STRESS, EXTINCTIONS AND EVOLUTIONARY CHANGE: FROM LIVING ORGANISMS TO FOSSILS

1. Natural populations are exposed to environmental stress of varying intensities. This provides a reference point for extrapolations from the living biota to fossils and vice versa. 2. Evolutionary change is likely when there are resources in excess of maintenance and survival needs. It is largely precluded at species borders by the metabolic costs of stress; from this follows climatic tracking by species. 3. A relatively small increase in abiotic stress could underlie extinctions of stress-sensitive endemic species and the spread of stress-resistant generalist and widespread species. Widespread fossil species appear resistant to extinction under the stress level of normal background extinctions. 4. Synergistic interactions among generalized stresses should increase the likelihood of extinctions, especially for stresses with energetic consequences. 5. Some marine organisms survived the K-T mass extinction event because of stress-evasion mechanisms such as stress-resistant life-cycle stages with low metabolic rates. 6. In moderately stressed and narrowly fluctuating environments, sufficient genetic variability and metabolic energy should be available to permit adaptation. In these environments phyletic gradualism is expected. 7. In highly stressed and widely fluctuating environments, a punctuated evolutionary pattern is expected whereby stasis occurs most of the time. 8. Evolutionary patterns therefore can vary depending on the details of the interaction between stress, environmental fluctuations, energy availability and genetic variability. 9. Little evolutionary change is expected when the availability of energy is severely restricted. Examples include cave animals in stable but stressed environments and ‘living fossils’ in widely fluctuating but stressed environments. 10. Since the primary effect of abiotic stress may be at the level of energy carriers, a reductionist approach permits generalisations in considering extinctions and conditions under which diversification is likely.     

Ecosystem engineering as an energy transfer process: a simple agent-based model

Ecosystem engineers are defined as organisms who modulate the availability of resources for themselves and other organisms by physically changing the environment. Ecosystem engineering is a well-recognised ecological interaction, but there is a limited number of general models due to the recent development of the field. Agent-based models are often used to study how organisms respond to changing environments and are suitable for modelling ecosystem engineering. To our knowledge, agent-based methodology has not yet been used to model ecosystem engineering. In this paper, we develop a simple agent-based population dynamics model of ecosystem engineering as an energy transfer process. We apply energy budget approach to conceptually explain how ecosystem engineers transfer energy to the environment and define various types of energy transfers relative to their effects on the engineers and other organisms. We simulate environments with various levels of resource abundance and compare the results of the model without ecosystem engineering agents to the model with ecosystem engineering agents. We find that in environments with higher levels of resources, the presence of ecosystem engineers increases the average carrying capacity and the strength of population fluctuations, while in environments with lower levels of resources, ecosystem engineering mitigates fluctuations, increases average carrying capacity and makes environments more resilient. Finally, we discuss about the further application of agent-based modelling for the theoretical and experimental development of the ecosystem engineering concept.     

Environments and evolution: interactions between stress, resource inadequacy and energetic efficiency

Evolutionary change is interpreted in terms of the near-universal ecological scenario of stressful environments. Consequently, there is a premium on the energetically efficient exploitation of resources in a resource-inadequate world. Under this environmental model, fitness can be approximated to energetic efficiency especially towards the limits of survival. Furthermore, fitness at one stage of the life-cycle should correlate with fitness at other stages, especially for development time, survival and longevity; 'good genotypes' under stress should therefore be at a premium. Conservation in the wild depends primarily on adaptation to abiotically changing habitats since towards the limits of survival, genomic variation is rarely restrictive. The balance between energetic costs under variable environments and energy from resources provides a model for interpreting evolutionary stasis, punctuational and gradual change, and specialist diversification. Ultimately, a species should be in an equilibrium between the physiology of an organism and its adaptation to the environment. The primary key to understanding evolutionary change should therefore be ecological, highlighting energy availability in a stressed world; this approach is predictive for various patterns of evolutionary change in the living and fossil biota.     

The importance of physiological limits in determining biogeographical range shifts due to global climate change: the heat-shock response

Physiological processes that set an organism's thermal limits are in part determining recent shifts in biogeographic distribution ranges due to global climate change. Several characteristics of the heat-shock response (HSR), such as the onset, maximal, and upper limit of heat-shock protein (Hsp) synthesis, contribute to setting the acute upper thermal limits of most organisms. Aquatic animals from stable, moderately variable, or highly variable thermal environments differ in their HSR. Some animals living in extremely stable thermal environments lack the response altogether. In contrast, rocky intertidal animals that experience highly variable thermal conditions start synthesizing Hsps, that is, the onset of synthesis, below the highest temperatures that they experience. Thus, these organisms experience thermal conditions in their environment that are close to the upper thermal limits in which they can defend themselves against cellular thermal insults by employing the HSR. Subtidal animals are characterized by moderately variable thermal environments, and their cells start synthesizing Hsps above the highest temperatures that they experience. The upper thermal limits against which they can defend themselves are thus much higher than the highest body temperatures they currently experience. Furthermore, the ability to acclimate to changing thermal conditions seems greatest among animals from moderately variable environments and limited in animals from stable and highly variable environments. Thus, these findings suggest that organisms with the narrowest (stenothermal) and the widest (highly eurythermal) temperature tolerance ranges live closest to their thermal limits and have a limited ability to acclimate, suggesting that they will be most affected by global climate change.     

Development of energetics in evolution of the living world (Stages,Digits, Postulates)

This work deals with the current stage of study of energy exchange between living organ-isms and the environment. In the epoch of molecular biology, study of energy exchange might have seemed a study of old, well known concepts. However, the retrospective insight into the energy exchange of quite a few organisms allows obtaining new data about development of energetics of the living world, approaches to interesting comparisons, opens the earlier unknown quantitative relations in energetics of living organisms, provides a possibility of analyzing causes of very high values of energy consumption by living organisms, causes of different sensitivity of living organisms to deficit of energy, etc. Based on all these data, there have been noted 12 principal moments or postulates in development of energetics of the living world from the most ancient to the present time.     

When to store energy in a stochastic environment

The ability to store energy enables organisms to deal with temporarily harsh and uncertain conditions. Empirical studies have demonstrated that organisms adapted to fluctuating energy availability plastically adjust their storage strategies. So far, however, theoretical studies have investigated general storage strategies only in constant or deterministically varying environments. In this study, we analyze how the ability to store energy influences optimal energy allocation to storage, reproduction, and maintenance in environments in which energy availability varies stochastically. We find that allocation to storage is evolutionarily optimal when environmental energy availability is intermediate and energy stores are not yet too full. In environments with low variability and low predictability of energy availability, it is not optimal to store energy. As environments become more variable or more predictable, energy allocation to storage is increasingly favored. By varying environmental variability, environmental predictability, and the cost of survival, we obtain a variety of different optimal life-history strategies, from highly iteroparous to semelparous, which differ significantly in their storage patterns. Our results demonstrate that in a stochastically varying environment simultaneous allocation to reproduction, maintenance, and storage can be optimal, which contrasts with previous findings obtained for deterministic environments.     

De Novo Evolution of Complex, Global and Hierarchical Gene Regulatory Mechanisms

Gene regulatory networks exhibit complex, hierarchical features such as global regulation and network motifs. There is much debate about whether the evolutionary origins of such features are the results of adaptation, or the by-products of non-adaptive processes of DNA replication. The lack of availability of gene regulatory networks of ancestor species on evolutionary timescales makes this a particularly difficult problem to resolve. Digital organisms, however, can be used to provide a complete evolutionary record of lineages. We use a biologically realistic evolutionary model that includes gene expression, regulation, metabolism and biosynthesis, to investigate the evolution of complex function in gene regulatory networks. We discover that: (i) network architecture and complexity evolve in response to environmental complexity, (ii) global gene regulation is selected for in complex environments, (iii) complex, inter-connected, hierarchical structures evolve in stages, with energy regulation preceding stress responses, and stress responses preceding growth rate adaptations and (iv) robustness of evolved models to mutations depends on hierarchical level: energy regulation and stress responses tend not to be robust to mutations, whereas growth rate adaptations are more robust and non-lethal when mutated. These results highlight the adaptive and incremental evolution of complex biological networks, and the value and potential of studying realistic in silico evolutionary systems as a way of understanding living systems.     

Root fungal endophytes: identity,phylogeny and roles in plant tolerance to metal stress

Metal trace elements accumulate in soils mainly because of anthropic activities, leading living organisms to develop strategies to handle metal toxicity. Plants often associate with root endophytic fungi, including nonmycorrhizal fungi, and some of these organisms are associated with metal tolerance. The lack of synthetic analyses of plant-endophyte-metal tripartite systems and the scant consideration for taxonomy led to this review aiming (1) to inventory non-mycorrhizal root fungal endophytes described with respect to their taxonomic diversity and (2) to determine the mutualistic roles of these plant-fungus associations under metal stress. More than 1500 species in 100 orders (mainly Hypocreales and Pleosporales) were reported from a wide variety of environments and hosts. Most reported endophytes had a positive effect on their host under metal stress, but with various effects on metal uptake or translocation and no clear taxonomic consistency. Future research considering the functional patterns and dynamics of these associations is thus encouraged.     

Adaptation and Acclimation of Photosynthetic Microorganisms to Permanently Cold Environments

Persistently cold environments constitute one of our world's largest ecosystems, and microorganisms dominate the biomass and metabolic activity in these extreme environments. The stress of low temperatures on life is exacerbated in organisms that rely on photoautrophic production of organic carbon and energy sources. Phototrophic organisms must coordinate temperature-independent reactions of light absorption and photochemistry with temperature-dependent processes of electron transport and utilization of energy sources through growth and metabolism. Despite this conundrum, phototrophic microorganisms thrive in all cold ecosystems described and (together with chemoautrophs) provide the base of autotrophic production in low-temperature food webs. Psychrophilic (organisms with a requirement for low growth temperatures) and psychrotolerant (organisms tolerant of low growth temperatures) photoautotrophs rely on low-temperature acclimative and adaptive strategies that have been described for other low-temperature-adapted heterotrophic organisms, such as cold-active proteins and maintenance of membrane fluidity. In addition, photoautrophic organisms possess other strategies to balance the absorption of light and the transduction of light energy to stored chemical energy products (NADPH and ATP) with downstream consumption of photosynthetically derived energy products at low temperatures. Lastly, differential adaptive and acclimative mechanisms exist in phototrophic microorganisms residing in low-temperature environments that are exposed to constant low-light environments versus high-light- and high-UV-exposed phototrophic assemblages.     

Adaptation and acclimation of photosynthetic microorganisms to permanently cold environments.

Persistently cold environments constitute one of our world's largest ecosystems, and microorganisms dominate the biomass and metabolic activity in these extreme environments. The stress of low temperatures on life is exacerbated in organisms that rely on photoautrophic production of organic carbon and energy sources. Phototrophic organisms must coordinate temperature-independent reactions of light absorption and photochemistry with temperature-dependent processes of electron transport and utilization of energy sources through growth and metabolism. Despite this conundrum, phototrophic microorganisms thrive in all cold ecosystems described and (together with chemoautrophs) provide the base of autotrophic production in low-temperature food webs. Psychrophilic (organisms with a requirement for low growth temperatures) and psychrotolerant (organisms tolerant of low growth temperatures) photoautotrophs rely on low-temperature acclimative and adaptive strategies that have been described for other low-temperature-adapted heterotrophic organisms, such as cold-active proteins and maintenance of membrane fluidity. In addition, photoautrophic organisms possess other strategies to balance the absorption of light and the transduction of light energy to stored chemical energy products (NADPH and ATP) with downstream consumption of photosynthetically derived energy products at low temperatures. Lastly, differential adaptive and acclimative mechanisms exist in phototrophic microorganisms residing in low-temperature environments that are exposed to constant low-light environments versus high-light- and high-UV-exposed phototrophic assemblages.     

Low dose ionizing radiation produces too few reactive oxygen species to directly affect antioxidant concentrations in cells

It has been hypothesized that radiation-induced oxidative stress is the mechanism for a wide range of negative impacts on biota living in radioactively contaminated areas around Chernobyl. The present study tests this hypothesis mechanistically, for the first time, by modelling the impacts of radiolysis products within the cell resulting from radiations (low linear energy transfer β and γ), and dose rates appropriate to current contamination types and densities in the Chernobyl exclusion zone and at Fukushima. At 417 μGy h(-1) (illustrative of the most contaminated areas at Chernobyl), generation of radiolysis products did not significantly impact cellular concentrations of reactive oxygen species, or cellular redox potential. This study does not support the hypothesis that direct oxidizing stress is a mechanism for damage to organisms exposed to chronic radiation at dose rates typical of contaminated environments.     

Adaptation of organisms to extreme conditions of deep-sea hydrothermal vents

The deep-sea hydrothermal vents are located along the volcanic ridges and are characterized by extreme conditions such as unique physical properties (temperature, pression), chemical toxicity, and absence of photosynthesis. However, life exists in these particular environments. The primary producers of energy and organic molecules in these biotopes are chimiolithoautotrophic bacteria. Many animals species live in intimate and complex symbiosis with these sulfo-oxidizing and methanogene bacteria. These symbioses imply a strategy of nutrition and a specific metabolic organization involving numerous interactions and metabolic exchanges, between partners. The organisms of these ecosystems have developed different adaptive strategies. In these environments many microorganisms are adapted to high temperatures. Moreover to survive in these environments, living organisms have developed various strategies to protect themselves against toxic molecules such as H2S and heavy metals.     

Physiological variation in the dog-whelk Nucella lapillus, L. either side of a cline in allozyme and karyotype frequencies

Genetic constitution in the intertidal gastropod Nucella lapillus influences variation in shell shape and growth rate which in turn are correlated with such habitat variables as wave action and temperature. We have investigated the response to hyperosmotic stress of samples from a cline in karyotype and allozyme frequencies and shell shape. Animals with a shell shape associated with environments where temperature and desiccation stress are important respond less to hyperosmotic stress than animals living in a high wave energy environment. With regard to the interaction between shell shape, physiology and habitat, animals with elongate shells associated with protected shores are shown to exhibit a reduced response to hyperosmotic stress compared to animals with a more spherical shell shape; this is discussed in relation to the production of an adaptive phenotype.     

Stress, order and survival

Much of the sophisticated chemistry of life is accomplished by multicomponent complexes, which act as molecular machines. Intrinsic to their accuracy and efficiency is the energy that is supplied by hydrolysis of nucleoside triphosphates. Conditions that deplete energy sources should therefore cause decay and death. But studies on organisms that are exposed to prolonged stress indicate that this fate could be circumvented through the formation of highly ordered intracellular assemblies. In these thermodynamically stable structures, vital components are protected by a physical sequestration that is independent of energy consumption.     

Some metabolic aspects of ecology

Summary

Due to the similarity of the major metabolic pathways in all living organisms it might be thought unlikely that the study of metabolic variation would add to an understanding of ecological differences. However, a quantitative study of the regulation of metabolism and its end-products indicates a link between cell biochemistry and the capability of certain species to succeed in particular environments. A study of anaerobic metabolism in animals and plants shows a number of similarities in those species able to survive in oxygen-poor environments. These include the production of nontoxic end-products and the avoidance of a large oxygen debt. As a large variety of organisms is examined, these comparisons suggest that the metabolic solutions to a given environmental stress, whether in plants or animals, can be strikingly similar. Therefore, although the main metabolic pathways in most living organism are the same, the regulation of metabolism varies with the ecology of the species. Examples are discussed in which a knowledge of metabolism in relation to ecology could have practical applications in forestry and agriculture.     

Ecology of planktonic heterotrophic flagellates. A review.

In aquatic environments heterotrophic flagellates are an important component within the microbial loop and the food web, owing to their involvement in the energy transfer and flux and as an intermediate link between bacteria and primary producers, and greater organisms, such as other protists and metazoan consumers. In the microbial loop heterotrophic flagellates highly contribute to fast biomass and nutrient recycling and to the production in aquatic environments. In fact, these protists consume efficiently viruses, bacteria, cyanobacteria and picophytoplankton, and are grazed mainly by other protists, rotifers and small crustaceans. In this paper the knowledge about these unicellular organisms is reviewed, taking into particular account their ecological relationships and trophic role within the plankton community of marine and freshwater environments.     

evolution: the dialogue between life and death

Organisms have the ability to harness energy from the environment to create order and to reproduce. From early error-prone systems natural selection acted to produce present day organisms with high accuracy in the synthesis of macromolecules. The environment imposes strict limits on reproduction, so evolution is always accompanied by the discarding of a large proportion of the less fit cells, or organisms. Sexual reproduction depends on an immortal germline and a soma which may be immortal or mortal. Higher animals living in hazardous environments have evolved aging and death of the soma for the benefit of the ongoing germline.     

Significance of palaeoberesellids (Chlorophyta) in Dinantian sedimentation, UK

Palaeoberesellids are septate, tubular microfossils usually attributed to the green algae. They occur widely in Upper Palaeozoic carbonate sediments, where they are normally seen in thin sections as cross-sections or short lengths of thallus. Detailed study of late Dinantian (Asbian) limestones from two areas of the UK. South Wales and northwest England, show that palaeoberesellids. particularly Kamaenella . are the most important carbonate-producing organisms in shallow. low to moderate energy environments and supplied grains to higher energy environments as a result of storm breakage and transport. Where palaeoberesellids were the dominant organisms they formed low-growing 'thickets' on the sea-floor which trapped fine sediment. to create a bafflestone texture. The late Dinantian was a time of great instability with rapid sea-level changes. Palaeoberesellids were opportunistic organisms which thrived in such an environment. The volume of carbonate produced by these organisms in shallow water may have been a contributory factor in the progradation of shallow marine facies and the establishment of relatively flat-topped shelves from the ramps of the Early Dinantian.     

Oxidants and antioxidants in aquatic animals.

1. Oxidative stress, potentially, is experienced by all aerobic life when antioxidant defenses are overcome by prooxidant forces, and is the basis of many physiological abberations. 2. Environmental contaminants may enhance oxidative stress in aquatic organisms, e.g. highly elevated rates of ideopathic lesions and neoplasia among fish inhabiting polluted environments is increasingly related to oxidative stress associated with environmental pollution. 3. Metabolism of redox cycling xenobiotics in aquatic organisms is very similar to that of mammals suggesting similarities in the health consequences of exposure to such compounds. 4. The expression of specific lesions known to arise specifically from oxidative stress, e.g. lipid peroxidation, oxidized bases in DNA and accumulation of lipofuscin pigments are present in many aquatic animals exposed to contaminants. 5. Aquatic organisms contain the major antioxidant enzymes SOD, catalase and glutathione peroxidase, albeit there are marked quantitative differences among the various species reported.