On limiting nutrient patchiness and phytoplankton growth: a conceptual approach

A theoretical framework is developed to explore the effectsof limiting nutrient patchiness on phytoplankton growth. Growthrate is represented as a function of the average ambient substrateconcentration in the medium, the degree of patchiness and thepatch duration. Phytoplankton growth, in relation to the externalsubstrate concentration, is mediated by the cell quota for thelimiting nutrient. Two general conclusions can be drawn from this study. Firstthe degree of patchiness in the environment can affect individualgrowth rates and thus alter community structure even thoughthere is no change in the average ambient nutrient concentration.Second, for patch-adapted populations, the apparent K_s for growthcan be lowered significantly by making the distribution of thelimiting nutrient patchy with respect to time. The insightswhich this model provides into future experimental methodologiesare also discussed. ¹ Address for reprints ² also Dept. Botany ³ also Institute of Applied Mathematics     

Autumn colours and the nutrient retranslocation hypothesis: a theoretical assessment

The adaptive value of the bright colours of leaves in autumn is still debated. It is possible that autumn colours are an adaptation to protect the tree against photoinibition and photooxidation, which allows a more efficient recovery of nutrients. It has been proposed that the preference of aphids for trees that retranslocate nitrogen more efficiently can explain the high diversity of aphids on tree species with bright autumn colours. This scenario however does not take into account the impact of insects on the fitness of the trees and has not been analysed theoretically. Its assumptions and predictions, therefore, remain uncertain. I show with a model of insect-tree interaction that the system can actually evolve under particular conditions. I discuss the differences with the coevolution theory of autumn colours, available evidence and possible tests.     

Potential effects of elevated atmospheric carbon dioxide on benthic autotrophs and consumers in stream ecosystems: a test using experimental stream mesocosms

Elevated atmospheric carbon dioxide (eCO₂) has been shown to have a variety of ecosystem‐level effects in terrestrial systems, but few studies have examined how eCO₂ might affect aquatic habitats. This limits broad generalizations about the effects of a changing climate across biomes. To broaden this generalization, we used free air CO₂ enrichment to compare effects of eCO₂ (i.e., double ambient ～720 ppm) relative to ambient CO₂ (aCO₂～360 ppm) on several ecosystem properties and functions in large, outdoor, experimental mesocosms that mimicked shallow sand‐bottom prairie streams. In general, we showed that eCO₂ had strong bottom‐up effects on stream autotrophs, which moved through the food web and indirectly affected consumer trophic levels. These general effects were likely mediated by differential CO₂ limitation between the eCO₂ and aCO₂ treatments. For example, we found that eCO₂ decreased water‐column pH and increased dissolved CO₂ in the mesocosms, reducing CO₂‐limitation at times of intense primary production (PP). At these times, PP of benthic algae was about two times greater in the eCO₂ treatment than aCO₂ treatment. Elevated PP enhanced the rate of carbon assimilation relative to nutrient uptake, which reduced algae quality in the eCO₂ treatment. We predicted that reduced algae quality would negatively affect benthic invertebrates. However, density, biomass and average individual size of benthic invertebrates increased in the eCO₂ treatment relative to aCO₂ treatment. This suggested that total PP was a more important regulator of secondary production than food quality in our experiment. This study broadens generalizations about ecosystem‐level effects of a changing climate by providing some of the first evidence that the global increase in atmospheric CO₂ might affect autotrophs and consumers in small stream ecosystems throughout the southern Great Plains and Gulf Coastal slope of North America.     

The role of mobile consumers in lake nutrient cycles: a brief review

Spawning habitats of anchovy Engraulis encrasicolus and sardinella Sardinella aurita were investigated in the Gulf of Gabès by means of two surveys carried out during the summer of 2005 and 2009. The spatial patterns of early developmental stages were analyzed through Lloyd’s patchiness index, single quotient analyses, and principal component analyses. The results showed that Gulf of Gabès is an important area for anchovy and sardinella spawning. Within this area, anchovy and sardinella showed preference for the warmest waters to spawn. The main spawning areas for anchovy were located in the inner parts of the Gulf, and a secondary spawning area was observed offshore. Contrastingly, the main spawning ground of sardinella was found in the center of the Gulf. In both years, early larvae of anchovy showed a greater degree of aggregation than that of sardinella. The developmental stages of both species showed low spatial overlap, indicating that the spawning habitats of these species are spatially differentiated in the Gulf of Gabès.     

Development of a pH gradient within a biofilm is dependent upon the limiting nutrient

Monospecies Citrobacter sp. biofilms were grown in a laminar flow cell using a carbon-limiting medium. Microelectrode measurements showed no change in pH between the bulk fluid and biofilm when the flow cell was supplied with the carbon-limiting medium under static or flowing conditions. When the biofilm was supplied with a phosphate-limiting medium the biofilm became more acidic than the bulk fluid and developed a gradient within. The implications for metals-bioremediation processes are discussed.     

Saltatory search: a theoretical analysis

Many animal search in a saltatory fashion: they move forward,pause briefly, and move forward again. Although many optimal-foragingmodels have been developed, most do not address how an animalsearches for food. We view search strategies as "time-distance"functions to allow not only for the possibility of oscillationsin body speed, as implied by saltatory search, but other movementpatterns as well, including cruise search. The key feature ofour models is distinguishing between the body position and thescan position (where the forager is looking). We see the varyingmovement of saltatory search as a consequence of the curvaturein the functions that relate body speed to benefits (Jensen'sinequality)     

When the exception becomes the rule: The disappearance of limiting similarity in the Lotka-Volterra model

We investigate the transition between limiting similarity and coexistence of a continuum in the competitive Lotka-Volterra model. It is known that there exist exceptional cases in which, contrary to the limiting similarity expectation, all phenotypes coexist along a trait axis. Earlier studies established that the distance between surviving phenotypes is in the magnitude of the niche width 2σ provided that the carrying capacity curve differs from the exceptional one significantly enough. In this paper we studied the outcome of competition for small perturbations of the exceptional (Gaussian) carrying capacity. We found that the average distance between the surviving phenotypes goes to zero when the perturbation vanishes. The number of coexisting species in equilibrium is proportional to the negative logarithm of the perturbation. Nevertheless, the niche width provides a good order of magnitude for the distance between survivors if the perturbations are larger than 10%. Therefore, we conclude that limiting similarity is a good framework of biological thinking despite the lack of an absolute lower bound of similarity.     

Changes in carbohydrate and nutrient contents throughout a reproductive cycle indicate that phosphorus is a limiting nutrient in the epiphytic bromeliad, Werauhia sanguinolenta

BACKGROUND AND AIMS: This study examined the physiological basis of the cost of reproduction in the epiphytic bromeliad Werauhia sanguinolenta, growing in situ in a tropical lowland forest in Panama. METHODS: Entire mature plants were sampled repeatedly over the course of 2 years, which represents the common interval between reproductive events. Due to the uncertainty concerning the appropriate currency of resource allocation to reproduction, the temporal changes of the contents of total non-structural carbohydrates (TNC) and of all major nutrient elements in different plant parts were studied (stems, green leaves, non-green leaf bases, roots and reproductive structures when present). KEY RESULTS: Although TNC varied with time in all compartments, this variation was more related to seasonal fluctuations than to reproductive status. The contents of the nutrient elements, N, P, K, Mg and S, on the other hand, showed significant differences between reproductive and non-reproductive individuals, while Ca did not change with reproductive status. Differences in nutrient contents were most pronounced in stems. Seeds were particularly enriched in P, much less so in N and the other nutrient elements. Model calculations of nutrient fluxes indicate that a plant needs about 2 years to accumulate the amount of P invested in a fruit crop, while the estimated uptake rates for N were much faster. CONCLUSIONS: Since most mature individuals of this species fruit every other year, it is hypothesized that P is the prime limiting factor for reproduction. These findings therefore add to an increasing body of evidence that P rather than N is limiting growth and reproduction in vascular epiphytes.     

The energetics of coprophagy: a theoretical analysis

Lagomorphs, many rodents and some other small mammals eat their faeces during the part of the day when they are not foraging for fresh food. One of the possible benefits of this habit of coprophagy is that it may enable them to extract more energy from their food. A computer model is used to assess the likely benefits and explore their relationships to food, feeding rate and gut morphology. The predicted benefits are much larger for hindgut fermenters than for foregut fermenters, and especially large for hindgut fermenters with relatively small fermentation chambers. They are larger for poor foods (with lower proportions of cell contents) than for richer ones. At low feeding rates the energetic advantage of coprophagy may disappear if the faeces from food eaten during one feeding period emerge largely during the next, but this can be avoided by adjusting the rate of passage of gut contents during the intervening rest period.     

Regulation of phosphorus stoichiometry and growth rate of consumers: theoretical and experimental analyses with Daphnia

Initial theories of ecological stoichiometry were based on the assumption that the mass-specific content of key nutrient elements (such as P), changes little within a consumer species. However, evidence has shown that this content changes substantially according to feeding conditions. To clarify how the specific P content (S _P) of a consumer species depends on food conditions and relates to the growth rate, we constructed a multiple mass-balance model incorporating feeding and metabolic costs and stoichiometrically regulated releases for C and P. The validity of the model was then tested experimentally by examining the growth rates and S _P of Daphnia pulicaria under various food conditions. The experimental observation agreed qualitatively well with the model, showing that the S _P of consumers relates positively to growth rate at high food C:P ratios but negatively at low food C:P ratios. Thus, within a consumer species, individuals with high S _P do not necessarily grow at high rates. The concordance in results between the model and our observation suggests that maintenance costs for both P and C are substantial regardless of food conditions and play crucial roles in determining the relationship between the S _P and growth rate of consumers.     

A theoretical experimental method to determine the locus of desensitization

Consider a ligand-gated channel with n agonist binding sites which can undergo desensitization. We present a theoretical experimental procedure for pinpointing the principal receptor state from which there is a transition to the desensitized state. The method is based on the observation that the dependence of the slope of the time constant of desensitization vs agonist concentration, at low concentrations, represents the state from which desensitization occurs. In those receptors where desensitization occurs from the open state (or the one immediately preceding it), the method also enables us to determine the number of binding sites.     

Friend or foe: Hybrid proline-rich proteins determine how plants respond to beneficial and pathogenic microbes

Plant plastids generate signals, including some derived from lipids, that need to be mobilized to effect signaling. We used informatics to discover potential plastid membrane proteins involved in microbial responses in Arabidopsis (Arabidopsis thaliana). Among these are proteins co-regulated with the systemic immunity component AZELAIC ACID INDUCED 1, a hybrid proline-rich protein (HyPRP), and HyPRP superfamily members. HyPRPs have a transmembrane domain, a proline-rich region (PRR), and a lipid transfer protein domain. The precise subcellular location(s) and function(s) are unknown for most HyPRP family members. As predicted by informatics, a subset of HyPRPs has a pool of proteins that target plastid outer envelope membranes via a mechanism that requires the PRR. Additionally, two HyPRPs may be associated with thylakoid membranes. Most of the plastid- and nonplastid-localized family members also have pools that localize to the endoplasmic reticulum, plasma membrane, or plasmodesmata. HyPRPs with plastid pools regulate, positively or negatively, systemic immunity against the pathogen Pseudomonas syringae. HyPRPs also regulate the interaction with the plant growth-promoting rhizobacteria Pseudomonas simiae WCS417 in the roots to influence colonization, root system architecture, and/or biomass. Thus, HyPRPs have broad and distinct roles in immunity, development, and growth responses to microbes and reside at sites that may facilitate signal molecule transport.

Hybrid proline-rich proteins that reside at plastid membranes and other sites have broad and distinct roles in immunity, development, and growth responses to microbes.     

Time constant-related capnograph distortion: a theoretical analysis

A theoretical analysis of time constant-related distortion in side-stream capnographs was carried out using two models of respiratory patterns. It is demonstrated that under circumstances likely to be encountered clinically, significant distortion may occur. This distortion may produce both spurious rebreathing and under estimation of the true end-tidal CO₂ concentration.     

Some theoretical insights into the hologenome theory of evolution and the role of microbes in speciation

Research on symbiotic communities (microbiomes) of multicellular organisms seems to be changing our understanding of how species of plants and animals have evolved over millions of years. The quintessence of these discoveries is the emergence of the hologenome theory of evolution, founded on the concept that a holobiont (a host along with all of its associated symbiotic microorganisms) acts a single unit of selection in the process of evolution. Although the hologenome theory has become very popular among certain scientific circles, its principles are still being debated. In this paper, we argue, firstly, that only a very small number of symbiotic microorganisms are sufficiently integrated into multicellular organisms to act in concert with them as units of selection, thus rendering claims that holobionts are units of selection invalid. Secondly, even though holobionts are not units of selection, they can still constitute genuine units from an evolutionary perspective, provided we accept certain constraints: mainly, they should be considered units of co-operation. Thirdly, we propose a reconciliation of the role of symbiotic microorganisms with the theory of speciation through the use of a developed framework. Mainly, we will argue that, in order to understand the role of microorganisms in the speciation of multicellular organisms, it is not necessary to consider holobionts units of selection; it is sufficient to consider them units of co-operation.     

When density dependence is not instantaneous: theoretical developments and management implications

Most organisms live in changing environments or do not use the same resources at different stages of their lives or in different seasons. As a result, density dependence will affect populations differently at different times. Such sequential density dependence generates markedly different population responses compared to the unrealistic assumption that all events occur simultaneously. Various field studies have also shown that the conditions that individuals experience during one period can influence success and per capita vital rates during the following period. These carry-over effects further complicate any general principles and increase the diversity of possible population dynamics. In this review, we describe how studies of sequential density dependence have diverged in directions that are both taxon-specific and have non-overlapping terminology, despite very similar underlying problems. By exploring and highlighting these similarities, we aim to improve communication between fields, clarify common misunderstandings, and provide a framework for improving conservation and management practices, including sustainable harvesting theory.     

Use of a multiple addition bioassay to determine limiting nutrients in eagle lake,California

This investigation uses anin vitro enrichment bioassay (in which all essential nutrients except one are added to each culture) to determine which nutrients are important to algal growth in Eagle Lake. The technique was developed when it was discovered that addition of individual nutrients produced little if any growth response. Laboratory bioassays correlated well with comparative studies in the lake. A great deal of variation was found throughout the year but P, N, Fe, and S were found to be limiting at one time or another. The north and south basins of the lake, which differ in morphometry, were also found to differ in the intensity spectrum of limitation. While P was the most important nutrient in both basins, the other nutrients were more limiting in the north basin than the south, and Fe, which was least limiting in the south, was very important in the north. The multiple enrichment bioassay has several advantages over other bioassays. Supported in part by Research Corporation