Abstract
Abstract. The Critical Zone (CZ) is the Earth's outer shell where all the fundamental physical, chemical, and biological processes critical for sustaining life occur and interact. As microbes in the CZ drive many of these biogeochemical cycles, understanding their impact on life-sustaining processes starts with an understanding of their biodiversity. In this review, we summarize the factors controlling where terrestrial CZ microbes (prokaryotes and micro-eukaryotes) live and what is known about their diversity and function. Microbes are found throughout the CZ, down to 5 km below the surface, but their functional roles change with depth due to habitat complexity, e.g. variability in pore spaces, water, oxygen, and nutrients. Abundances of prokaryotes and micro-eukaryotes decrease from 1010 or 107 cells g soil−1 or rock−1, or ml water−1 by up to eight orders of magnitude with depth. Although symbiotic mycorrhizal fungi and free-living decomposers have been studied extensively in soil habitats, where they occur up to 103 cells g soil−1, little is known regarding their identity or impact on weathering in the deep subsurface. The relatively low abundance of micro-eukaryotes in the deep subsurface suggests that they are limited in space, nutrients, are unable to cope with oxygen limitations, or some combination thereof. Since deep regions of the CZ have limited access to recent photosynthesis-derived carbon, microbes there depend on deposited organic material or a chemolithoautotrophic metabolism that allows for a complete food chain, independent from the surface, although limited energy flux means cell growth may take tens to thousands of years. Microbes are found in all regions of the CZ and can mediate important biogeochemical processes, but more work is needed to understand how microbial populations influence the links between different regions of the CZ and weathering processes. With the recent development of "omics" technologies, microbial ecologists have new methods that can be used to link the composition and function of in situ microbial communities. In particular, these methods can be used to search for new metabolic pathways that are relevant to biogeochemical nutrient cycling and determine how the activity of microorganisms can affect transport of carbon, particulates, and reactive gases between and within CZ regions.
Highlights
Relevant to biogeochemical nutrient cycling and determine how the activity of microorganisms can affect transport of carbon, particulates, and reactive gases between and within Critical Zone (CZ) regions
As microbes in the CZ drive many of these biogeochemical cycles, understanding their impact on life-sustaining processes starts with an understanding of their biodiversity
We summarize the factors controlling where terrestrial CZ microbes live and what is known about their diversity and function
Summary
The Earth’s Critical Zone (CZ) is the heterogeneous environment where complex interactions between rock, soil, water, air, and living organisms regulate the availability of lifesustaining resources (NRC, 2001). The CZ microbial world includes prokaryotes (Bacteria and Archaea), eukaryotes (fungi, algae, and protozoa), and viruses These microbes have developed an extraordinary diversity of metabolic potential and adapted to a wide range of habitats that vary in nutrient and water availability, depth, and temperature. The CZ is a unified biosphere, studies have traditionally divided it into five distinct geological zones: soils, the shallow subsurface, groundwater, caves, and the deep subsurface Such zonation is likely irrelevant to the microbes who live there to whom, the defining features of a habitat are space, temperature, water, nutrients, and energy sources that can support microbial functional groups (Madsen, 2008). We aim to facilitate a fuller understanding of complex Earth processes by stimulating microbiologists and ecologists to evaluate their data within the global CZ network
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have