Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - STOICHIOMET (Blending Stoichiometric and Metabolic Theories from Genes to Populations: Resource Stochiometry and Temperature Effects on Consumers with Contrasting Life-history Strategies)

Teaser

Biota and ecosystems worldwide are undergoing unprecedented environmental changes. Over the past 100 years, the global temperature has increased 0.6ºC, and is predicted to continue up to 1.8-4ºC. At the same time, anthropogenic perturbations are adding nitrogen (N) and...

Summary

Biota and ecosystems worldwide are undergoing unprecedented environmental changes. Over the past 100 years, the global temperature has increased 0.6ºC, and is predicted to continue up to 1.8-4ºC. At the same time, anthropogenic perturbations are adding nitrogen (N) and phosphorus (P) compounds, strongly affecting the key biogeochemical cycles. Although their single effects are strongly supported, there is an ample debate on the direction of their joint impacts at each biological level. Hence, we are needed of multifactorial and multilevel approaches that consider them simultaneously to increase the reliability of our predictions for species ecology and evolution.

Ecological/Biological Stoichiometry (ES/BS; the study of the balance of energy and multiple chemical elements in ecological interactions/living systems) and Metabolic Theory of Ecology/Biology (MTE/MTB; the study of metabolism as a driving force for ecological patterns/biological processes) have focused on how altered regimes of nutrients and temperature impact, respectively, on consumer growth. Thus, key concepts such as the knife-edge hypothesis (KEH), reporting unimodal responses of growth to nutrients, have greatly advanced our understanding of energy flow and nutrient cycling in ecosystems. Further, both approaches have been particularly interested in how selective pressures on growth have led to changes in metabolism, elemental composition, genome size and structure, and therefore, species evolution. A cornerstone of BS is the growth rate hypothesis (GRH), which proposes that organisms lacking major P storage capacity have elevated demands for increased P allocation to P-rich ribosomal RNA under rapid growth, driving variation in their P content (and therefore C:P and N:P ratios). In addition to this, P-allocation hypothesis (PAH) states that reduced genomes and P allocation from DNA to RNA is the evolutionary consequence of the strong and sustained selection for fast growth in these organisms undergoing P limitation. Both hypotheses establish close connections among individual growth, ribosomal metabolism, elemental composition, and genome size.

Genome size is, at the same time, strongly correlated with cell and body sizes for a wide range of organisms. Because they scale with growth, metabolic, and developmental rates, factors impinging on these trigger far-reaching consequences on life-history strategies, and therefore, evolutionary and ecological processes. Experimental and empirical evidence indicates smaller genome, cell, and body sizes at higher temperatures due to global warming, consistently with the Bergmann-type temperature-size rule. However, these patterns may be masked by other environmental factors such as higher nutrient availability, inducing expectedly larger “sizes”. Thus, it is fundamental to question (i) if the effects of global warming are more intense than those of higher nutrient availability, or quite the opposite; and (ii) if new responses of “sizes” and “rates” emerge when both stressors interact jointly.

STOICHIOMET aimed at studying single and interactive effects of increased nutrients and temperature in keystone species growth to:

I. Test concurrently central concepts of stoichiometric and metabolic theories (KEH, GRH, PAH, Bergmann’s rule).

II. Model the effects of these factors on GRs and coupled traits to integrate both theoretical frameworks.

Work performed

I. GRH in primary producers across thermal, irradiance, and trophic gradients.

GRH has been proven for a wide variety of organisms under diverse ecological conditions, but its validity for photoautotrophs remains elusive. We tested experimentally if GRH was applicable for Chlamydomonas reinhardtii growing under diverse conditions of temperature and irradiance, and assessed the validity of our experimental observations in natural systems. Our results of the regressions among growth rate, protein:RNA, and N:P ratios provide strong evidence for GRH in primary producers as well. Thus, environmental factors operating on autotrophs’ growth rates determine their elemental composition at the base of the food web with major consequences for its architecture and function.

II. Temperature-specific knife-edge responses of freshwater consumer Daphnia to calcium (Ca) gradients.

Lake-water Ca concentrations are currently declining in boreal lakes. Others are reflecting unusually high Ca concentrations in regions increasingly affected by Ca deposition. Organisms in these systems may well be vulnerable to suffer detrimental effects of either Ca limitation or excess. But, its biological impact remains unpredictable in a global warming scenario because of the non-additive nature of environmental stressors. KEH was proposed to define the role of extreme nutrient conditions, reporting unimodal responses of consumer life-history traits to these gradients. Although valid for multiple conditions, there is still no empirical corroboration of KEH for Ca. Thus, we tested experimentally if the temperature-specific response of the keystone grazer Daphnia magna would also satisfy KEH across a Ca gradient. Daphnia magna’s performance in terms of survival, growth or reproduction, was highest at intermediate levels of the Ca gradient, but strongly impinged at both extremes, consistently with KEH. By contrast, temperature effects were mostly given at intermediate levels, being much more detrimental at higher temperatures. Apparently, organisms inhabiting systems not impacted by Ca limitation or oversaturation seem to be more sensitive to global warming-induced higher temperatures. This impact seems to be masked with either declining or saturating Ca in the ecosystem.

III. Growth rate-C:N:P stoichiometry-Genome size couplings in the marine copepod Calanus hyperboreus across natural gradients of nutrients and temperature.

Bergmann-type temperature-size rule predicts that higher temperatures with global warming induce smaller sizes, favouring competitive advantage for fast growth. However, with glacier retreat, nutrients in the ice are being released and available for the plankton community, potentially relaxing selective pressures for fast growth and allowing organisms to reach larger “sizes”. To disentangle how “sizes”, C:N:P stoichiometry and growth rates in the copepod Calanus hyperboreus are performed by the contrasting roles of nutrients and temperature, we conducted a field survey on arctic waters of the Beaufort Sea. In doing so, we are also testing concurrently key concepts of MTB and BS: Bergmann’s rule, GRH and PAH. Results will be available in brief.

Final results

Stoichiometric and metabolic approaches have greatly advanced our understanding of the environmental factors that control ecological and biological processes, but did it separately. Findings of STOICHIOMET project of the effects of nutrients and temperature (energy) on consumers, have theoretically integrated central concepts of both frameworks (GRH, PAH, KEH, Bergmann’s rule). In doing so, it provided empirical evidence for the mechanistic connections of stoichiometric and metabolic principles, contributing to our progress towards a more synthetic theory of Ecology.

Website & more info

More info: https://www.mn.uio.no/ibv/english/people/aca/francijb/.