Digging Deeper, Heterotrophs Below the Surface

Surface environments contain heterotrophic microorganisms that metabolize small molecules to gain energy. Extracellular enzymes then break down the material outside the cell, making up the rate of microbial carbon oxidation. Surface environments tend to have a faster metabolism than subsurface environments, so the main take away from the study, Steen and colleagues were driven to determine how heterotrophs in the subsurface have adapted to using the degraded organic matter to live. Little is known about the heterotrophic activity in subsurface sediments, and whether they use the same mechanisms as surface environments to undergo carbon oxidation. To get an insight as to how the heterotrophic microorganisms in subsurface sediments acquire the organic matter, peptidases in the sediment cores were assayed, and were then compared with potential extracellular peptidase production from existing genetic data sets. The study was done to give us a better understanding as to how microorganisms survive in low energy environments and into the processes that shape the pool of organic carbon that is preserved or oxidized over periods of time.

Figure 1. The X axis represents the sum of Vmax relative to modeled carbon oxidation rates, while the y axis represents the depth of the sediment in centimeters below sea floor (cmbsf). Figure taken from Steen et al. 2019.

The potential activity of the extracellular peptidases decreased slower than the rate of organic carbon oxidation as the sediment level lowered. As shown in figure 1, the lower the sediment level resulted in the ratio of ΣVmax  OC oxidation rate increase. The evidence of the study raises the question as to which aquatic environments are more suitable for gaining reproductive advantage by growing on a molecule that cannot be modified by organic carbon.

Original Article:
Steen A, Kevorkian R, Bird J, Dambrowski N, Brett B, Hagen S, et al. (2019) Kinetics and Identities of Extracellular Peptidases in Subsurface Sediments of the White Oak River Estuary, North Carolina. Applied & Environmental Microbiology. 85 (19). DOI: 10.1128/AEM.00102-19