Experimental Evolution Shows Body Size Decrease in Response to Hypoxia, with a Complex Effect on Plastic Size Response to Temperature.

AbstractThere is a scientific debate whether oxygen concentration may be a factor driving the pattern of size decrease at higher temperature. Central to this debate is the fact that oxygen availability relative to demand for living organisms decreases with increasing temperature. We examined whether rotifers Lecane inermis exposed to hypoxic conditions would evolve smaller sizes than rotifers exposed to normoxic conditions, using experimental evolution with the same fluctuating temperature but differentiated by three regimes of oxygen availability: normoxia, hypoxia throughout the whole thermal range, and hypoxia only at the highest temperature. Immediately after the six-month experiment (more than 90 generations), we tested the plasticity of size responses to temperature in three post-evolution groups, and we related these responses to fitness. The results show that normoxic rotifers had evolved significantly larger sizes than two hypoxic rotifer groups, which were similar in size. All three groups displayed similar plastic body size reductions in response to warming over the range of temperatures they were exposed to during the period of experimental evolution, but they showed different and complex responses at two temperatures below this range. Any type of plastic response to different temperatures resulted in a similar fitness pattern across post-evolution groups. We conclude that (i) these rotifers showed a genetic basis for the pattern of size decrease following evolution under both temperature-dependent and temperature-independent hypoxia; and (ii) plastic body size responds consistently to temperatures that are within the thermal range that the rotifers experienced during their evolutionary history, but responses become more noisy at novel temperatures, suggesting the importance of evolutionary responses to reliable environmental cues.

Integrative Approaches to Understanding Organismal Responses to Aquatic Deoxygenation.

AbstractOxygen bioavailability is declining in aquatic systems worldwide as a result of climate change and other anthropogenic stressors. For aquatic organisms, the consequences are poorly known but are likely to reflect both direct effects of declining oxygen bioavailability and interactions between oxygen and other stressors, including two-warming and acidification-that have received substantial attention in recent decades and that typically accompany oxygen changes. Drawing on the collected papers in this symposium volume ("An Oxygen Perspective on Climate Change"), we outline the causes and consequences of declining oxygen bioavailability. First, we discuss the scope of natural and predicted anthropogenic changes in aquatic oxygen levels. Although modern organisms are the result of long evolutionary histories during which they were exposed to natural oxygen regimes, anthropogenic change is now exposing them to more extreme conditions and novel combinations of low oxygen with other stressors. Second, we identify behavioral and physiological mechanisms that underlie the interactive effects of oxygen with other stressors, and we assess the range of potential organismal responses to oxygen limitation that occur across levels of biological organization and over multiple timescales. We argue that metabolism and energetics provide a powerful and unifying framework for understanding organism-oxygen interactions. Third, we conclude by outlining a set of approaches for maximizing the effectiveness of future work, including focusing on long-term experiments using biologically realistic variation in experimental factors and taking truly cross-disciplinary and integrative approaches to understanding and predicting future effects.

Body size variability across habitats in the Brachionus plicatilis cryptic species complex.

The body size response to temperature is one of the most recognizable but still poorly understood ecological phenomena. Other covarying environmental factors are frequently invoked as either affecting the strength of that response or even driving this pattern. We tested the body size response in five species representing the Brachionus plicatilis cryptic species complex, inhabiting 10 brackish ponds with different environmental characteristics. Principal Component Analysis selected salinity and oxygen concentration as the most important factors, while temperature and pH were less influential in explaining variation of limnological parameters. Path analysis showed a positive interclonal effect of pH on body size. At the interspecific level, the size response was species- and factor-dependent. Under the lack of a natural thermo-oxygenic relationship, the negative response of size to temperature, expected according to 'size-to-temperature response' rules, disappeared, but a positive response of size to oxygen, expected according to predictions selecting oxygen as a factor actually driving these rules, remained. Our results confirm the crucial role of oxygen in determining the size-to-temperature patterns observed in the field.

Phenology and production of pollen, nectar, and sugar in 1612 plant species from various environments.

To predict the quantity and quality of food available to pollinators in various landscapes over time, it is necessary to collect detailed data on the pollen, nectar, and sugar production per unit area and the flowering phenology of plants. Similar data are needed to estimate the contribution of plants to the functioning of food webs via the flow of energy and nutrients through the soil-plant-nectar/pollen-consumer pathway. Current knowledge on this topic is fragmented. This database represents the first compilation of data on the various food resources produced by 1612 plant species belonging to 755 genera and 133 families, including crop plants and wild plants, annuals and perennials, animal- and wind-pollinated plants, and weeds and trees growing in different ecosystems under various environmental conditions. The data set consists of 103 parameters related to the traits of plant species and geographical and environmental factors, allowing for precise calculations of the amounts of nectar, pollen, and energy provided by plants and available to consumers in the considered flora or ecosystem on a daily basis throughout the year. These parameters, gathered by us and extracted from the available literature, describe pollen, nectar, and sugar production (where applicable, in mass, volume, and concentration units), honey yield, the timing and duration of flowering, flower longevity, number of plants and flowers per unit area, weather conditions (temperature and precipitation), geographical location, landscape, and syntaxonomy. The data were obtained from various, mostly European, pedoclimatic zones, and the majority of the data were available for plant species and communities present in Central Europe, especially in Poland, where research on floral resources has a long tradition. These data are representative of the whole continent and may be used as a reference for plant communities occurring on continents other than Europe since the database allows for the consideration of differences in the production of resources by a single plant species growing in different communities. This data set provides a unique opportunity to test hypotheses related to the functioning of food webs, nutrient cycling, plant ecology, and pollinator ecology and conservation. The data are released under a CC-BY-NC-SA license, and this paper must be properly cited when using the database.

Aerobic scope does matter in the temperature-size rule, but only under optimal conditions.

We united theoretical predictions of the factors responsible for the evolutionary significance of the temperature-size rule (TSR). We assumed that (i) the TSR is a response to temperature-dependent oxic conditions, (ii) body size decrease is a consequence of cell shrinkage in response to hypoxia, (iii) this response enables organisms to maintain a wide scope for aerobic performance, and (iv) it prevents a decrease in fitness. We examined three clones of the rotifer Lecane inermis exposed to three experimental regimes: mild hypoxia, severe hypoxia driven by too high of a temperature, and severe hypoxia driven by an inadequate oxygen concentration. We compared the following traits in normoxia- and hypoxia-exposed rotifers: nuclear size (a proxy for cell size), body size, specific dynamic action (SDA, a proxy of aerobic metabolism) and two fitness measures, the population growth rate and eggs/female ratio. The results showed that (i) under mildly hypoxic conditions, our causative reasoning was correct, except that one of the clones decreased in body size without a decrease in nuclear size, and (ii) in more stressful environments, rotifers exhibited clone- and condition-specific responses, which were equally successful in terms of fitness levels. Our results indicate the importance of the testing conditions. The important conclusions were that (i) a body size decrease at higher temperatures enabled the maintenance of a wide aerobic scope under clone-specific, thermally optimal conditions, and (ii) this response was not the only option to prevent fitness reduction under hypoxia.

Diversity and function of the microbial community under strong selective pressure of rotifers.

We unearthed some interesting microecological discoveries while selecting for the most beneficial bacterial strains to be used as probiotics in Lecane inermis rotifer mass culture. For 3 years, we maintained the cultures of L. inermis, with selection for the highest growth rate and resistance to potential contamination. Then, we conducted further selection and isolation in two groups: rotifers inoculated with the bacterial consortium isolated from the rotifer cultures, and rotifers fed with a commercial bioproduct. Selection was conducted in demanding conditions, with particulate matter suspended in spring water as a substrate, without aeration and under strong consumer pressure, and led to selection of two cultivable strains isolated from the optimal rotifers culture. According to molecular analysis, these strains were Aeromonas veronii and Pseudomonas mosselii. Biolog® ECO plate tests showed that both investigated bacterial communities metabolized wide but similar range of substrates. Therefore, intensely selective conditions led to considerable reduction in bacterial community regarding taxonomy, but not in metabolic activity, showing a functional composition decoupling. Aside from this result, our novel selection method dedicated to the sustainable culture of two trophic levels, a directed selection procedure (DSC), could potentially lead to the development of biotechnologically valuable strains with high metabolic activity and the ability to metabolize different sorts of substrate without harmful impact on higher trophic levels.

What may a fussy creature reveal about body/cell size integration under stressful conditions?

There is a growing amount of empirical evidence on the important role of cell size in body size adjustment in ambient or changing conditions. Though the adaptive significance of their correspondence is well understood and demonstrated, the proximate mechanisms are still in a phase of speculation. We made interesting observations on body/cell size adjustment under stressful conditions during an experiment designed for another purpose. We found that the strength of the body/cell size match is condition-dependent. Specifically, it is stronger under more stressful conditions, and it changes depending on exposure to lower temperature vs. exposure to higher temperature. The question whether these observations are of limiting or adaptive character remains open; yet, according to our results, both versions are possible but may differ in response to stress caused by too low vs. too high temperatures. Our results suggest that testing the hypotheses on body/cell size match may be a promising study system for the recent scientific dispute on the evolutionary meaning of developmental noise as opposed to phenotypic plasticity.

Interaction Between a Bacterivorous Ciliate Aspidisca cicada and a Rotifer Lecane inermis: Doozers and Fraggles in Aquatic Flocs.

Activated sludge is a semi-natural habitat composed of macroaggregates made by flocculating bacteria and inhabited by numerous protozoans and metazoans, creating a complicated interactome. The activated sludge resembles the biological formation of naturally occurring floc habitats, such as "marine snow." So far, these two types of habitat have been analyzed separately, despite their similarities. We examined the effect of a bacterivorous ciliate, Aspidisca cicada, on the quality of the macroaggregate ecosystem by estimating (i) the floc characteristics, (ii) the proliferation of other bacterivores (rotifers), and (iii) the chemical processes. We found that A. cicada (i) positively affected floc quality by creating flocs of larger size; (ii) promoted the population growth of the rotifer Lecane inermis, an important biological agent in activated sludge systems; and (iii) increased the efficiency of ammonia removal while at the same time improving the oxygen conditions. The effect of A. cicada was detectable long after its disappearance from the system. We therefore claim that A. cicada is a very specialized scavenger of flocs with a key role in floc ecosystem functioning. These results may be relevant to the ecology of any natural and engineered aggregates.

The underestimated role of temperature-oxygen relationship in large-scale studies on size-to-temperature response.

The observation that ectotherm size decreases with increasing temperature (temperature-size rule; TSR) has been widely supported. This phenomenon intrigues researchers because neither its adaptive role nor the conditions under which it is realized are well defined. In light of recent theoretical and empirical studies, oxygen availability is an important candidate for understanding the adaptive role behind TSR. However, this hypothesis is still undervalued in TSR studies at the geographical level. We reanalyzed previously published data about the TSR pattern in diatoms sampled from Icelandic geothermal streams, which concluded that diatoms were an exception to the TSR. Our goal was to incorporate oxygen as a factor in the analysis and to examine whether this approach would change the results. Specifically, we expected that the strength of size response to cold temperatures would be different than the strength of response to hot temperatures, where the oxygen limitation is strongest. By conducting a regression analysis for size response at the community level, we found that diatoms from cold, well-oxygenated streams showed no size-to-temperature response, those from intermediate temperature and oxygen conditions showed reverse TSR, and diatoms from warm, poorly oxygenated streams showed significant TSR. We also distinguished the roles of oxygen and nutrition in TSR. Oxygen is a driving factor, while nutrition is an important factor that should be controlled for. Our results show that if the geographical or global patterns of TSR are to be understood, oxygen should be included in the studies. This argument is important especially for predicting the size response of ectotherms facing climate warming.

'Optimal thermal range' in ectotherms: Defining criteria for tests of the temperature-size-rule.

Thermal performance curves for population growth rate r (a measure of fitness) were estimated over a wide range of temperature for three species: Coleps hirtus (Protista), Lecane inermis (Rotifera) and Aeolosoma hemprichi (Oligochaeta). We measured individual body size and examined if predictions for the temperature-size rule (TSR) were valid for different temperatures. All three organisms investigated follow the TSR, but only over a specific range between minimal and optimal temperatures, while maintenance at temperatures beyond this range showed the opposite pattern in these taxa. We consider minimal and optimal temperatures to be species-specific, and moreover delineate a physiological range outside of which an ectotherm is constrained against displaying size plasticity in response to temperature. This thermal range concept has important implications for general size-temperature studies. Furthermore, the concept of 'operating thermal conditions' may provide a new approach to (i) defining criteria required for investigating and interpreting temperature effects, and (ii) providing a novel interpretation for many cases in which species do not conform to the TSR.

The Temperature-Size Rule in Lecane inermis (Rotifera) is adaptive and driven by nuclei size adjustment to temperature and oxygen combinations.

The evolutionary implications of the Temperature-Size Rule (TSR) in ectotherms is debatable; it is uncertain whether size decrease with temperature increase is an adaptation or a non-adaptive by-product of some temperature-dependent processes. We tested whether (i) the size of the rotifer Lecane inermis affects fecundity in a way that depends on the combination of low or high temperature and oxygen content and (ii) the proximate mechanism underlying TSR in this species is associated with nuclei size adjustment (a proxy of cell size). Small-type and large-type rotifers were obtained by culturing at different temperatures prior to the experiment and then exposed to combinations of two temperature and two oxygen conditions. Fecundity was estimated and used as a measure of fitness. Nuclei and body sizes were measured to examine the response to both environmental factors tested. The results show the following for L. inermis. (i) Body size affects fecundity in response to both temperature and oxygen, supporting a hypothesis regarding the contribution of oxygen in TSR. (ii) Large individuals are generally more fecund than small ones; however, under a combination of high temperature and poor oxygen conditions, small individuals are more fecund than large ones, in accordance with a hypothesis of the adaptive significance of TSR. (iii) The body size response to temperature is realised by nuclei size adjustment. (iv) Nuclei size changes in response to temperature and oxygen conditions, in agreement with hypotheses on the cellular mechanism underlying TSR and on a contribution of oxygen availability in TSR. These results serve as empirical evidence for the adaptive significance of TSR and validation of the cellular mechanism for the observed response.

Seasonal changes in the body size of two rotifer species living in activated sludge follow the Temperature-Size Rule.

Temperature-Size Rule (TSR) is a phenotypic body size response of ectotherms to changing temperature. It is known from the laboratory studies, but seasonal patterns in the field were not studied so far. We examined the body size changes in time of rotifers inhabiting activated sludge. We hypothesize that temperature is the most influencing parameter in sludge environment, leading sludge rotifers to seasonally change their body size according to TSR, and that oxygen content also induces the size response. The presence of TSR in Lecane inermis rotifer was tested in a laboratory study with two temperature and two food-type treatments. The effect of interaction between temperature and food was significant; L. inermis followed TSR in one food type only. The seasonal variability in the body sizes of the rotifers L. inermis and Cephalodella gracilis was estimated by monthly sampling and analyzed by multiple regression, in relation to the sludge parameters selected as the most influential by multivariate analysis, and predicted to alter rotifer body size (temperature and oxygen). L. inermis varied significantly in size throughout the year, and this variability is explained by temperature as predicted by the TSR, but not by oxygen availability. C. gracilis also varied in size, though this variability was explained by both temperature and oxygen. We suggest that sludge age acts as a mortality factor in activated sludge. It may have a seasonal effect on the body size of L. inermis and modify a possible effect of oxygen. Activated sludge habitat is driven by both biological processes and human regulation, yet its resident organisms follow general evolutionary rule as they do in other biological systems. The interspecific response patterns differ, revealing the importance of taking species-specific properties into account. Our findings are applicable to sludge properties enhancement through optimizing the conditions for its biological component.

Bioenergetic strategy of a xylem-feeder.

To understand the efficiency of energy flow through an organism living in a nutrient poor environment, the bioenergetics of a xylem-feeding beetle Aredolpona rubra was investigated. The larvae of different ages were kept at a constant high, constant low and seasonally variable temperature or in agar plates and agar+nitrogen plates. Bioenergetic parameters were measured during the course of 1 year. The results showed (i) a very strong influence of food moisture on the bioenergetic parameters of A. rubra, (ii) the influence of temperature depends on whether it is fluctuating or constant, (iii) opposite mechanisms regulate growth in a shortage of water and at a suboptimal temperature: in the former case, consumption does not change while the metabolic rate decreases, and at a suboptimal temperature the metabolic rate is dictated by temperature and the consumption rate is altered and (iv) a nitrogen-rich diet results in a decreased metabolic rate, suggesting the existence of energetically costly adaptations to low quality wood as a food source. The study results have broad implications for environmental influences on insect life histories.