(^) DOM and DOC must be defined operationally, which turns out to be simply the
organic matter (or organic carbon) that passes the finest filters available. Work on
quantifying DOM has used filters from 0.2 to 1 μm (Benner et al. 1993), with 0.7 μm
glass-fiber filters often used because filters with this nominal porosity are cheap and
readily cleaned. Recent developments provide filters down to 0.01 μm (Poretics®),
but not much experience is available with pores smaller than 0.2 μm. It is ferociously
difficult to get much water through the finest holes now available; these filters retain
particles previously said to be in colloidal suspension. While even living bacteria, for
example SAR11, can be smaller than, say, 0.7 μm pores, the amount of organic matter
they contain is small, despite their large numbers, compared to truly dissolved organic
matter. Thus, there is little difference in DOC determinations between 0.7 and 0.02
μm filters (Williams, P.M. et al., 1993). The DOC in the global oceans overall
amounts to about 6 × 10^17 g (Hedges 1992). This is a relatively small amount
compared to the total of inorganic carbon (entirely as oxidized, equilibrated CO 2 ,
HCO 3 − and CO 3 2−), about 38 × 10^18 g C, which is the largest reservoir of labile
carbon. Nonetheless, marine DOC is approximately equal to CO 2 in the atmosphere.
Most DOC is not particularly labile, which means that it is not immediately available
to bacteria for nutrition. Bacterial activity is dependent upon the roughly 1% that is
amenable to assimilation and metabolism and thus subject to rapid turnover. There is
also a seasonal cycle in concentration of DOM in surface waters (Williams, P.J.L.
1995), which implies that some components (termed semi-labile) have intermediate
turnover times, weeks to seasons, not hours to days. The DOM in the deep Pacific at
the downstream end of the deep-sea, density-driven circulation has radiocarbon ages
as great as 6000 years. So, it is very stable (recalcitrant to metabolism), with long
turnover times. The sink processes for this deep-sea DOC are poorly known.
(^) While DOC is a relatively big global organic-carbon reservoir, it is still very dilute
in seawater, about 30–150 μmol C liter−1 (∼1 mg C liter−1), the upper range near the
surface and inshore, the lower range at depth (Benner 2002). This has made
measurement of amounts very difficult, plagued by contamination, high blanks (water
suitably free of organic carbon is extremely difficult to make), adsorption to container
walls, and sample preservation problems. The now standard technique is high-
temperature oxidation (HTCO) of samples after acidification and removal of
carbonates, with quantification by infrared spectroscopy of the CO 2 produced. It is
also very difficult to accumulate significant amounts of material for compositional
analysis, and we lack suitable standards equivalent to refractory marine organics.
However, work on this progresses. Aluwihare et al. (1997) have concentrated the
high-molecular-weight fraction by dialysis and tangential flow ultrafiltration (0.1 μm
pores), showing that it accounts for about 30% of DOC and is about 80% highly
polymerized carbohydrate, including a wide range of simple sugars. Dissolved
ff
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