Interest in extracting mineral resources from the seafloor through deep‐sea mining has accelerated in the past decade, driven by consumer demand for various metals like zinc, cobalt, and rare earth elements. While there are ongoing studies evaluating potential environmental impacts of deep‐sea mining activities, these focus primarily on impacts to animal biodiversity. The microscopic spectrum of seafloor life and the services that this life provides in the deep sea are rarely considered explicitly. In April 2018, scientists met to define the microbial ecosystem services that should be considered when assessing potential impacts of deep‐sea mining, and to provide recommendations for how to evaluate and safeguard these services. Here, we indicate that the potential impacts of mining on microbial ecosystem services in the deep sea vary substantially, from minimal expected impact to loss of services that cannot be remedied by protected area offsets. For example, we (1) describe potential major losses of microbial ecosystem services at active hydrothermal vent habitats impacted by mining, (2) speculate that there could be major ecosystem service degradation at inactive massive sulfide deposits without extensive mitigation efforts, (3) suggest minor impacts to carbon sequestration within manganese nodule fields coupled with potentially important impacts to primary production capacity, and (4) surmise that assessment of impacts to microbial ecosystem services at seamounts with ferromanganese crusts is too poorly understood to be definitive. We conclude by recommending that baseline assessments of microbial diversity, biomass, and, importantly, biogeochemical function need to be considered in environmental impact assessments of deep‐sea mining.
Marine microorganisms play a fundamental role in the global carbon cycle by mediating the sequestration of organic matter in ocean waters and sediments. A better understanding of how biological factors, such as microbial community composition, influence the lability and fate of organic matter is needed. Here, we explored the extent to which organic matter remineralization is influenced by species‐specific metabolic capabilities. We carried out aerobic time‐series incubations of Guaymas Basin sediments to quantify the dynamics of carbon utilization by two different heterotrophic marine isolates (Vibrio splendidus 1A01; Pseudoalteromonas sp. 3D05). Continuous measurement of respiratory CO2 production and its carbon isotopic compositions (13C and 14C) shows species‐specific differences in the rate, quantity, and type of organic matter remineralized. Each species was incubated with hydrothermally‐influenced vs. unimpacted sediments, resulting in a ~2‐fold difference in respiratory CO2 yield across the experiments. Genomic analysis indicated that the observed carbon utilization patterns may be attributed in part to the number of gene copies encoding for extracellular hydrolytic enzymes. Our results demonstrate that the lability and remineralization of organic matter in marine environments is not only a function of chemical composition and/or environmental conditions, but also a function of the microorganisms that are present and active.
Aquatic sediments harbor diverse microbial communities that mediate organic matter degradation and influence biogeochemical cycles. The pool of bioavailable carbon continuously changes as a result of abiotic processes and microbial activity. It remains unclear how microbial communities respond to heterogeneous organic matrices and how this ultimately affects heterotrophic respiration. To explore the relationships between the degradation of mixed carbon substrates and microbial activity, we incubated batches of organic-rich sediments in a novel bioreactor (IsoCaRB) that permitted continuous observations of CO2 production rates, as well as sequential sampling of isotopic signatures (δ13C, Δ14C), microbial community structure and diversity, and extracellular enzyme activity. Our results indicated that lower molecular weight (MW), labile, phytoplankton-derived compounds were degraded first, followed by petroleum-derived exogenous pollutants, and finally by higher MW polymeric plant material. This shift in utilization coincided with a community succession and increased extracellular enzyme activities. Thus, sequential utilization of different carbon pools induced changes at both the community and cellular level, shifting community composition, enzyme activity, respiration rates, and residual organic matter reactivity. Our results provide novel insight into the accessibility of sedimentary organic matter and demonstrate how bioavailability of natural organic substrates may affect the function and composition of heterotrophic bacterial populations. This article is protected by copyright. All rights reserved.
The biogeochemical transformation and remineralization of organic matter (OM) in marine sediments is largely driven by the activity of complex and diverse microbial communities. A major unknown in carbon biogeochemistry is determining what controls the reactivity or lability of OM to microorganisms. Recent studies have found that the ability to use different carbon sources varies among microorganisms, suggesting that the reactivity of specific carbon pools can be specific to the taxa that utilize the pool. This project investigated the extent to which the reactivity and transformation of OM is species-specific. Using a novel bioreactor system (IsoCaRB), we carried out time-series incubations using bacterial isolates and sterilized organic-rich sediment collected from Guaymas Basin. The IsoCaRB system allows us to measure the production rate and natural isotopic (Δ14C and δ13C) signature of microbially-respired CO2 to constrain the type and age of organic matter that is accessible to each species. Separate incubations using marine bacterial isolates (Vibrio sp. and Pseudoalteromonas sp.) and sterilized sediment under oxic conditions showed that the rate and total quantity of organic matter metabolized by these two species differs. Approximately twice as much respired CO2 was collected during the Pseudoalteromonas sp. incubations compared to the Vibrio sp. incubations. Δ14C and δ13C signatures of respired CO2 show selective utilization of different carbon pools by the two species. These differences in OM degradation may be due to the gene copy number and substrate specificity of degradative enzymes. The results of this work indicate that organic matter transformation in marine sediments may depend on the metabolic capabilities of the microbial populations that are present and active.