller, subordinate species through size-selective predator effects, enabling these two competitive species to coexist in the same habitat. [ABSTRACT FROM AUTHOR]

The oxygenated hypolimnion accounts for a volumetrically significant part of the global freshwater systems. Previous studies have proposed the presence of hypolimnion-specific bacterioplankton lineages that are distinct from those inhabiting the epilimnion. To date, however, no consensus exists regarding their ubiquity and abundance, which is necessary to evaluate their ecological importance. The present study investigated the bacterioplankton community in the oxygenated hypolimnia of 10 deep freshwater lakes. Despite the broad geochemical characteristics of the lakes, 16S rRNA gene sequencing demonstrated that the communities in the oxygenated hypolimnia were distinct from those in the epilimnia and identified several predominant lineages inhabiting multiple lakes. Catalyzed reporter deposition fluorescence in situ hybridization revealed that abundant hypolimnion-specific lineages, CL500-11 (Chloroflexi), CL500-3, CL500-37, CL500-15 (Planctomycetes) and Marine Group I (Thaumarchaeota), together accounted for 1.5–32.9% of all bacterioplankton in the hypolimnion of the lakes. Furthermore, an analysis of single-nucleotide variation in the partial 16S rRNA gene sequence (oligotyping) suggested the presence of different sub-populations between lakes and water layers among the lineages occurring in the entire water layer (for example, acI-B1 and acI-A7). Collectively, these results provide the first comprehensive overview of the bacterioplankton community in the oxygenated hypolimnion of deep freshwater lakes.

The oxygenated hypolimnion accounts for a volumetrically significant part of the global freshwater systems. Previous studies have proposed the presence of hypolimnion-specific bacterioplankton lineages that are distinct from those inhabiting the epilimnion. To date, however, no consensus exists regarding their ubiquity and abundance, which is necessary to evaluate their ecological importance. The present study investigated the bacterioplankton community in the oxygenated hypolimnia of 10 deep freshwater lakes. Despite the broad geochemical characteristics of the lakes, 16S rRNA gene sequencing demonstrated that the communities in the oxygenated hypolimnia were distinct from those in the epilimnia and identified several predominant lineages inhabiting multiple lakes. Catalyzed reporter deposition fluorescence in situhybridization revealed that abundant hypolimnion-specific lineages, CL500-11 (Chloroflexi), CL500-3, CL500-37, CL500-15 (Planctomycetes) and Marine Group I (Thaumarchaeota), together accounted for 1.5–32.9% of all bacterioplankton in the hypolimnion of the lakes. Furthermore, an analysis of single-nucleotide variation in the partial 16S rRNA gene sequence (oligotyping) suggested the presence of different sub-populations between lakes and water layers among the lineages occurring in the entire water layer (for example, acI-B1 and acI-A7). Collectively, these results provide the first comprehensive overview of the bacterioplankton community in the oxygenated hypolimnion of deep freshwater lakes.

Abstract The presence of pico-sized cyanobacteria (genus Synechococcus) in hypolimnetic waters has been reported, and investigators have suggested that Synechococcuscontribute more to ecological processes in the hypolimnion than previously hypothesized; however, the ecological role of Synechococcusin food webs and/or matter cycling in the hypolimnion remains unknown. To address this issue, we assessed protistan grazing and the virus-mediated mortality of Synechococcusin the oxygenated hypolimnion of a large freshwater lake (Lake Biwa, Japan) during the stratification period. In addition, we compared the carbon flux through mortality of Synechococcusto that of heterotrophic bacteria to evaluate the role of Synechococcusin ecological processes within a hypolimnetic ecosystem. Our results suggest that the biomass of Synechococcusand heterotrophic bacteria in the hypolimnion was removed primarily by protistan grazing. The abundance of Synechococcuswas highest in August, when the average Synechococcus:bacteria carbon biomass and daily grazing loss ratios were 10.8% and 11.0%, respectively. Thus, the Synechococcusbiomass is likely an important seasonal component of the carbon flux in the hypolimnetic microbial loop. Our results provide the first data on carbon flux through the mortality of both Synechococcusand bacteria in a hypolimnetic ecosystem.

Progranulin (PGRN) is a multifunctional growth factor involved in many physiological and pathological processes in the brain such as sexual differentiation, neurogenesis, neuroinflammation, and neurodegeneration. Previously, we showed that PGRN was expressed broadly in the brain and the Purkinje cells in the cerebellum were one of the regions with the highest expression level of PGRN. Thus, in the present study, we investigated the possible roles of PGRN in the cerebellum by comparing wild-type (WT) and PGRN-deficient (KO) mice with immunohistochemical staining for calbindin, a marker of Purkinje cells. The results showed that the density of Purkinje cell dendrites in the molecular layer of the cerebellum was significantly higher in KO mice than in WT mice, although the number of cell bodies was comparable between the genotypes. Subsequently, as the cerebellum is the center of the motor function, we performed a rotarod test and found that KO mice remained on the rotating rod for significantly shorter periods than WT mice. However, KO and WT mice did not differ significantly with respect to the diameter of myofibers in a skeletal muscle. These results suggest that PGRN is involved in the development and/or maturation of neuronal networks comprising Purkinje cells in the cerebellum, which may be a prerequisite to normal motor function.

Seasonal changes of zooplankton communities were monitored at two coastal marine areas, Uchiumi and Fukuura Bays located at the Uwa Sea, east side of Bungo Channel. The two sites are close to each other (c.a. 20 km) with similar habitat environments, but receive different environmental stresses from aquaculture activities, pearl oyster farming for Uchiumi Bay and fish farming for Fukuura Bay. Since both bays have no inflowing river and small populations in the catchment, the impacts of different aquaculture activities on meso- and microzooplankton communities were compared. As results, Fukuura Bay showed higher ammonium and phosphate concentrations, suggesting that fish farming could accelerate eutrophication more seriously. The dominant species of mesozooplankton were similar at the two sampling sites with dominance of small coastal calanoid copepod, Paracalanus spp., but the abundances of larger calanoid species, Calanidae, Eucalanidae, Temoridae and Acartiidae, were markedly higher in Uchiumi Bay. Small cyclopoid copepods, Oithona spp. were abundant at both sites, but the densities varied seasonally, and often more abundant at Fukuura Bay. The abundances of microzooplankton including rotifers and small Oithona copepodids were higher at Fukuura Bay. Results suggest that fish farms accompanying discharge of nutrients from additional food supply for fish may seriously modify zooplankton community structure by decreasing the abundance of mesozooplankton through accelerating the eutrophication of coastal waters.

Abstract: Nutrient limitation of the primary production of phytoplankton at some stations in southern and central Lake Baikal was studied by nutrient enrichment experiments in August 2002. Chlorophyll (Chl.) a concentrations ranged from 0.7 to 5.8μgl−1. Inorganic nutrient concentrations were low: soluble reactive phosphorus ranged from 0.05 to 0.20μmoll−1, ammonia from 0.21 to 0.41μmoll−1, and nitrite plus nitrate from 0.33 to 0.37μmoll−1. In the five enrichment experiments, phosphate spikes and phosphate plus nitrate spikes always stimulated primary production. Nitrate spikes also stimulated primary production in four of the experiments. Significant differences were detected between the controls and phosphate spikes and between the controls and phosphate plus nitrate spikes. Thus, the first limiting nutrient is thought to be phosphorus, but once phosphorus is supplied to the surface water, the limiting nutrient will quickly shift from phosphorus to nitrogen.

Abstract: We determined vertical changes in current velocity and dissolved oxygen concentration in biofilms on artificial and natural substrates using microelectrodes. We used biofilms developed on glass slides dipped in an artificial stream for 3 months, artificial clay tiles dipped in an outdoor artificial stream for 3 months, and natural pebbles. In the biofilm on a glass slide, current velocity significantly decreased from the surface of the biofilm and became 0 cm s−1 at the surface of the glass slide. Vertical profile of current velocity versus depth indicated a presence of a viscous sublayer of 0.2-mm thickness above the surface of glass slide. Dissolved oxygen (DO) concentration increased within the biofilm and attained the maximum (123%) at the surface of the glass slide, indicating active photosynthesis by sessile diatoms at the layer corresponding to the observed viscous sublayer. In the biofilm on an artificial tile, DO increased to 163% saturation at 24.849-mm depth, followed by rapid decrease (6%) at the surface of the tile. A similar result of remarkable decrease in DO saturation was also found in the biofilm on a natural pebble. These results suggest that smoothness of the substrate surface is related to the vertical profile of DO saturation. The thickness of the viscous sublayer and oxygen-depleted area (up to several hundred micrometers, μm) was sufficient for the presence of bacteria, protists, and other metazoan animals, suggesting high activity and diversity of those heterotrophs in the bottom part of biofilms.

We estimated the food sources of macroinvertebrates using carbon, nitrogen, and sulfur stable isotopes in a headwater stream. Stream food webs, including macroinvertebrates, rely on production from autochthonous and allochthonous photosynthesis. We found that a freshwater grazer, the snail Semisulcospira libertina, may assimilate different food sources, based on stable carbon and nitrogen isotope evidence from snail muscle and the much lighter sulfur isotope signature than those of other associated macroinvertebrates. Previous studies have shown the importance of methanotrophic and sulfur bacteria in reductive environments (clay and organic-rich sediments) as food sources for macroinvertebrates. Our results show that the production by chemoautotrophic bacteria contributes to the food sources of a snail in a stream. Thus, the chemoautotrophic bacteria are important in the freshwater food webs, even in mostly aerobic habitats. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract: The abundance, growth, and grazing loss rates of picophytoplankton were investigated in August 2002 in Barguzin Bay, Lake Baikal. Water samples for incubation were taken once at a near-shore station and twice at an offshore station. Contributions of picophytoplankton to total phytoplankton were high (56.9–83.9%) at the offshore station and low (5.8–6.8%) at the near-shore station. The picophytoplankton community in the offshore station comprised mainly phycoerythrin (PE)-rich cyanobacteria, with eukaryotic picophytoplankton being less abundant. In contrast, as well as PE-rich cyanobacteria and eukaryotic picophytoplankton, phycocyanin (PC)-rich cyanobacteria were found in the near-shore station. At the offshore station, growth and grazing loss rates on 25 August were 0.56 and 0.43 day−1, respectively, and on 29 August, 0.69 and 0.83 day−1, respectively. At the near-shore station, growth and grazing loss rates were 1.61 and 0.70 day−1, respectively. These results show that there is a difference in the abundance, composition, and ecological role in the microbial food web of picophytoplankton between the near-shore and the offshore areas in Barguzin Bay.

Abstract We examined the vertical abundance of bacteria, phytoplankton and protists along a transect of six stations from near-shore (Stn. 1) to off-shore (Stn. 6) in Barguzin Bay of Lake Baikal, in the summer of 2002. Chlorophyll concentrations at Stn. 1 were higher (>10μg l−1) than at the other five stations (<3μg l−1). Planktonic and sessile diatoms dominated at Stn. 1, while pico-phytoplankon was dominant at other stations. Densities of heterotrophic bacteria were high in both the epilimnion and the thermocline at all stations. Nanoflagellates were abundant in the epilimnion, and ciliates in the thermocline, but no horizontal trend could be found for these heterotrophs. At Stn. 1, not only filter feeding (Strombidium and Strobilidium) and raptorial (Balanion) ciliates but also predatory ciliates (Prorodon and Spathidiosus) dominated, while at other stations only the filter feeding and raptorial ciliates were dominant. In off-shore stations (Stns. 5 and 6), significant correlations were detected between concentrations of chlorophyll a and density of filter feeding or raptorial ciliates, suggesting tight food linkages between phytoplankton and these ciliates. The concentration of dissolved organic carbon (DOC) was significantly correlated with chlorophyll a concentration, although there was no significant correlation between DOC concentration and bacterial density. We suggest that there is a shift of the dominant food linkage from a herbivorous food chain in near-shore areas to a microbial food web in off-shore areas in Barguzin Bay of Lake Baikal.

Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water (bottom intrusion). No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001. These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton (diatoms and dinoflagellates) roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton (APP) cell density, which reached maxima in August of every year at very similar levels (4.0–5.0 × 105cells ml−1). Thus, the seasonal abundance of APP was apparently independent of the occurrence of bottom intrusions. Seasonal changes in cell densities of heterotrophic bacteria showed similar trends to the APP, and temperature-dependent growth of both was indicated. The present study suggests that the matter cycling in the bay varies as a result of shifts in the dominant food linkages, from a microbial food web to a herbivorous food web, due to intermittent nutrient supplies from bottom intrusions.

Seasonal changes in abundance of planktonic microorganisms, together with some physico-chemical variables, were monitored monthly from May 1999 to March 2002 in the surface water of a coastal bay where nutrients are mainly supplied by intermittent intrusions of deeper water (bottom intrusion). No significant bottom intrusion was detected in 1999 but large or frequent bottom intrusions were found from June to October in 2000, and again from mid-June only to late July in 2001. These results indicate that there is a different nutrient supply every year, and peaks in the abundance of dominant eukaryotic phytoplankton (diatoms and dinoflagellates) roughly corresponded to the occurrences of bottom intrusions. By contrast, there was a cyclic seasonal pattern of autotrophic picoplankton (APP) cell density, which reached maxima in August of every year at very similar levels (4.0-5.0 × 105cells ml–1). Thus, the seasonal abundance of APP was apparently independent of the occurrence of bottom intrusions. Seasonal changes in cell densities of heterotrophic bacteria showed similar trends to the APP, and temperature-dependent growth of both was indicated. The present study suggests that the matter cycling in the bay varies as a result of shifts in the dominant food linkages, from a microbial food web to a herbivorous food web, due to intermittent nutrient supplies from bottom intrusions.

Seasonal changes in abundance of the testate amoeba Penardochlamys sp. and its food vacuole contents were investigated in relation to blooms of the cyanobacteria Microcystis spp. in a hypertrophic pond from April 1999 to March 2000. The behavior of the amoeba feeding on M. aeruginosa and M. wesenbergii was also observed in the laboratory. The amoeba was detectable from late May to November 1999 during the blooms of Microcystis spp. Cell densities of the amoeba fluctuated between 1.4 and 350 cells ml −1 with some sporadic peaks, which did not coincide with rapid decreases in the abundance of Microcystis spp. Food vacuoles contained only Microcystis cells; other prey items were not found, suggesting that this amoeba utilized only the cyanobacteria as food. The amoeba was frequently found attached to Microcystis colonies, but was not associated with other suspended particles. Observation of the amoeba feeding revealed the feeding mechanism and that the amoeba was able to graze on both species of Microcystis. These results suggest that the trophic coupling of these organisms is substantial, although grazing by the amoeba is not sufficient to regulate the dynamics of Microcystis populations in this hypertrophic pond.

In microbiological studies in streams, pebble samples have until now been taken out of the water following the conventional method. However, this allows the loss of microorganisms as a result of the removal of overlying water. In the present study, to minimize the loss of microorganisms, we have developed a new sampling method, called the submerged method, for collecting microorganisms living on pebbles in streams. The abundance of microorganisms on natural pebbles and artificial clay tiles, both of which had biofilms developing on their surfaces, was measured using both the conventional and submerged methods and the results from the two methods were compared. The cell densities of bacteria (0.10–14.00 × 10 8 cells cm -2), heterotrophic nanoflagellates (0.36–50.30 × 10 4 cells cm -2), and ciliates (0.071–88.27 × 10 2 cells cm -2) measured by the submerged method tended to be higher than those obtained by the conventional method, although there were only a few cases in which a significant difference existed between microbial abundances determined by the two methods. Also supported by microscopic observation, these results suggest the presence of planktonic and/or weakly attached microorganisms on substrate materials in streams. Significant correlations between the concentration of chlorophyll a and the cell densities of heterotrophic microorganisms and significant correlations among heterotrophic microorganisms suggest the presence of active microbial food webs in streams.

Planktonic microbial interactions in the central basin of Lake Baikal were examined on a summer day in 1999. The subsurface maxima of bacterial abundance and chlorophyll concentration were recorded at the same depth, whereas the vertical distribution of heterotrophic nanoflagellates was the inverse of those of bacteria and picophytoplankton. Release of extracellular organic car-bon (EOC) from phytoplankton was estimated by the NaH 14CO 3 method as 2.4?µg?C?l -1?day -1. Bacterial production (4.3?µg?C?l -1?day -1), estimated in a bottle incubation experiment using size-fractionated water samples, exceeded the EOC released. Thus, other supplying sources of organic matter are needed for the bacterial production. Grazing (2.6?µg?C?l -1?day -1) was also estimated in the experiment and accounted for 60% of the bacterial production. This is the first report on the microbial food web in the central basin of Lake Baikal.

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 0.3×105and 10.1×105cells 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 μ 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, Strobilidiumsp. and Urotrichaspp. The relative contribution of the < 2 μm fraction to total chlorophyll concentration was seasonally high (up to 16.2%), indicating seasonal importance of APP abundance as food for heterotrophs.

In this paper we introduce a new drawing style of a plane graph Gcalled a box-rectangular drawing. It is defined to be a drawing of Gon an integer grid such that every vertex is drawn as a rectangle, called a box, each edge is drawn as either a horizontal line segment or a vertical line segment, and the contour of each face is drawn as a rectangle. We establish a necessary and sufficient condition for the existence of a box-rectangular drawing of G. We also give a linear-time algorithm to find a box-rectangular drawing of Gif it exists.

Many combinatorial problems can be efficiently solved for partialk-trees (graphs of treewidth bounded byk). The edge-coloring problem is one of the well-known combinatorial problems for which no efficient algorithms were previously known, except a polynomial-time algorithm of very high complexity. This paper gives a linear-time sequential algorithm and an optimal parallel algorithm which find an edge-coloring of a given partialk-tree with the minimum number of colors for fixedk.

The carbon: nitrogen: phosphorus (C : N: P) ratio and nutrient regeneration rate of an isolated heterotrophic flagellate fed on four strains of bacteria, with different elemental ratios, were experimentally examined. Depending on the C : N : P ratios of the media, bacterial C : N and C : P molar ratios changed from 2.0 to 5.3 and from 32 to 130, respectively. Elemental contents (fmol per cell) of the flagellate changed widely: 148-708 for carbon, 51-235 for nitrogen and 1.6-9.1 for phosphorus. However, changes in C : N (2.5-5.3) and C : P (69-109) ratios of the flagellate were relatively small, compared with the large variations in its elemental contents. The ammonium regeneration rate of the flagellate was closely related to the ingested bacterial nitrogen (µmol l-1), while its phosphate regeneration rate depended on bacterial C : P ratios. There was no significant relationship between the N : P ratio of the nutrients regenerated by the flagellate and that of the bacteria.

Temporal changes in population densities of algae and concentrations of chlorophyll-a, particulate organic carbon, and both soluble inorganic and particulate organic fractions of Si, N and P, were surveyed in summer and autumn 1991 in the south basin of Lake Biwa, Japan, at three stations different in the nutritional status. Phytoplankton community during the survey period was characterized by overwhelming dominance by a diatom (Melosira (syn. Aulacoseira) (granulata (EHRENB.) RALFS), which was presumably brought about by exceptionally windy weather in this year. Diatom population density and concentrations of biogenic silica and chlorophyll-a were higher in the more eutrophied stations. It seemed unlikely that the diatom population growth was limited by the supply of silicate, except in a short period in October at the station of median nutritional status. Rather, an observation that diatoms tended to decrease in August when dissolved inorganic nitrogen was commonly depleted suggested the diatom population size to have been determined principally by the nitrogen availability. A comparison of relative turnover periods among soluble inorganic N, P and Si supported further that the growth of phytoplankton as a whole was limited by nitrogen supply during August and the first half of September, and by phosphorus supply in the other period. Some potential factors are discussed which would determine the availability of nutrients during the successional passage from summer to autumn in this basin.