We conducted experiments to determine isotope changes in the deposit-feeding chironomid larvae Chironomus acerbiphilus during feeding, starvation and metamorphosis. Isotope changes in chironomid larvae occurred mainly during growth and rarely afterward. This finding indicates that chironomid isotope turnover mainly occurs in conjunction with growth and suggests that chironomid larvae only break down newly assimilated food for energy during periods of no growth. Chironomid delta(13)C values significantly increased throughout the starvation experiment, indicating that chironomids preferentially break down components with lower delta(13)C content during starvation. We found significant changes in chironomid isotope ratios ((15)N enrichment) during pupation. This evidence suggests that the physiological condition of animals (such as during an active growth phase or pre- or post-molting) is important to their stable isotope ratios.
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.
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 d 13 C 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 d 13 C 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.
Lake Katanuma is a strongly acidic volcanic lake (average pH 2.2) located in NE Japan in which only 2 algal species are found: Pinnularia acidojaponica, a benthic diatom, and Chlamydomonas acidophila, a phytoplankton species. Although the δ 13 C of phytoplankton generally varies seasonally in lake ecosystems, in Lake Katanuma the mean δ 13 C of particulate organic matter (POM, mainly phytoplankton) is constrained to a narrow range from -26.4 to -23.7 ‰. A major reason for this is the continuous supply of dissolved CO 2 gas available to C. acidophila from fumaroles at the lake bottom. The δ 13 C of P. acidojaponica in Lake Katanuma varied seasonally and was positively correlated with P. acidojaponica abundance at 1 and 4 m depths. This suggests that the higher P. acidojaponica biomass at 1 and 4 m produced the 13 C-enrichment in the high-density algal mats because of the limited dissolved CO 2 gas. The mean δ 13 C of benthic diatoms was higher than that of phytoplankton in Lake Katanuma, although the diatoms seemed to assimilate the same carbon source (CO 2 gas) in the lake water. KEY WORDS: Carbon stable isotope · POM · Phytoplankton · Benthic diatoms · Inorganic carbon · Seasonal variation · Acidic conditions Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 33: [87][88][89][90][91][92][93][94] 2003 phytoplankton species Chlamydomonas acidophila and the benthic diatom species Pinnularia acidojaponica are found in the lake.Usually, the phytoplankton and benthic algal communities in lake ecosystems are composed of many species. The complexity of the species composition may make it difficult to analyze the relationships between the carbon isotope ratios of algae and their physiological and environmental conditions in situ, since the isotope ratios of microalgae are mainly measured for an entire algal community (Yoshioka et al. 1989, Takahashi et al. 1990b). The seasonal variation in the δ 13 C of planktonic and benthic microalgal communities is inevitably influenced by changes in species composition (Fry 1996). Only few studies have reported seasonal variation in the carbon isotope ratio at the species level (Zohary et al. 1994).This study examines the seasonal changes in the δ 13 C of phytoplankton and benthic diatoms in a strongly acidic lake and compares the difference in the δ 13 C between the 2 algal species that dominate it. In Lake Katanuma, because of its limited phytoplankton and benthic algal species, we could analyze the relationships between the carbon isotope ratios of the phytoplankton and benthic diatoms and physiological and environmental factors more accurately than in an environment with a greater variety of species. Moreover, we could determine the difference in the δ 13 C of benthic diatoms and phytoplankton in the same natural environment. MATERIALS AND METHODSStudy area. Lake Katanuma is a volcanic lake located in NE Japan (38°44' N, 140°43' E) and is situated 306 m above sea level. Lake Katanuma is strongly acidic, with an average pH...
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