INTRODUCTION

Forests over limestone, or more popularly known as karst ecosystems, are one of the most ecologically fragile ecosystems in the world. These landscapes are threatened by anthropogenic activities and natural processes. Karst ecosystems are typically thin, coarse, highly erosive and degenerative (He et al., 2008). Nearly 25% of the world’s population lives in karst areas (Fleury, 2009; Drew, 2017). In the Philippines, it covers about 30,000 km2 or 10% of the country’s total land area, and a large portion of it is not yet studied and protected. Karst soils have excessive amounts of exchangeable calcium that limit the growth of vegetation. The adaptive capability of plants to grow in such an environment, partially or completely comes from symbiotic or associative microorganisms (Li et al., 2018). Most research in karst ecosystems is aimed at their geomorphology, but in the past years, microorganisms have become subject of increasing interest.

Soil microorganisms help in recovering and maintaining the health of ecosystems, particularly Karst ecosystems (Chen et al., 2012). Soil microbial communities (SMC) play a key role in release and retention of soil nutrients and soil fertility. These microorganisms also play an essential role in shaping the biodiversity and functioning of terrestrial ecosystems by driving biogeochemical processes and mediating nutrient turnover (Doran and Zeiss, 2000; Bardgett and van der Putten, 2014). Other uses of soil microbes include production of phytohormones, nutrient cycling, detoxification of contaminated soil, biocontrol of soil-borne phytopathogens, and organic matter decomposition. However, soil microbial processes and microbial resource limitation of a karst ecosystem remain poorly understood (Chen et al., 2018). Soil microorganisms also play a significant role in soil biogeochemical cycling, but their growth and activities are often limited by resource availability. Studying soil processes that are driven by these microorganisms will help elucidate controls on soil fertility and improve the ability to predict the responses of an ecosystem to global changes. Moreover, baseline data generated from this study may provide valuable insight on this ecosystem and may pave the way in long-term monitoring of microbial diversity. Despite the importance of SMC-plant interactions in regulating the structure and function of karst ecosystems, the diversity of SMC in such a landform remains largely unstudied.

The most studied karst terrains are in China, Europe, and the Yucatan Peninsula in Mexico (Santillán et al., 2021). In this paper, we report for the first time the taxonomic diversity and metabolic functions of bacterial and fungal communities present in forest over limestone ecosystems in the Basey, Samar, Philippines, using metagenomic analysis. Taxonomic and functional profile of SMC in a karst ecosystem will potentially accelerate research on natural microbial communities, thereby promoting the adaptive capacity of host plants to abiotic stresses, such as high calcium stress. Karst forests are more prone to rapid degradation processes as compared to non-karst forests, such as soil loss and reduced water holding capacity, resulting in irreversible changes in vegetation cover (Peng et al., 2013 and Tang et al., 2013). Therefore, understanding the microbial community structure is vital for effective vegetation restoration in karst areas. Data from this study will also be valuable for determining which microbial strains can be used for field application in karst topography.