Sink or link? The bacterial role in benthic carbon cycling in the Arabian Sea's oxygen minimum zone

Abstract

Abstract. The bacterial loop, the consumption of dissolved organic matter (DOM) by bacteria and subsequent transfer of bacterial carbon to higher trophic levels, plays a prominent role in pelagic food webs. However, its role in sedimentary ecosystems is not well documented. Here we present the results of isotope tracer experiments performed under in situ oxygen conditions in sediments from inside and outside the Arabian Sea's oxygen minimum zone (OMZ) to study the importance of the microbial loop in this setting. Particulate organic matter, added as phytodetritus, was processed by bacteria, protozoa and metazoans, while dissolved organic matter was processed only by bacteria and there was very little, if any, transfer to higher trophic levels within the 7 day experimental period. This lack of significant transfer of bacterial-derived carbon to metazoan consumers indicates that the bacterial loop is rather inefficient, in sediments both inside and outside the OMZ. Moreover, metazoans directly consumed labile particulate organic matter resources and thus competed with bacteria for phytodetritus.

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

“…While the exclusion of metazoan meiofauna could have altered our results somewhat, this likely had less effect on our results than the exclusion of the Foraminfera. Previous investigations have shown metazoan meiofauna do not respond as strongly to phytodetritus C input as macrofauna or Foraminifera (Gooday et al ; Moodley et al , ; Nomaki et al ; Guilini et al ; Pozzato et al ; Mevenkamp et al ; Stratmann et al ). Among the Foraminifera, monothalmids dominate assemblages in the eastern CCFZ, including the UK1 and OMS areas (Nozawa et al ; Goineau and Gooday ).…”

Dark carbon fixation in the Arabian Sea oxygen minimum zone contributes to sedimentary organic carbon (SOM)

“…Anammox activity has been shown to occur both within the OMZ water column (Jensen et al, ; Lüke et al, ; Pitcher et al, ; Villanueva et al, ) as well as in sediments (Devol, ; Sokoll et al, ). To test whether the anammox‐derived biomarkers are formed in the sediment, the labelled cores were incubated for 7 days with particulate and dissolved organic matter (Table ), which resulted in the substantial incorporation of 13 C into bacterial fatty acids, as well as generating 13 C‐enriched CO 2 due to heterotrophic respiration (up to 14 % of the added C was respired, Pozzato, et al, , ). Anammox bacteria are autotrophic, and it is therefore likely that an active sedimentary community would result in some uptake of this 13 C labelled CO 2 formed by respiration.…”

Origin of remineralized organic matter in sediments from the Rhone River prodelta (NW Mediterranean) traced by Δ 14 C and δ 13 C signatures of pore water DIC

“…The d 13 C signatures of DIC (d 13 C-DIC) have been measured at the Royal Netherlands Institute for Sea Research (NIOZ). A headspace was created in the sample vials, CO 2 was extracted via acidification with 99% H 3 PO 4 and the ratios of the carbon isotopes were measured by isotope ratio mass spectroscopy (IR-MS) right after extraction (Pozzato et al, 2013). The isotopic ratio 13 C/ 12 C is reported on the d 13 C notation and is expressed relative to the V-PDB standard in ‰:…”