Japanese beetle (Popilia japonica) copyright enterlinedesign/Adobe Stock

Like a Southern drawl popping up on the West Coast, our accents mark us as newcomers. With time they fade, leaving only traces of our past in the occasional slip of a word.

Northern Arizona University researchers led by Bruce Hungate, director of the Center for Ecosystem Science and Society, have found the microscopic equivalent of an accent in an invasive pest: the Japanese beetle. This microscopic “accent” is the amount of a rare but stable hydrogen isotope in the beetles’ body tissues. The results, published in PLOS ONE, can help invasive pest managers answer the question of whether a beetle detected in new territory is new or part of an established population in the area.

“Knowing the timing of arrival of these invasive organisms can be really helpful in managing them, and the stable isotope gives us a very useful chemical clock,” Hungate said. “It’s a powerful addition to the tools we have to understand where these organisms are from and the dynamics of their movements.”

Japanese beetles wreak havoc by feeding on over 300 plants, contributing to the billions of dollars per year in economic costs caused by invasive species. Japanese beetles are well established in the eastern United States. Control efforts at airports on both coasts aim to keep the beetles from spreading westward, with only partial success.

Researchers studied colonizing beetles trapped at Portland International Airport in Oregon over the past decade to develop this new technique.

The study used isotopes as a sleuthing tool. Hydrogen, like other elements in nature, occurs in slightly different forms, called isotopes. One of the heavier isotopes of hydrogen, deuterium, is rare but stable, meaning it does not decay. The amount of this isotope—its signature—in local water sources varies from place to place, and has been found to match the signature in tissues of plants and animals consuming the local water.

Researchers found a close relationship between the stable hydrogen isotope signature in beetle tissue and local water from 71 sites around the country. Combined with the signatures of water at known sources of Japanese beetles in the East, these results provide a sort of “geographic fingerprint” to determine where the beetle is from.

To model the time since arrival, researchers transplanted Eastern beetles to a Western environment and measured the signature change over time. The hydrogen isotope signature in beetles began to change after two weeks and took about five weeks to equilibrate to the new environment. This offered a new clue: beetles trapped at points of entry to an area, like airports, are likely to be new arrivals if their signature is distinctly different from the signature of water in the local area.

The transplant experiment also asked a more refined question about beetle origins: Would the signature from the hard, chitin-rich tissue of the beetle’s wing covers change more slowly, preserving clues about the beetle’s origin longer, like the trace of accent we retain after we are no longer newcomers? They found that signatures did shift more slowly in hard tissue, adding it as a potential tool for tracing the origin of beetles.

The resulting model pointed to the southeastern United States as the origin of beetles trapped at the Portland International Airport. And beetles trapped after 2011 appeared to have been more recent arrivals than beetles trapped in earlier years, suggesting that efforts to prevent beetles from establishing viable populations at the Portland International Airport seemed to be working.

Reference:
Hungate et al. (2016) PLOS ONE. “Hydrogen Isotopes as a Sentinel of Biological Invasion by the Japanese Beetle, Popillia japonica (Newman).

Blue and orange microbes

Soil is a complex ecosystem with diverse microenvironments ranging from bulk soil with low quality substrates and no or very limited microbial growth, to high quality C-rich environments near decomposing litter and rhizosphere where microbial growth and death rates are high. Soil contains different microbial communities, supports varying microbial activities and carbon (C) and nitrogen (N) availabilities. This research, a collaboration among NAU and two DOE National Labs (PNNL & LLNL) addresses whether the microbial taxa exhibit differences in growth and death rates and underlying biochemistry in these environments, and to what degree these responses are genetically determined or environmentally induced. In this work, we have gathered data to test the overarching hypothesis that microbes in these environments exhibit diverse metabolic capabilities and activities, from gluconeogenesis in bulk soil where mainly small organic acids are present to glycolysis in environments rich with carbohydrates; from amino acid catabolism in bulk soil to amino acid anabolism near plant litter or roots; from K-growth strategy with highly evolved and active secondary metabolism to r-growth strategy with high C availability; and from an emphasis on peroxidases and laccases to decompose recalcitrant materials in bulk soil to cellulases to decompose plant materials.

This work integrates data from the metagenome, transcriptome, proteome, metabolome and fluxome of the microbial communities in bulk soil, soil amended with decomposing litter, and rhizosphere soils. This work also involves combining ¹⁶O and ¹⁸O-water labeling, isopycnic centrifugation, enabling calculation of taxon-specific growth and death rates of the microbial community, and correlating these rates with metabolic capabilities associated with the central C metabolic network, catabolic and anabolic amino acid pathways, extracellular enzymes, and secondary compound metabolism in these three environments (determined by metabolic flux analysis, a technique developed at NAU). Together, these data present the opportunity to link, for the first time, in vivo taxon-specific microbial growth and death rates to metabolic capabilities and activities in strongly contrasting soil environments. Information on microbial growth and death rates, and their metabolic capabilities and activities in undisturbed soil environments is an essential step towards developing more mechanistic soil C cycling models.

Supporting Grants:

Dijkstra, NSF Ecosystem

This research is being conducted by Paul Dijkstra and others.

Microscopic microbes

When new carbon enters soil, especially carbon that is easily assimilated and decomposed by soil microorganisms, a chain reaction occurs leading to the breakdown of older soil carbon, carbon that would otherwise have remained stable. Current theory does not explain this chain reaction, sometimes called the “priming effect.” But understanding this is important, because soil carbon is a major reservoir in the global carbon cycle, storing about three times the amount of carbon contained in the atmosphere as carbon dioxide. Some soil processes promote carbon storage, locking it away in stable forms, resistant to decay. The priming effect has the opposite effect, converting carbon that was thought to be stable to carbon dioxide, and contributing to the atmospheric pool, amplifying rising carbon dioxide due to human burning of coal, oil, and gasoline. Carbon stability is a major uncertainty about the future carbon sink in soil, so this research addresses important questions about the global carbon cycle.

Minerals and soil carbon

Carbon dioxide (CO₂) is released to the atmosphere when humans burn oil, coal, and gasoline, and is the major cause of global warming. Soils can store carbon (C), helping counteract rising carbon dioxide, but the future of the soil C sink is uncertain. Will it be converted to soil organic C, which can stay put for thousands of years, or will soil microorganisms convert it back to CO₂, returning it to the atmosphere? This is a major uncertainty about the future C sink on land. Recent work suggests a surprising response, called the priming effect, in which adding C to soil boosts the metabolism of microorganisms, causing them to produce even more CO₂ than expected.

Yet, this phenomenon is variable and very poorly understood. Proposed mechanisms fail to explain what conditions modulate the occurrence and magnitude of the priming response. Preliminary data suggest that the soil mineral assemblage, reflecting the chemical and geological properties of soil, interacts strongly with the soil microbial community to influence the priming effect. This research will test the idea that the priming response depends on interactions between the soil mineral assemblage and the soil microbial community. Thus, this research lies at the interface among geology, biology, and chemistry. The research will investigate priming responses in nine soils, spanning a broad range of climatic and environmental conditions. Laboratory Projects will evaluate how priming responds to variation in the mineral assemblage, and samples from the projects will be tested for carbon cycling and microbial community characteristics. The work will use state-of-the-art techniques, including in-line isotope-ratio measurements using a cavity ring-down instrument, and new stable isotope probing techniques paired with gene microarrays capable of identifying microorganisms performing specific ecological functions. This project emphasizes integrating research and teaching, will provide interdisciplinary training for undergraduate students at institutions with strong histories of minority enrollment. Students will gain experience with the cutting-edge methods, and with a research field with strong implications for policy decisions surrounding global climate change and carbon management.

Grants supporting this work

• National Science Foundation, Geosciences, Collaborative Research: Biological and mineralogical controls over soil carbon cycling across multiple ecosystems: a focus on the priming effect, $414,075, 9/11 – 8/13

Soil biodiversity and the carbon cycle

Microbial diversity is vast, and recent discoveries place soils as home to the most diverse of the Earth’s microbial communities. Soil microbial diversity spans the tree of life – Bacteria and Archaea, the ancient single-celled life forms important for everything from decomposition to disease, and Eukarya, the part of the tree of life that includes plants and animals, as well as microorganisms like protists and fungi.The overwhelming biodiversity of soil microorganisms motivates the search for understanding its functional significance.

The current project probes a surprising response of microorganisms to changes in soil carbon availability: when new carbon enters soil, especially carbon that is easily assimilated and decomposed by soil microorganisms, a chain reaction occurs leading to the breakdown of older soil carbon, carbon that would otherwise have remained stable. Current theory does not explain this chain reaction.

This project will test whether taxonomic biodiversity and the genetic biodiversity it supports – in other words, who is there and what are they doing – can explain this unusual carbon cycling phenomenon.

Project details

This project explores new dimensions connecting the diversity of the tree of life with the carbon cycle. The work will use long-term study sites in soils spanning a climatic gradient in Arizona.

For this project, carbon cycling will be measured using a cavity ring-down spectrometer to measure decomposition of new and old soil carbon. Taxonomic and genetic biodiversity will be measured using genetic sequencing, focusing on taxonomy of organisms within the tree of life, and genetic potential of organisms using carbon cycle genes.

One feature of the proposed work will be the use of dual stable isotope tracers, which will enable identifying which microorganisms are involved in particular carbon cycling functions. The work will test the idea that parts of the carbon cycle are emergent consequences of interactions among organisms, with biodiversity as a fundamental driver, thereby connecting genes to communities to ecosystems. By focusing on a common yet unexplained carbon cycle phenomenon, the work is well positioned to advance the science of soil biodiversity.

The work is important because soil carbon is a major reservoir in the global carbon cycle, storing about three times the amount of carbon contained in the atmosphere as carbon dioxide. Some soil processes promote carbon storage, locking it away in stable forms, resistant to decay.

The phenomenon addressed in the proposed work has the opposite effect, converting carbon that was thought to be stable to carbon dioxide, and contributing to the atmospheric pool, amplifying rising carbon dioxide due to human burning of coal, oil, and gasoline. Carbon stability is a major uncertainty about the future C sink in soil, so the proposed work addresses important questions both in biodiversity science and in the global carbon cycle.

The project also integrates science research and education. The proposed work will train two PhD students, a postdoctoral research associate, and numerous undergraduates in state-of-the art techniques in carbon cycling and molecular biology applied to biodiversity. The work will extend to K-12 education, adding microbial biodiversity into a project developing curricular materials about the carbon cycle, and informal science through an existing partnership with the Museum of Northern Arizona.

The project will support undergraduate internships to conduct basic research. Students will have unusual access to research at the interface between molecular and ecosystems biology, and will help advance the field by testing basic ecological principles applied to understanding the carbon cycle. The work is designed to engage undergraduate students in the process of science. Microbial biodiversity is the biological template upon which much of the carbon cycle unfolds, yet evidence of how diversity alters the soil carbon cycle remains elusive. This project will address this fundamental knowledge gap.

Grants supporting this work

• National Science Foundation, Dimensions: Collaborative Research: The taxonomic, genomic, and functional diversity of soil carbon dynamics, $1,487,750, 1/13 – 12/17