Mixed chemostat cultures of obligately aerobic and fermentative or methanogenic bacteria grown under oxygen-limiting conditions

Abstract Defined mixed cultures of an obligately aerobic Pseudomonas testosteroni and anaerobic Veillonella alcalescens strain were grown under oxygen and lactate limitation in chemostats with different oxygen supply rates. The aerobic and the anaerobic bacteria were shown to coexist and to complete for common substrates over a wide range of oxygen supply rates. Under similar conditions but with formate as the major substrate chemostat enrichments gave rise to undefined mixed cultures of aerobic, fermentative and methanogenic bacteria. The relevance of these observations to natural mineralization processes is discussed.     

REVIEW: The evolution of polyembryony in parasitoid wasps

Polyembryony has evolved independently in four families of parasitoid wasps. We review three main hypotheses for the selective forces favouring this developmental mode in parasitoids: polyembryony (i) reduces the costs of egg limitation; (ii) reduces the genetic conflict among offspring; and (iii) allows offspring to adjust their numbers to the quality of the host. Using comparative data and verbal and mathematical arguments, we evaluate the relative importance of the different selective forces through different evolutionary stages and in the different groups of polyembryonic wasps. We conclude that reducing the cost of egg limitation is especially important when large broods are favoured. Reducing genetic conflict may be most important when broods are small, thus might have been important during, or immediately following, the initial transition from monoembryony to polyembryony. Empirical data provide little support for the brood‐size adjustment hypothesis, although it is likely to interact with other selective forces favouring polyembryony.     

Effects of predator treatments,individual traits,and environment on moose survival in Alaska

We studied moose (Alces alces) survival, physical condition, and abundance in a 3-predator system in western Interior Alaska, USA, during 2001–2007. Our objective was to quantify the effects of predator treatments on moose population dynamics by investigating changes in survival while evaluating the contribution of potentially confounding covariates. In May 2003 and 2004, we reduced black bear (Ursus americanus) and brown bear (U. arctos) numbers by translocating bears ≥240 km from the study area. Aircraft-assisted take reduced wolf (Canis lupus) numbers markedly in the study area during 2004–2007. We estimated black bears were reduced by approximately 96% by June 2004 and recovered to within 27% of untreated numbers by May 2007. Brown bears were reduced approximately 50% by June 2004. Late-winter wolf numbers were reduced by 75% by 2005 and likely remained at these levels through 2007. In addition to predator treatments, moose hunting closures during 2004–2007 reduced harvests of male moose by 60% in the study area. Predator treatments resulted in increased calf survival rates during summer (primarily from reduced black bear predation) and autumn (primarily from reduced wolf predation). Predator treatments had little influence on survival of moose calves during winter; instead, calf survival was influenced by snow depth and possibly temperature. Increased survival of moose calves during summer and autumn combined with relatively constant winter survival in most years led to a corresponding increase in annual survival of calves following predator treatments. Nonpredation mortalities of calves increased following predator treatments; however, this increase provided little compensation to the decrease in predation mortalities resulting from treatments. Thus, predator-induced calf mortality was primarily additive. Summer survival of moose calves was positively related to calf mass (β > 0.07, SE = 0.073) during treated years and lower (β = −0.82, SE = 0.247) for twins than singletons during all years. Following predator treatments, survival of yearling moose increased 8.7% for females and 21.4% for males during summer and 2.2% for females and 15.6% for males during autumn. Annual survival of adult (≥2 yr old) female moose also increased in treated years and was negatively (β = −0.21, SE = 0.078) related to age. Moose density increased 45%, from 0.38 moose/km² in 2001 to 0.55 moose/km² in 2007, which resulted from annual increases in overall survival of moose, not increases in reproductive rates. Indices of nutritional status remained constant throughout our study despite increased moose density. This information can be used by wildlife managers and policymakers to better understand the outcomes of predator treatments in Alaska and similar environments. © 2011 The Wildlife Society.     

Physical site characteristics limit pollination and fruit set in the dioecious hydrophilous species,Vallisneria americana

Experimental and observational studies of the submersed, freshwater macrophyte Vallisneria americana Michx. revealed that depth, wind and wave exposure, and current velocity may all influence fruit set. In this dioecious species, long-pedunculate female flowers are pollinated by free-floating male flowers at the water surface. Average fruit set in the natural populations studied varied from zero to 97% of the flowers observed. With increasing water depth in New York and Pennsylvania lakes, female plants continued to flower, though these flowers were unable to reach the surface, and consequently, did not set fruit. Fruit set was also lower in relatively open sites exposed to wind and waves, presumably because male flowers do not remain in the vicinity of female flowers long enough for effective pollination. This was particularly striking at a site with low male flower densities, but fruit set was increased to 100% at that site by confining the floating male flowers within a field enclosure. In a river, fruit set was negatively correlated with surface current velocity, and was reduced to zero in current velocities greater than 0.30 m · s^–1. Fruit set in V. americana appears to be restricted or precluded by physical environmental conditions in a variety of sites.     

Genotypic variation in source and sink traits affects the response of photosynthesis and growth to elevated atmospheric CO2

This study aimed to understand the response of photosynthesis and growth to e-CO₂ conditions (800 vs. 400 μmol mol⁻¹) of rice genotypes differing in source–sink relationships. A proxy trait called local C source–sink ratio was defined as the ratio of flag leaf area to the number of spikelets on the corresponding panicle, and five genotypes differing in this ratio were grown in a controlled greenhouse. Differential CO₂ resources were applied either during the 2 weeks following heading (EXP1) or during the whole growth cycle (EXP2). Under e-CO₂, low source–sink ratio cultivars (LSS) had greater gains in photosynthesis, and they accumulated less nonstructural carbohydrate in the flag leaf than high source–sink ratio cultivars (HSS). In EXP2, grain yield and biomass gain was also greater in LSS probably caused by their strong sink. Photosynthetic capacity response to e-CO₂ was negatively correlated across genotypes with local C source–sink ratio, a trait highly conserved across environments. HSS were sink-limited under e-CO₂, probably associated with low triose phosphate utilization (TPU) capacity. We suggest that the local C source–sink ratio is a potential target for selecting more CO₂-responsive cultivars, pending validation for a broader genotypic spectrum and for field conditions.     

Stimulation of xanthan production by Xanthomonas campestris using citric acid

The biosynthesis of xanthan by Xanthomonas campestris was found to be affected by the addition of citric acid in fed batch mode. Under oxygen-limiting conditions, the addition of up to 2.6 g citric acid per litre improved cell viability as well as increasing xanthan yield by up to 80%. Comparative xanthan formation profiles at different operating conditions indicate that at higher aeration (when there was no oxygen limitation), citric acid addition did not improve xanthan production.     

Emergence of nutrient co‐limitation through movement in stoichiometric meta‐ecosystems

Evidence that ecosystems and primary producers are limited in their productivity by multiple nutrients has caused the traditional nutrient limitation framework to include multiple limiting nutrients. The models built to mimic these responses have invoked local mechanisms at the level of the primary producers. In this paper, we explore an alternative explanation for the emergence of co‐limitation by developing a simple, stoichiometrically explicit meta‐ecosystem model with two limiting nutrients, autotrophs and herbivores. Our results show that differences in movement rates for the nutrients, autotrophs and herbivores can allow for nutrient co‐limitation in biomass response to emerge despite no local mechanisms of nutrient co‐limitation. Furthermore, our results provide an explanation to why autotrophs show positive growth responses to nutrients despite ‘nominal’ top‐down control by herbivores. These results suggest that spatial processes can be mechanisms for nutrient co‐limitation at local and regional scales, and can help explain anomalous results in the co‐limitation literature.