Andrew P. Ouimette

Hydrothermal element fluxes from Copahue, Argentina: A “beehive” volcano in turmoil

Copahue volcano erupted altered rock debris, siliceous dust, pyroclastic sulfur, and rare juvenile fragments between 1992 and 1995, and magmatic eruptions occurred in July– October 2000. Prior to 2000, the Copahue crater lake, acid hot springs, and rivers carried acid brines with compositions that reflected close to congruent rock dissolution. The ratio between rock-forming elements and chloride in the central zone of the volcano-hydrothermal system has diminished over the past few years, reflecting increased water/rock ratios as a result of progressive rock dissolution. Magmatic activity in 2000 provided fresh rocks for the acid fluids, resulting in higher ratios between rock-forming elements and chloride in the fluids and enhanced Mg fluxes. The higher Mg fluxes started several weeks prior to the eruption. Model data on the crater lake and river element flux determinations indicate that Copahue volcano was hollowed out at a rate of about 20 000–25 000 m 3 /yr, but that void space was filled with about equal amounts of silica and liquid elemental sulfur. The extensive rock dissolution has weakened the internal volcanic structure, making flank collapse a volcanic hazard at Copahue.

Conservation of biotrophy in Hygrophoraceae inferred from combined stable isotope and phylogenetic analyses.

The nutritional modes of genera in Hygrophoraceae (Basidiomycota: Agaricales), apart from the ectomycorrhizal Hygrophorus and lichen-forming taxa, are uncertain. New δ15N and δ13C values were obtained from 15 taxa under Hygrophoraceae collected in central Massachusetts and combined with isotopic datasets from five prior studies including a further 12 species using a data standardization method to allow cross-site comparison. Based on these data, we inferred the probable nutritional modes for species of Hygrophorus, Hygrocybe, Humidicutis, Cuphophyllus and Gliophorus. A phylogeny of Hygrophoraceae was constructed by maximum likelihood analysis of nuclear ribosomal 28S and 5.8S sequences and standardized δ15N and δ13C values were used for parsimony optimization on this phylogeny. Our results supported a mode of biotrophy in Hygrocybe, Humidicutis, Cuphophyllus and Gliophorus quantitatively unlike that in more than 450 other fungal taxa sampled in the present and prior studies. Parsimony optimization of stable isotope data suggests moderate conservation of nutritional strategies in Hygrophoraceae and a single switch to a predominantly ectomycorrhizal life strategy in the lineage leading to Hygrophorus. We conclude that Hygrophoraceae of previously unknown nutritional status are unlikely to be saprotrophs and are probably in symbiosis with bryophytes or other understory plants.

Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus sylvestris

Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ15N signatures. Here, we studied how biomass allocation and δ15N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% per day or 4% per day relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater (Laccaria laccata) or lesser (Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ15N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ15N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ15N and fungal biomass on root tips were positively correlated in ammonium treatments (r2 = 0.52) but not in nitrate treatments (r2 = 0.00). Fungal biomass on root tips was enriched in 15N an estimated 6–8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ15N. We conclude that: (1) transfer of 15N-depleted N from mycorrhizal fungi to plants produces low plant δ15N signatures and high root tip and fungal δ15N signatures; (2) limited nitrate reduction in fungi restricted transfer of 15N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ15N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without 15N fractionation. These factors probably control plant δ15N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants.

Fungal functioning in a pine forest: evidence from a 15N-labeled global change experiment

• We used natural and tracer nitrogen (N) isotopes in a Pinus taeda free air CO₂ enrichment (FACE) experiment to investigate functioning of ectomycorrhizal and saprotrophic fungi in N cycling. • Fungal sporocarps were sampled in 2004 (natural abundance and (15) N tracer) and 2010 (tracer) and δ(15)N patterns were compared against litter and soil pools. • Ectomycorrhizal fungi with hydrophobic ectomycorrhizas (e.g. Cortinarius and Tricholoma) acquired N from the Oea horizon or deeper. Taxa with hydrophilic ectomycorrhizas acquired N from the Oi horizon (Russula and Lactarius) or deeper (Laccaria, Inocybe, and Amanita). (15)N enrichment patterns for Cortinarius and Amanita in 2010 did not correspond to any measured bulk pool, suggesting that a persistent pool of active organic N supplied these two taxa. Saprotrophic fungi could be separated into those colonizing pine cones (Baeospora), wood, litter (Oi), and soil (Ramariopsis), with δ(15)N of taxa reflecting substrate differences. (15)N enrichment between sources and sporocarps varied across taxa and contributed to δ(15)N patterns. • Natural abundance and (15)N tracers proved useful for tracking N from different depths into fungal taxa, generally corresponded to literature estimates of fungal activity within soil profiles, and provided new insights into interpreting natural abundance δ(15)N patterns.

Tracing metabolic pathways of lipid biosynthesis in ectomycorrhizal fungi from position-specific 13C-labelling in glucose.

Six position-specific (13)C-labelled isotopomers of glucose were supplied to the ectomycorrhizal fungi Suillus pungens and Tricholoma flavovirens. From the resulting distribution of (13)C among fungal PLFAs, the overall order and contribution of each glucose atom to fatty acid (13)C enrichment was: C6 (approximately 31%) > C5 (approximately 25%) > C1 (approximately 18%) > C2 (approximately 18%) > C3 (approximately 8%) > C4 (approximately 1%). These data were used to parameterize a metabolic model of the relative fluxes from glucose degradation to lipid synthesis. Our data revealed that a higher amount of carbon is directed to glycolysis than to the oxidative pentose phosphate pathway (60% and 40% respectively) and that a significant part flows through these pathways more than once (73%) due to the reversibility of some glycolysis reactions. Surprisingly, 95% of carbon cycled through glyoxylate prior to incorporation into lipids, possibly to consume the excess of acetyl-CoA produced during fatty acid turnover. Our approach provides a rigorous framework for analysing lipid biosynthesis in fungi. In addition, this approach could ultimately improve the interpretation of isotopic patterns at natural abundance in field studies.

Stable Isotopes and Radiocarbon Assess Variable Importance of Plants and Fungi in Diets of Arctic Ground Squirrels

ABSTRACT Arctic ground squirrels (Urocitellus parryii) rely primarily on dietary protein derived from plants to fuel gluconeogenesis during hibernation, yet fungal sporocarps may be an important, yet overlooked, protein source. Fungivory levels depend on sporocarp productivity, which varies with the dominant plant species and is higher on acidic than on non-acidic soils. To test whether these factors altered fungal consumption, we used stable isotopes to investigate arctic ground squirrel diets at two sites in northern Alaska, Toolik (primarily moist acidic tundra) and Atigun (primarily moist non-acidic tundra). Radiocarbon estimates can also indicate fungivory levels because ectomycorrhizal fungi assimilate soil-derived organic nitrogen whose 14C levels are higher than current photosynthesis. We measured radiocarbon in hair and δ13C and δ15N in hair, feces, ectomycorrhizal sporocarps, graminoids, and dicots. Feces were higher in δ13C and δ15N at Toolik than at Atigun, and fecal δ15N increased in August at Toolik, coincident with sporocarp production and fungal spores in feces. Mixing models indicated that graminoids contributed 64%, dicots 35%, and sporocarps 1% to Atigun hair protein, whereas graminoids contributed 37%, dicots 16%, and sporocarps 47% to Toolik hair protein. Acidic soils appeared to correlate with higher sporocarp production and fungivory at Toolik than at Atigun. Atigun hair resembled atmospheric CO2 in 14C, whereas Toolik hair had higher 14C, consistent with greater fungal consumption at Toolik. Late-season sporocarps may be a key protein source for some squirrels and may provide an integrated signal of the soil organic nitrogen assimilated by ectomycorrhizal fungi.