Elevated atmospheric CO2 stimulates soil fungal diversity through increased fine root production in a semiarid shrubland ecosystem

Long-term free-air-CO2-enrichment increases carbon distribution in the stable fraction in the deep layer of non-clay soils

Elevated CO2 (eCO2) in the atmosphere can increase plant C input into soils. However, in dryland cropping systems, it remains unclear how eCO2 may alter soil organic C content and stability in relation to potential changes in microbial community composition and whether these changes may depend on soil type and depth. Using an eight-year free-air-CO2-enrichment (SoilFACE) system, this study addressed these questions in three farming soils including a sandy Calcarosol, a clay Vertosol and a silt loam Chromosol at depths of 0-40 cm. Long-term eCO2 did not change soil C content or its distribution in different C fractions in the top 30-cm soil. The majority of the relatively abundant bacterial taxa significantly affected by eCO2 in the 0-10 cm layer were copiotrophic; this also occurred to fungal community, except for the Calcarosol where some saprotrophs showed a decreasing trend. These changes in microbial taxa indicate that eCO2 accelerated the decomposition of both new and pre-existing C pools in the topsoil. Although eCO2 did not change soil C content in the 30-40 cm layer, it increased soil C content in the stable C fraction associated with particles < 50 μm in the Calcarosol (by 39%) and particles < 2 μm in the Chromosol (by 29%). In the 30-40 cm layer of the Calcarosol, many fungal saprotrophs were enriched, and the abundance of fungal community increased under eCO2. Further investigation is warranted on whether the enhanced stability subsoil C under eCO2 results from the leaching of stable organic molecules from the topsoil to the subsoil for buildup in the non-clay Calcarosol and Chromosol. Overall, these findings suggest that eCO2 is likely to enhance soil C stability in the deeper parts of the profile of non-clay soils.

Fungal communities in soils under global change

Soil fungi play indispensable roles in all ecosystems including the recycling of organic matter and interactions with plants, both as symbionts and pathogens. Past observations and experimental manipulations indicate that projected global change effects, including the increase of CO₂ concentration, temperature, change of precipitation and nitrogen (N) deposition, affect fungal species and communities in soils. Although the observed effects depend on the size and duration of change and reflect local conditions, increased N deposition seems to have the most profound effect on fungal communities. The plant-mutualistic fungal guilds - ectomycorrhizal fungi and arbuscular mycorrhizal fungi - appear to be especially responsive to global change factors with N deposition and warming seemingly having the strongest adverse effects. While global change effects on fungal biodiversity seem to be limited, multiple studies demonstrate increases in abundance and dispersal of plant pathogenic fungi. Additionally, ecosystems weakened by global change-induced phenomena, such as drought, are more vulnerable to pathogen outbreaks. The shift from mutualistic fungi to plant pathogens is likely the largest potential threat for the future functioning of natural and managed ecosystems. However, our ability to predict global change effects on fungi is still insufficient and requires further experimental work and long-term observations. Citation: Baldrian P, Bell-Dereske L, Lepinay C, Větrovský T, Kohout P (2022). Fungal communities in soils under global change. Studies in Mycology 103: 1-24. doi: 10.3114/sim.2022.103.01.

Taxonomic revision of the genus Zygorhizidium: Zygorhizidiales and Zygophlyctidales ord. nov. (Chytridiomycetes, Chytridiomycota)

During the last decade, the classification system of chytrids has dramatically changed based on zoospore ultrastructure and molecular phylogeny. In contrast to well-studied saprotrophic chytrids, most parasitic chytrids have thus far been only morphologically described by light microscopy, hence they hold great potential for filling some of the existing gaps in the current classification of chytrids. The genus Zygorhizidium is characterized by an operculate zoosporangium and a resting spore formed as a result of sexual reproduction in which a male thallus and female thallus fuse via a conjugation tube. All described species of Zygorhizidium are parasites of algae and their taxonomic positions remain to be resolved. Here, we examined morphology, zoospore ultrastructure, host specificity, and molecular phylogeny of seven cultures of Zygorhizidium spp. Based on thallus morphology and host specificity, one culture was identified as Z. willei parasitic on zygnematophycean green algae, whereas the others were identified as parasites of diatoms, Z. asterionellae on Asterionella, Z. melosirae on Aulacoseira, and Z. planktonicum on Ulnaria (formerly Synedra). According to phylogenetic analysis, Zygorhizidium was separated into two distinct order-level novel lineages; one lineage was composed singly of Z. willei, which is the type species of the genus, and the other included the three species of diatom parasites. Zoospore ultrastructural observation revealed that the two lineages can be distinguished from each other and both possess unique characters among the known orders within the Chytridiomycetes. Based on these results, we accommodate the three diatom parasites, Z. asterionellae, Z. melosirae, and Z. planktonicum in the distinct genus Zygophlyctis, and propose two new orders: Zygorhizidiales and Zygophlyctidales.

Effects of elevated CO2 and endophytic bacterium on photosynthetic characteristics and cadmium accumulation in Sedum alfredii

Elevated CO₂ and use of endophytic microorganisms have been considered as efficient and novel ways to improve phytoextraction efficiency. However, the interactive effects of elevated CO₂ and endophytes on hyperaccumulator is poorly understood. In this study, a hydroponics experiment was conducted to investigate the combined effect of elevated CO₂ (eCO₂) and inoculation with endophyte SaMR12 (ES) on the photosynthetic characteristics and cadmium (Cd) accumulation in hyperaccumulator Sedum alfredii. The results showed that eCO₂ × ES interaction promoted the growth of S. alfredii, shoot and root biomass net increment were increased by 264.7 and 392.3%, respectively, as compared with plants grown in ambient CO₂ (aCO₂). The interaction of eCO₂ and ES significantly (P < 0.05) increased chlorophyll content (53.2%), Pn (111.6%), Pn_max (59.8%), AQY (65.1%), and Lsp (28.8%), but reduced Gs, Tr, Rd, and Lcp. Increased photosynthetic efficiency was associated with higher activities of rubisco, Ca²⁺-ATPase, and Mg²⁺-ATPase, and linked with over-expression of two photosystem related genes (SaPsbS and SaLhcb2). PS II activities were significantly (P < 0.05) enhanced with Fv/Fm and Φ(II) increased by 12.3 and 13.0%, respectively, compared with plants grown in aCO₂. In addition, the net uptake of Cd in the shoot and root tissue of S. alfredii grown in eCO₂ × ES treatment was increased by 260.7 and 434.9%, respectively, due to increased expression of SaHMA2 and SaCAX2 Cd transporter genes. Our results suggest that eCO₂ × ES can promote the growth of S. alfredii due to increased photosynthetic efficiency, and improve Cd accumulation and showed considerable potential of improving the phytoextraction ability of Cd by S. alfredii.

The Response Patterns of Arbuscular Mycorrhizal and Ectomycorrhizal Symbionts Under Elevated CO2: A Meta-Analysis

Elevated carbon dioxide (eCO2), a much-discussed topic in global warming, influences development and functions of mycorrhizal fungi and plants. However, due to the inconsistent results reported in various publications, the response patterns of symbionts associated with the arbuscular mycorrhizal (AM) or with ectomycorrhizal (ECM) fungi to eCO2 remains still unclear. Therefore, we performed a meta-analysis to identify how eCO2 affected mycorrhizal fungi and if there is a significant different response between AM and ECM symbionts. Our results demonstrated that eCO2 increased mycorrhizal plants biomass (+26.20%), nutrient contents [+2.45% in nitrogen (N), and +10.66% in phosphorus (P)] and mycorrhizal fungal growth (+22.87% in extraradical hyphal length and +21.77% in mycorrhizal fungal biomass), whereas plant nutrient concentrations decreased (-11.86% in N and -12.01% in P) because the increase in plant biomass was greater than that in nutrient content. The AM plants exhibited larger increases in their biomass (+33.90%) and in their N (+21.99%) and P contents (+19.48%) than did the ECM plants (+20.57% in biomass, -4.28% in N content and -13.35% in P content). However, ECM fungi demonstrated increased responses of mycorrhizal fungal biomass (+29.98%) under eCO2 compared with AM fungi (+6.61%). These data indicate different patterns in the growth of AM and ECM symbionts under eCO2: AM symbionts contributed more to plant growth, while ECM symbionts were more favorable to mycorrhizal fungal growth. In addition, the responses of plant biomass to eCO2 showed no significant difference between short-term and long-term groups, whereas a significant difference in the responses of mycorrhizal fungal growth was found between the two groups. The addition of N increased plant growth but decreased mycorrhizal fungal abundance, and P addition increased total plant biomass and extraradical hyphal length, but shoot biomass largely increased in low P conditions. Mixtures of mycorrhizal fungi affected the total plant and root biomasses more than a single mycorrhizal fungus. Clarifying the different patterns in AM and ECM symbionts under eCO2 would contribute to a better understanding of the interactions between mycorrhizal fungi and plant symbionts under the conditions of global climate change as well as of the coevolution of flora with Earth's environment.

Diversity and Hidden Host Specificity of Chytrids Infecting Colonial Volvocacean Algae

Chytrids are zoosporic fungi that play an important, but yet understudied, ecological role in aquatic ecosystems. Many chytrid species have been morphologically described as parasites on phytoplankton. However, the majority of them have rarely been isolated and lack DNA sequence data. In this study we isolated and cultivated three parasitic chytrids, infecting a common volvocacean host species, Yamagishiella unicocca. To identify the chytrids, we characterized morphology and life cycle, and analyzed phylogenetic relationships based on 18S and 28S rDNA genes. Host range and specificity of the chytrids was determined by cross-infection assays with host strains, characterized by rbcL and ITS markers. We were able to confirm the identity of two chytrid strains as Endocoenobium eudorinae Ingold and Dangeardia mamillata Schröder and described the third chytrid strain as Algomyces stechlinensis gen. et sp. nov. The three chytrids were assigned to novel and phylogenetically distant clades within the phylum Chytridiomycota, each exhibiting different host specificities. By integrating morphological and molecular data of both the parasitic chytrids and their respective host species, we unveiled cryptic host-parasite associations. This study highlights that a high prevalence of (pseudo)cryptic diversity requires molecular characterization of both phytoplankton host and parasitic chytrid to accurately identify and compare host range and specificity, and to study phytoplankton-chytrid interactions in general.

Higher fungal diversity is correlated with lower CO2 emissions from dead wood in a natural forest

Wood decomposition releases almost as much CO₂ to the atmosphere as does fossil-fuel combustion, so the factors regulating wood decomposition can affect global carbon cycling. We used metabarcoding to estimate the fungal species diversities of naturally colonized decomposing wood in subtropical China and, for the first time, compared them to concurrent measures of CO₂ emissions. Wood hosting more diverse fungal communities emitted less CO₂, with Shannon diversity explaining 26 to 44% of emissions variation. Community analysis supports a ‘pure diversity’ effect of fungi on decomposition rates and thus suggests that interference competition is an underlying mechanism. Our findings extend the results of published experiments using low-diversity, laboratory-inoculated wood to a high-diversity, natural system. We hypothesize that high levels of saprotrophic fungal biodiversity could be providing globally important ecosystem services by maintaining dead-wood habitats and by slowing the atmospheric contribution of CO₂ from the world’s stock of decomposing wood. However, large-scale surveys and controlled experimental tests in natural settings will be needed to test this hypothesis.

Resilience of Fungal Communities to Elevated CO2

Soil filamentous fungi play a prominent role in regulating ecosystem functioning in terrestrial ecosystems. This necessitates understanding their responses to climate change drivers in order to predict how nutrient cycling and ecosystem services will be influenced in the future. Here, we provide a quantitative synthesis of ten studies on soil fungal community responses to elevated CO2. Many of these studies reported contradictory diversity responses. We identify the duration of the study as an influential parameter that determines the outcome of experimentation. Our analysis reconciles the existing globally distributed experiments on fungal community responses to elevated CO2 and provides a framework for comparing results of future CO2 enrichment studies.

Fungal Community Responses to Past and Future Atmospheric CO2 Differ by Soil Type

Soils sequester and release substantial atmospheric carbon, but the contribution of fungal communities to soil carbon balance under rising CO2 is not well understood. Soil properties likely mediate these fungal responses but are rarely explored in CO2 experiments. We studied soil fungal communities in a grassland ecosystem exposed to a preindustrial-to-future CO2 gradient (250 to 500 ppm) in a black clay soil and a sandy loam soil. Sanger sequencing and pyrosequencing of the rRNA gene cluster revealed that fungal community composition and its response to CO2 differed significantly between soils. Fungal species richness and relative abundance of Chytridiomycota (chytrids) increased linearly with CO2 in the black clay (P < 0.04, R(2) > 0.7), whereas the relative abundance of Glomeromycota (arbuscular mycorrhizal fungi) increased linearly with elevated CO2 in the sandy loam (P = 0.02, R(2) = 0.63). Across both soils, decomposition rate was positively correlated with chytrid relative abundance (r = 0.57) and, in the black clay soil, fungal species richness. Decomposition rate was more strongly correlated with microbial biomass (r = 0.88) than with fungal variables. Increased labile carbon availability with elevated CO2 may explain the greater fungal species richness and Chytridiomycota abundance in the black clay soil, whereas increased phosphorus limitation may explain the increase in Glomeromycota at elevated CO2 in the sandy loam. Our results demonstrate that soil type plays a key role in soil fungal responses to rising atmospheric CO2.