Spatial biogeography of microbes in soils vs. aquatic ecosystems in U.S.’s major natural biomes

Introduction Microbial macroecology has gained recognition as a critical component of microbial ecology. Using data from the National Ecological Observatory Network (NEON), this study examined the spatial patterns of microbial abundance and their environmental controls in soil and aquatic ecosystems across major natural biomes in the United States. Methods Microbial cell density in aquatic ecosystems and soil microbial biomass carbon in terrestrial ecosystems were analyzed, and their relationships with environmental variables were evaluated using correlation analyses. Results In aquatic ecosystems, microbial cell density ranged from 1.8 × <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>5</mml:mn> </mml:msup> </mml:math> to 4.1 × <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>6</mml:mn> </mml:msup> </mml:math> cells mL −1 and was positively associated with specific conductance ( r = 0.32, p &amp;lt; 0.01) and water temperature ( r = 0.19, p = 0.03), but negatively associated with dissolved oxygen ( r = −0.21, p = 0.01). Across all locations, the cell density averaged approximately 1.4 × <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>6</mml:mn> </mml:msup> </mml:math> cells mL −1 . In terrestrial ecosystems, soil microbial biomass carbon ranged from 27 to 2.5 × <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M4"> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>4</mml:mn> </mml:msup> </mml:math> μg C g −1 dry soil and was positively correlated with soil moisture ( r = 0.57, p &amp;lt; 0.01), soil carbon content ( r = 0.67, p &amp;lt; 0.01), and soil nitrogen content ( r = 0.66, p &amp;lt; 0.01). It was negatively associated with soil temperature ( r = −0.42, p &amp;lt; 0.01) and soil pH ( r = −0.42, p &amp;lt; 0.01). Across all locations, the microbial biomass carbon averaged approximately 2.9 × <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M5"> <mml:msup> <mml:mn>10</mml:mn> <mml:mn>3</mml:mn> </mml:msup> </mml:math> μg C g −1 . Bacteria were dominant across both aquatic and terrestrial environments, ranging from 28% to 88%, while Eukarya ranged from 0% to 48%. Archaea made a minor contribution to the microbial community across all sites. Unclassified microbes varied across the United States, ranging from less than 1% at the Lower Tombigbee River in southwest Alabama to 57% at Sycamore Creek in Arizona. Discussion In aquatic systems, cell density increased with specific conductance and water temperature but decreased where dissolved oxygen was high. In terrestrial ecosystems, biomass was higher in soils with greater soil nitrogen content, soil carbon content, and moisture, indicating that nutrient-rich and humid environments favored microbial growth. In contrast, abundances declined in warmer and more alkaline soils. These biogeographic patterns show divergent environmental factors driving microbial abundance in various ecosystems, reflecting high microbial adaptation to surrounding physical and chemical conditions.