Assessment of ‘top-down’ and ‘bottom-up’ forces as determinants of rotifer distribution among lakes in Ontario,Canada

Predation and food supply are generally perceived as important determinants of spatial and temporal variations of populations. The population dynamics of freshwater rotifers have been well researched in this aspect. However, their spatial variations have attracted less attention and have not been studied by simultaneously considering both predation and food supply. We studied spatial variations of rotifer abundance among 34 Canadian boreal lakes. A large part of the variance of rotifer abundance was associated with variables related to trophic status including chlorophyll a and total phosphorus. However, abundances of mesozooplankton such as potential predators and competitors did not correlate with rotifer abundance and did not explain the residual of the regression between rotifer abundance and chlorophyll a. The results of the present study indicated that variation in rotifer abundance among lakes was caused by bottom-up forces related to food supply and not by top-down predatory interactions. This provides a contrast to previous empiric and experimental studies that reported that temporal variations of rotifer abundance were mainly regulated by top-down interactions. This discrepancy suggests that overall differences in rotifer abundance among lakes are mainly determined by bottom-up forces while temporal changes in single lakes are shaped by top-down forces. Meanwhile, the composition of rotifer species was correlated with mesozooplankton abundance as well as trophic status. Rotifer species with long spines or rigid loricae were found in the lakes where mesozooplankton were abundant, which suggests that defensive morphology could have affected the rotifer species distribution among the study lakes.     

Fluvial transport of carbon along the river-to-ocean continuum and its potential impacts on a brackish water food web in the Iwaki River watershed, northern Japan

Riverine transport of dissolved inorganic carbon (DIC) from land to the ocean is an important carbon flux that influences the carbon budget at the watershed scale. However, the dynamics of DIC in an entire river network has remained unknown, especially in mountainous Japanese watersheds. We examined the effects of watershed land use and geology on the transports of inorganic carbon as well as weathered silica (Si) and calcium (Ca) in the Iwaki River system where agricultural and residential areas have developed in the middle and lower parts of the watershed. The concentration and stable carbon isotope ratios (δ¹³C) of DIC showed the longitudinal increase of ¹³C-depleted inorganic carbon along the river. As a result, most streams and rivers were supersaturated in dissolved CO₂ that will eventually be emitted to the atmosphere. The possible origin of ¹³C-depleted carbon is CO₂ derived from the decomposition of organic matter in agricultural and urban landscapes, as well as from in-stream respiration. In addition, agricultural and urban areas, respectively, exported the large amount of dissolved Si and Ca to the rivers, suggesting that CO₂ increased by respiration accelerates the chemical weathering of silicate and carbonate materials in soils, river sediments, and/or urban infrastructure. Furthermore, riverine bicarbonate flux is likely to enter shell carbonates of Corbicula japonica, an aragonitic bivalve, in the downstream brackish lake (Lake Jusan). These results revealed that the flux of DIC from the human-dominated watersheds is a key to understanding the carbon dynamics and food-web structure along the land-to-river-to-ocean continuum.     

The production-to-respiration ratio and its implication in Lake Biwa,Japan

Production-to-respiration (P:R) ratio was estimated at an offshore site of Lake Biwa in order to examine whether the plankton and benthic community is subsidized with allochthonous organic carbon, and to clarify the role of this lake as potential source or sink of carbon dioxide. The respiration rate of protozoan and metazoan plankton was calculated from their biomass and empirical equations of oxygen consumption rates, and that of bacterioplankton was derived from their production rate and growth efficiency. In addition, the carbon mineralization rate in the lake sediments was estimated from the accumulation rate of organic carbon, which was determined using a ²¹⁰Pb dating technique. On an annual basis, the sum of respiration rates of heterotrophic plankton was comparable to net primary production rate measured by the ¹³C method. However, when the mineralization rate in the lake sediments was included, the areal P:R ratio was 0.89, suggesting that Lake Biwa is net heterotrophic at the offshore site with the community being subsidized with allochthonous organic carbon. Such a view was supported by the surface water pCO₂ that was on average higher than that of the atmosphere. However, the estimate of net CO₂ release rate was close to that of carbon burial rate in the sediments. The result suggests that the role of Lake Biwa in relation to atmospheric carbon is almost null at the offshore site, although the community is supported partially by organic carbon released from the surrounding areas.     

Metabolic stoichiometry and the fate of excess carbon and nutrients in consumers

Animals encountering nutritionally imbalanced foods should release elements in excess of requirements in order to maintain overall homeostasis. Quantifying these excesses and predicting their fate is, however, problematic. A new model of the stoichiometry of consumers is formulated that incorporates the separate terms in the metabolic budget, namely, assimilation of ingested substrates and associated costs, protein turnover, other basal costs, such as osmoregulation, and the use of remaining substrates for production. The model indicates that release of excess C and nonlimiting nutrients may often be a significant fraction of the total metabolic budget of animals consuming the nutrient-deficient forages that are common in terrestrial and aquatic systems. The cost of maintenance, in terms of not just C but also N and P, is considerable, such that food quality is important even when intake is low. Many generalist consumers experience short-term and unpredictable fluctuations in their diets. Comparison of model output with data for one such consumer, Daphnia, indicates that mechanisms operating postabsorption in the gut are likely the primary means of regulating excess C, N, and P in these organisms, notably respiration decoupled from biochemical or mechanical work and excretion of carbon and nutrients. This stoichiometrically regulated release may often be in organic rather than inorganic form, with important consequences for the balance of autotrophic and heterotrophic processes in ecosystems.     

Understanding the stoichiometric limitation of herbivore growth: the importance of feeding and assimilation flexibilities

Ecological stoichiometry suggests that herbivore growth is limited by phosphorus when this element in the diet is < 8.6 μg P mg C^?1 (C : P atomic ratio > 300). However, in nature, it is not necessarily related to the relative phosphorus content in diets. This may be the result of complex feeding and assimilation responses to diets. We examined these possibilities using herbivorous plankton fed mono‐specific and mixed algae varying in phosphorus content of 1.6 to 8.1 μg P mg C^?1. The herbivores showed a 10‐fold growth rate difference among the diets. Growth rates related poorly with phosphorus content in the diets (r² = 0.07), better with P ingestion rate (r² = 0.41) and best with phosphorus assimilation rate (r² = 0.69). Inclusion of assimilation rates for carbon and fatty acids increased 7% of the explained growth variance. These results indicate that the feeding and assimilation flexibilities play pivotal roles in acquiring a deficient element and in regulating growth rate.     

Influence of food density on respiration rate of two crustacean plankters,Daphnia galeata and Bosmina longirostris

Summary The influence of food density on respiration rate was measured for two cladoceran plankton species, Daphnia galeata and Bosmina longirostris, over the range 0 to 2.5 mg C 1^-1, using the modified Winkler technique in order to examine how this affects the respiration rate and whether the functional response is the same in the two species. The respiration rate for animals of equivalent body size did not differ significantly between the two species in the absence of food, but was significantly lower in Bosmina longirostris than Daphnia galeata at high food density. Within a species, the response of respiration rate to changing food density did not differ among individuals of different body size. The respiration rate of D. galeata increased with increasing food density and reached a plateau at a high food density. A similar response curve was also found with the respiration rate of B. longirostris, although the response was less conspicuous. This response curve cannot be explained by the energy cost of known feeding behavior in cladocerans. Since the respiration rate related linearly with the assimilation rate, increase in food density seemed to increase the respiration rate by increasing the energetic cost required to process food biochemically, known as specific dynamic action.     

Role of the DELSEED Loop in Torque Transmission of F1-ATPase

F₁-ATPase is an ATP-driven rotary motor that generates torque at the interface between the catalytic β-subunits and the rotor γ-subunit. The β-subunit inwardly rotates the C-terminal domain upon nucleotide binding/dissociation; hence, the region of the C-terminal domain that is in direct contact with γ—termed the DELSEED loop—is thought to play a critical role in torque transmission. We substituted all the DELSEED loop residues with alanine to diminish specific DELSEED loop-γ interactions and with glycine to disrupt the loop structure. All the mutants rotated unidirectionally with kinetic parameters comparable to those of the wild-type F₁, suggesting that the specific interactions between DELSEED loop and γ is not involved in cooperative interplays between the catalytic β-subunits. Glycine substitution mutants generated half the torque of the wild-type F₁, whereas the alanine mutant generated comparable torque. Fluctuation analyses of the glycine/alanine mutants revealed that the γ-subunit was less tightly held in the α₃β₃-stator ring of the glycine mutant than in the wild-type F₁ and the alanine mutant. Molecular dynamics simulation showed that the DELSEED loop was disordered by the glycine substitution, whereas it formed an α-helix in the alanine mutant. Our results emphasize the importance of loop rigidity for efficient torque transmissions.     

Novel anti-carbohydrate antibodies reveal the cooperative function of sulfated N- and O-glycans in lymphocyte homing

Cell surface glycans play pivotal roles in immune cell trafficking and immunity. Here we present an efficient method for generating anti-carbohydrate monoclonal antibodies (mAbs) using gene-targeted mice and describe critical glycans in lymphocyte homing. We immunized sulfotransferase GlcNAc6ST-1 and GlcNAc6ST-2 doubly deficient mice with sulfotransferase-overexpressing Chinese hamster ovary cells and generated two mAbs, termed S1 and S2. Both S1 and S2 bound high endothelial venules (HEVs) in the lymphoid organs of humans and wild-type mice, but not in those of doubly deficient mice. Glycan array analysis indicated that both S1 and S2 specifically bound 6-sulfo sialyl Lewis X and its defucosylated structure. Interestingly, S2 inhibited lymphocyte homing to peripheral lymph nodes by 95%, whereas S1 inhibited it by only 25%. S2 also significantly inhibited contact hypersensitivity responses and L-selectin-dependent leukocyte adhesion to HEVs. Immunohistochemical and Western blot analyses indicated that S1 preferentially bound sulfated O-glycans, whereas S2 bound both sulfated N- and O-glycans in HEVs. Furthermore, S2 strongly inhibited the N-glycan-dependent residual lymphocyte homing in mutant mice lacking sulfated O-glycans, indicating the importance of both sulfated N- and O-glycans in lymphocyte homing. Thus, the two mAbs generated by a novel method revealed the cooperative function of sulfated N- and O-glycans in lymphocyte homing and immune surveillance.     

Finding copepod footprints: a protocol for molecular identification of diapausing eggs in lake sediments

Even though calanoid copepods produce diapausing eggs that stay alive in lake sediments, these eggs have rarely been used paleolimnologically, as they lack diagnostic morphological features. In this study, we developed a method to identify copepod diapausing eggs in Japan as a clue toward reconstructing past plankton populations. We first determined a 28S ribosomal DNA (rDNA) (i.e., nc28S) regional sequence library (240 bp) of various calanoid copepod species using ethanol-fixed plankton samples collected from across the Japanese archipelago. Then we applied the UltraSHOT method to extract DNA from an individual diapausing egg. Finally, the nc28S region of diapausing eggs collected from various lakes was sequenced and compared with the regional library for species identification. In total, 21 haplotypes of the nc28S region were recovered from planktonic samples of 11 Japanese freshwater calanoid copepod species. Despite the short length of this region, no identical haplotypes were shared among the species analyzed, including the Acanthodiaptomus pacificus complex treated as a species. Even different lineages of A. pacificus could be separated. These results indicate that the nc28S region can be used as a barcode in Japan. A total of 112 diapausing eggs collected from various lakes and ponds was processed, and the nc28S region of each was successfully sequenced. All of these egg sequences matched one or the other of the nc28S haplotypes in the regional library mentioned above. The set of protocols we applied (i.e., preparing a comprehensive regional sequence library and sequencing egg DNA) is thus useful for involving copepod diapausing eggs in paleolimnological studies in lakes. The nc28S region treated in this study has a strong potential to uncover the paleodiversity of copepods, at least in Japan.