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

Abstract

The 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.

Preliminary Observations of the Abyssal Megafauna of Kiribati

“…Standing stocks of the larger sized fauna decrease dramatically with water depth (Rex et al, 2006) as diminishing food supply appears to prevent growth to larger body sizes (McClain et al, 2005). This hypothesis is consistent with the large dominance of foraminifera in the abyssal megabenthos, as their protoplasm volume represents a minor fraction of their visible test (Levin and Gooday, 1992), and because benthic bacteria dominate carbon consumption of these ecosystems (Sweetman et al, 2019).…”

Cold Seeps in a Warming Arctic: Insights for Benthic Ecology

“…Other sources of food are zooplankton and fecal pellets that become available to the benthos via pelagic-benthic coupling processes (Graf, 1989;Grebmeier and Barry, 1991). In benthic deep-sea systems, inorganic carbon fixed via prokaryotes (so-called "dark fixation") may also provide significant carbon sources (Molari et al, 2013;Sweetman et al, 2019). Occasionally, large pulses of food arrive at the bottom of the ocean in the form of carcasses of large animals such as whale/shark falls, or mass mortality of jellyfish (e.g., Smith and Baco, 2003;Higgs et al, 2014;Dunlop et al, 2018) or even from terrestrial sources by wood-falls or large inputs of sediment (e.g., Dando et al, 1992;Bienhold et al, 2013;Holding et al, 2017;Sen et al, 2017).…”

Challenges to the sustainability of deep-seabed mining

“…Finally, the connections of these habitats to the wider global functioning is poorly understood, although new studies of in situ carbon fixation on abyssal plains 28 and hydrothermal vent contributions to surface productivity 29 have begun shedding light on these connections.…”

“…Seafloor substrates targeted for mining may hold genetic resources that could be lost 26,35 . Deep-seabed mining could cause disruption of carbon cycling by removal of autotrophic microbes that fix carbon, and fauna that bury carbon in sediments 28 . Loss of tourism from the threat of mining is feared in diverse settings such as Papua New Guinea, Fiji, Portugal and Spain 21 .…”