Seasonal variation and potential roles of dark septate fungi in an arid grassland

High temperatures and extended drought in temperate and tropical arid ecosystems promote the colonization of diverse microenvironments by dark septate fungi (DSF). These fungi contribute to soil nutrient cycling, soil stabilization, and plant survival, but the roles of individual DSF species, their distributions, and their community diversity are poorly understood. The objective of this study was to evaluate the distribution, seasonal variation, and potential roles of DSF on plant growth. We collected biocrust (lichen-, moss-, and cyanobacterium-dominated biocrusts) soils at different depths and rhizosphere soils from two grasses, Bromus tectorum and Pleuraphis jamesii, in an arid grassland near Moab, Utah, USA. Seasonal variation of DSF was evaluated using culture-based approaches and compared with fungal community profiles from next-generation sequencing (NGS). Culturing showed that DSF were 30% more abundant in biocrusts compared with the focal rhizospheres. The abundance of DSF varied seasonally in belowground samples (rhizosphere and below-biocrust), with a significant increase during the summer months. Pleosporales was the dominant order (35%) in both biocrust and rhizosphere soils out of 817 isolated fungi. Dominant DSF genera in culture included Alternaria, Preussia, Cladosporium, Phoma, and an unknown Pleosporales. Similar results were observed in biocrust and rhizosphere soils NGS. Further, seed germination experiments using dominant taxa were conducted to determine their potential roles on germination and seedling growth using maize as a model plant. Cladosporium and unknown Pleosporales isolates showed plant growth–promoting ability. The variation in abundance of DSF, their differential occurrence in different microenvironments, and their ability to grow in a xerotolerant medium reflect adaptations to summer environmental conditions and to changes in the abundance of organic matter, as well as a potential increase in plant investment in these fungi when heat and drought stresses are more severe.温带与热带干旱生态系统中的高温与长期干旱，会促进暗色有隔真菌（dark septate fungi，DSF）在各类微生境中定殖。这类真菌参与土壤养分循环、土壤固持与植物存活，但学界对单个DSF物种的功能、其分布格局及群落多样性的认知仍十分有限。本研究旨在探明DSF的分布特征、季节动态及其对植物生长的潜在作用。我们在美国犹他州莫阿布附近的干旱草原中，采集了不同深度的生物结皮（分别以地衣、苔藓及蓝细菌为优势类群的生物结皮）土壤，以及两种草本植物——旱雀麦（Bromus tectorum）与詹姆斯单穗草（Pleuraphis jamesii）的根际土壤。本研究采用分离培养法分析DSF的季节动态，并与下一代测序（next-generation sequencing，NGS）获得的真菌群落特征进行对比。分离培养结果显示，生物结皮中的DSF丰度较目标根际土壤高30%。地下样品（根际土壤及结皮下层土壤）中的DSF丰度存在显著季节波动，夏季月份的丰度显著升高。在817株分离获得的真菌中，格孢菌目（Pleosporales）在生物结皮与根际土壤中均为优势类群，占比达35%。分离培养得到的优势DSF属包括链格孢属（Alternaria）、普雷斯亚霉属（Preussia）、枝孢属（Cladosporium）、茎点霉属（Phoma）以及1株未分类的格孢菌目菌株。下一代测序分析的生物结皮与根际土壤结果也呈现出相似趋势。此外，本研究以玉米为模式植物，利用优势DSF类群开展种子萌发实验，以明确其对种子萌发及幼苗生长的潜在作用。实验结果显示，枝孢属菌株与未分类的格孢菌目菌株具有植物促生能力。DSF的丰度波动、其在不同微生境中的差异化分布，以及其在耐旱培养基中的生长能力，均反映出其对夏季环境条件与有机质丰度变化的适应性；同时也表明，在高温与干旱胁迫加剧时，植物对这类真菌的资源投入可能会有所增加。