Incorporation of aged dissolved organic carbon (DOC) by oceanic particulate organic carbon (POC): An experimental approach using natural carbon isotopes

Abstract

Incorporation of ¹⁴C-depleted (old) dissolved organic carbon (DOC) on/into particulate organic carbon (POC) has been suggested as a possible mechanism to explain the low Δ¹⁴C-POC values observed in the deep ocean [Druffel, E.R.M., Williams, P.M., 1990. Identification of a deep marine source of particulate organic carbon using bomb ¹⁴C. Nature, 347, 172–174.]. A shipboard incubation experiment was performed in the Sargasso Sea to test this hypothesis. Finely ground dried plankton was incubated in seawater samples from the deep Sargasso Sea, both with and without a biological poison (HgCl₂). Changes in parameters such as biochemical composition and carbon isotopic signatures of bulk POC and its organic compound classes were examined to study the roles of sorptive processes and biotic activity on POC character. Following a 13-day incubation, the relative abundance of the acid-insoluble organic fraction increased. Abundances of extractable lipids and total hydrolyzable amino acids decreased for both treatments, but by a greater extent in the non-poisoned treatment. The Δ¹⁴C values of POC recovered from the non-poisoned treatment were significantly lower than the value of the unaltered plankton material used for the incubation, indicating incorporation of ¹⁴C-depleted carbon, most likely DOC. The old carbon was present only in the lipid and acid-insoluble fractions. These results are consistent with previous findings of old carbon dominating the same organic fractions of sinking POC from the deep Northeast Pacific [Hwang, J., Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299, 881–884.]. However, the Δ¹⁴C values of POC recovered from the poisoned treatment did not change as much as those from the non-poisoned treatment suggesting that biological processes were involved in the incorporation of DOC on/into POC.

Introduction

Most particulate organic carbon (POC) in the open ocean is produced by photosynthesis using dissolved inorganic carbon (DIC) as its precursor in the surface ocean. Only a small fraction of POC produced by photosynthesis reaches the deep ocean (Martin et al., 1987, Wakeham et al., 1997). The POC is believed to reach the abyssal seafloor within approximately two months of its production (Deuser and Ross, 1980, Honjo, 1982).

Natural abundance radiocarbon signatures (Δ¹⁴C, per mil deviation of the ¹⁴C / ¹²C ratio relative to a nineteenth century wood standard, corrected to δ¹³C of − 25‰; Stuiver and Polach, 1977) have been applied to studies of the sources, sinks, and cycling of marine POC in several studies (Williams and Druffel, 1987, Druffel et al., 1992, Williams et al., 1992). A major conundrum of these isotope data is the observed decrease of 50‰ to 100‰ in Δ¹⁴C for both suspended and sinking POC with depth (Druffel et al., 1992, Druffel and Williams, 1990). If surface-produced POC was the only source of POC to deep waters and was transported to the deep ocean on time scales of months, as for sinking POC (Deuser and Ross, 1980, Honjo, 1982), to several years, as for suspended POC (Bacon and Anderson, 1982, Druffel et al., 2003), then there should be no discernible gradient in Δ¹⁴C-POC with depth.

One suggestion for the vertical gradient in  Δ¹⁴C of suspended POC is the incorporation of ¹⁴C-depleted (old) DOC into POC pool (Druffel and Williams, 1990). The Δ¹⁴C of DOC is so low that incorporation of a relatively small fraction of deep DOC (∼14% of total POC mass with a Δ¹⁴C of − 525‰ in the Pacific) would be sufficient to decrease  Δ¹⁴C of suspended POC from 60‰ (surface water value) to − 20‰ (at 3450 m depth, Druffel et al., 1998a). Another mechanism suggested is incorporation of low-Δ¹⁴C DIC by either chemoautotrophy or anapleurotic reactions (direct use of bicarbonate ion to produce citric acid cycle intermediates that are drawn from the cycle for syntheses of other compounds) at depth by organisms associated with particles and aggregates (Rau et al., 1986). Finally, incorporation of old, resuspended sediments into the suspended POC may be a dominant mechanism on continental margins and surrounding slope regions (Bauer and Druffel, 1998, Druffel et al., 1998a, Sherrell et al., 1998, Hwang et al., 2004). However, the Δ¹⁴C depth gradient is also observed at remote sites (e.g., North Central Pacific, Sargasso Sea), indicating that resuspended material is unlikely to be the only source of old POC, especially at depths 1–2 km above bottom (Druffel et al., 1998b).

Several studies suggest that incorporation of DOC on/into particles does occur in the ocean. Sorption of DOC onto mineral particles was observed in laboratory experiments (Hedges, 1977, Wang and Lee, 1993) and has been suggested as a way of preserving labile organic matter (Keil and Hedges, 1993, Mayer, 1994). Spontaneous formation of particulate organic matter microgels from dissolved organic matter polymers in filtered seawater has also been reported (Chin et al., 1998). Highly sticky surface of transparent exopolymer particles (TEP) facilitates particle aggregation (Passow and Alldredge, 1995), implying that particles of similar properties as TEP may help the incorporation of DOC.

In the present study, an incubation experiment was conducted to better evaluate the process of carbon exchange between DOC and POC pools in open ocean waters. Changes in biochemical composition and isotopic signatures of bulk POC and its component organic compound classes were used to demonstrate that incorporation of DOC on/into POC does occur and should be considered in models of oceanic carbon fate and transformations.