Shuichi Shikano

Changes of traits in a bacterial population associated with protozoal predation

In an attempt to understand the significance of predation in the evolution of prey species, the ecological and morphological characteristics of bacterial species under predation by a ciliated protozoa,Cyclidium sp., were investigated. Serial transfer at 7 day intervals was applied to the bacterial populations in the presence or absence ofCyclidium. Although cells of the parental bacterial strain are typically short rods up to 1.5 μm long, cells of much greater length, up to 20 μm long (type L) were found in populations exposed to predation fromCyclidium. However, the wildtype, shorter length bacteria persisted even after the appearance of type L. Type L was not observed in the singl bacterial culture throughout the serial transfers. Type L appeared to improve the ability to escape predation by elongating cell size, but growth rate and saturation density were decreased.

Advances in soil microbial ecology and the biodiversity

Recent studies on the colony formation of soil bacteria opened the way to categorize soil bacteria into colony forming curve (CFC) groups of different growth rates. A bacterial culture collection comprising organisms from every CFC group is called an ecocollection. Outlines of ECs of paddy soil 1992 and grassland soil 1987 and 1992 were described. Phylogenetic studies by 16S rDNA sequencing showed a great diversity of culture strains of the ecocollections (EC). A set of alternative concepts was proposed; the active and the quiescent forms of bacterial cells in soil. The former is able to be cultivated and thus counted by the plate method, while the latter is not unless it transforms into the former. Based on the results several points required for extensive cataloguing of soil bacteria were noted.

Carbon, nitrogen, and sulfur isotope changes and hydro-geological processes in a saline lake chain

The ionic concentrations,conductivity and pH of water in the Lake Chany complex in West Siberian Russia change from the mouth to the interior of the lake. This difference is indicative of marked evaporation of lake water from the closed water body system in the dry climate of Western Siberia. The carbon isotope composition of particulate organic matter (POM, composed mainly of phytoplankton) clearly changes, along with the pH of the water, reflecting the concentration of dissolved CO2. Carbon and nitrogen isotope signatures of Chironomus plumosus larvae, a benthic invertebrate that may feed on bulk lake sediment, systematically increase, along with those of POM and sediment organic matter (SOM), through the lake chain. Both sulfate-sulfur and nitrogen isotope compositions of the POM and SOM increase with distance from the estuary into the Lake Chany complex. Heavier sulfur and nitrogen isotope recycling from the sediment, caused by microbial sulfate reduction and denitrification, respectively, may have led to the increased sulfate-sulfur and nitrogen isotope compositions of the POM and SOM.

Differences in nitrogen and carbon stable isotopes between planktonic and benthic microalgae

We compiled published data on the nitrogen and carbon stable isotope ratios of phytoplankton and benthic microalgae from lentic systems and explored the primary factors determining the isotope values among systems. Also, we investigated seasonal changes in nitrogen stable isotope ratios of phytoplankton and benthic microalgae in the strongly acidic lake, Lake Katanuma, which has only two dominant species, Pinnularia acidojaponica as a benthic diatom and Chlamydomonas acidophila, a planktonic green alga. From the published dataset, it may be concluded that δ13C of benthic diatoms were more enriched than those of phytoplankton at the same sites, although the nitrogen isotope of phytoplankton and benthic microalgae were similar. This differences in δ13C between benthic microalgae and phytoplankton could be explained by the boundary layer effect. On the other hand, nitrogen isotope values of both benthic microalgae and phytoplankton were primarily controlled by the same environmental factor, and boundary layer effects are not the primary factor determining the nitrogen isotope values of microalgae. Also, we showed temporal dynamics in nitrogen isotopes of benthic and planktonic microalgae species in Lake Katanuma, and the trends of nitrogen isotopes are similar between benthic and planktonic microalgae, as concluded from the published dataset.

Seasonal increase of methane in sediment decreases δ13C of larval chironomids in a eutrophic shallow lake

Recent studies have shown that larval chironomids assimilate 13C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval δ13C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval δ13C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10–11- and 20–21-cm layers peaked in October. Negative correlations between methane concentrations in the 5–6/10–11-cm layers and larval δ13C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

Spatial changes in carbon and nitrogen stable isotopes of the plankton food web in a saline lake ecosystem

We investigated spatial changes in the isotope ratios of the plankton food web in Lake Chany, Siberia, Russia, especially at an estuarine transition zone of the lake. The δ13C values of particulate organic matter (POM) varied among the sampling sites, and increased with increasing pH of the lake water. This may reflect a shift by phytoplankton from using CO2 to using bicarbonate for photosynthesis with increasing pH. The δ13C values of zooplankton community also changed at each site along with those of the POM. This was indicative of carbon isotope changes of plankton food webs between the stations along an environmental gradient.

Effects of acute γ-irradiation on community structure of the aquatic microbial microcosm

To characterise indirect effects of ionising radiation on aquatic microbial communities, effects of acute gamma-irradiation were investigated in a microcosm consisting of populations of green algae (Chlorella sp. and Scenedesmus sp.) and a blue-green alga (Tolypothrix sp.) as producer; a ciliate protozoan (Cyclidium glaucoma), rotifers (Lecane sp. and Philodina sp.) and an oligochaete (Aeolosoma hemprichi) as consumer; and more than four species of bacteria as decomposers. Population changes in the constituent organisms were observed over 160 days after irradiation. Prokaryotic community structure was also examined by denaturing gradient gel electrophoresis (DGGE) of 16S rDNA. Principle response curve analysis revealed that the populations of the microcosm as a whole were not significantly affected at 100Gy while they were adversely affected at 500-5000Gy in a dose-dependent manner. However, some effects on each population, including each bacterial population detected by DGGE, did not depend on radiation doses, and some populations in the irradiated microcosm were larger than those of the control. These unexpected results are regarded as indirect effects through interspecies interactions, and possible mechanisms are proposed originating from population changes in other organisms co-existing in the microcosm. For example, some indirect effects on consumers and decomposers likely arose from interspecies competition within each trophic level. It is also likely that prey-predator relationships between producers and consumers caused some indirect effects on producers.

Parasite-Induced Changes in Nitrogen Isotope Signatures of Host Tissues

To estimate isotopic changes caused by trematode parasites within a host, we investigated changes in the carbon and nitrogen isotope ratios of the freshwater snail Lymnaea stagnalis infected by trematode larvae. We measured carbon and nitrogen stable isotopes within the foot, gonad, and hepatopancreas of both infected and uninfected snails. There was no significant difference in the δ13C and δ15N values of foot and gonad between infected and uninfected snails; thus, trematode parasite infections may not cause changes in snail diets. However, in the hepatopancreas, δ15N values were significantly higher in infected than in uninfected snails. The 15N enrichment in the hepatopancreas of infected snails is caused by the higher 15N ratio in parasite tissues. Using an isotope-mixing model, we roughly estimated that the parasites in the hepatopancreas represented from 0.8 to 3.4% of the total snail biomass, including the shell.

Effect at the ecosystem level of elevated atmospheric CO2in an aquatic microcosm

We studied the responses of an aquatic microcosm in two different eutrophic conditions to elevated atmospheric CO2concentration. We used microcosms, consisting of Escherichia coli(bacteria), Tetrahymena thermophila(protozoa) and Euglena gracilis(algae), in salt solution with 50 and 500 mg l−1of proteose peptone (eutrophic and hypereutrophic conditions, respectively) under ambient and elevated CO2(1550±100 μl l−1) conditions. The density of E. gracilisincreased significantly under elevated CO2in both eutrophic and hypereutrophic microcosms. In the eutrophic microcosm, the other elements were not affected by elevated CO2. In the hypereutrophic microcosm, however, the concentrations of ammonium and phosphate decreased significantly under elevated CO2. Furthermore, the density of T. thermophilawas maintained in higher level than that in the microcosm with ambient CO2and the density of E. coliwas decreased by CO2enrichment. Calculating the carbon biomasses of T. thermophilaand E. colifrom their densities, the changes in their biomasses by CO2enrichment were little as compared with large increase of E. graciliscarbon biomass converted from chlorophyll a. From the responses to elevated CO2in the subsystems of the hypereutrophic microcosm consisting of either one or two species, the increase of E. graciliswas a direct effect of elevated CO2, whereas the changes in the density of E. coliand T. thermophilaand the decreases in the concentration of ammonium and phosphate are considered to be indirect effects rather than direct effects of elevated CO2. The indirect effects of elevated CO2were prominent in the hypereutrophic microcosm.

Analysis of factors controlling responses of an aquatic microcosm to organic loading

The changes in the numbers of individuals of the constituent species and ammonia concentrations in 6 subsystems (Bacteria, bacteria-Cyclidium, bacteria-Aeolosoma, bacteria-Chlorella, bacteria-Cyclidium-Chlorella, bacteria-Aeolosoma-Chlorella) of an aquatic microcosm were examined after the addition of organic substance (peptone) at both the young and mature stages of succession. Organic loading led to decreases in the numbers of bacteria and Aeolosoma after temporary increases in their numbers and an oscillation in the numbers of Cyclidium but the densities of these populations were maintained at constant values in the presence of Chlorella. When organic substance was added at a young stage where the photosynthetic activities of algae were high, invertebrates, especially Aeolosoma, were damaged by unionized ammonia which was formed as the pH value increased. In contrast, organic loading at the mature stage enhanced the population densities and biomasses of all constituent species.