Key role of bacteria in the short‐term cycling of carbon at the abyssal seafloor in a low particulate organic carbon flux region of the eastern Pacific Ocean

Andrew K Sweetman,Dick Van Oevelen,Tanja Stratmann,Markus Lindh,Andrew J Gooday,Matthew J Church,Brianne Maillot,Christine N Shulse,Craig R Smith,Kirstin S Meyer

doi:10.1002/lno.11069

Abstract

AbstractThe cycling of carbon (C) by benthic organisms is a key ecosystem function in the deep sea. Pulse‐chase experiments are designed to quantify this process, yet few studies have been carried out using abyssal (3500–6000 m) sediments and only a handful of studies have been undertaken in situ. We undertook eight in situ pulse‐chase experiments in three abyssal strata (4050–4200 m water depth) separated by tens to hundreds of kilometers in the eastern Clarion‐Clipperton Fracture Zone (CCFZ). These experiments demonstrated that benthic bacteria dominated the consumption of phytodetritus over short (~ 1.5 d) time scales, with metazoan macrofauna playing a minor role. These results contrast with the only other comparable in situ abyssal study, where macrofauna dominated phytodetritus assimilation over short (2.5 d) time scales in the eutrophic NE Atlantic. We also demonstrated that benthic bacteria were capable of converting dissolved inorganic C into biomass and showed that this process can occur at rates that are as high as the bacterial assimilation of algal‐derived organic C. This demonstrates the potential importance of inorganic C uptake to abyssal ecosystems in this region. It also alludes to the possibility that some of the C incorporation by bacteria in our algal‐addition studies may have resulted from the incorporation of labeled dissolved inorganic carbon initially respired by other unstudied organisms. Our findings reveal the key importance of benthic bacteria in the short‐term cycling of C in abyssal habitats in the eastern CCFZ and provide important information on benthic ecosystem functioning in an area targeted for commercial‐scale, deep‐sea mining activities.

Highlights

Benthic community structure Mean bacterial biomass measured in benthic chambers that were incubated with algae or dissolved inorganic carbon (DIC) during the AB01 and AB02 campaigns were 530 Æ 104 mg C m−2 (n = 4, standard error of the mean [SEM]) and 277 Æ 52 mg C m−2 (n = 6, SEM), respectively (Fig. 2)
Our results reveal a key role for bacteria in the initial degradation of fresh phytodetritus at the abyssal seafloor of the eastern Clarion-Clipperton Fracture Zone (CCFZ), which is consistent with findings from earlier investigations from other deep-sea regions
Our in situ DICaddition experiments indicated that fixation of inorganic C by benthic bacteria does occur at the abyssal seafloor in this region

Summary

Introduction

In contrast to shipboard studies that have shown that bacteria are important regulators of short-term degradation of organic material in the deep sea (Rowe and Deming 1985; Lochte and Turley 1988; Boetius and Lochte 1996; Kanzog et al 2009; Hoffmann et al 2017), the study by Witte et al (2003a) identified macrofauna as the dominant group of organisms responsible for the initial stages of organic matter remineralisation Such conflicting results highlight our limited understanding of the role of different organism size classes in the degradation of POC at the abyssal seafloor. This represents a serious environmental issue, heightened by the lack of present knowledge about abyssal seafloor biology and ecology in this region

Methods

Results

Discussion

Conclusion