nd, no data.
Food-chain Transfer via Dissolved Organic
Matter (DOM)
(^) If bacterial production is 10–25% of primary production, and even if bacterial growth
efficiency is high enough that bacteria utilize 30–50% of primary production in
growth or metabolism, then a very large fraction of photosynthate must be released
(directly or indirectly, e.g. via grazing or viral lysis) from phytoplankton cells into the
ocean. This is simply examined by an extension of the ^14 C-uptake technique for
quantifying photosynthesis. Seawater is sampled from the euphotic zone, and ^14 C-
labeled bicarbonate is added, followed by incubation under natural or simulated-
natural temperature and illumination. Phytoplankton are filtered out and evaluated for
(^14) C content, producing the primary productivity estimate. The filtrate is saved and
acidified to remove carbon dioxide, including the labeled tracer. The sample is mixed
in scintillation fluor, and then the ^14 C which comes from labeled, dissolved organic
matter (DOM) not removed by the acidification is measured. In natural waters, DOC
as a percentage of assimilated, particulate carbon varies from a few percent to 80%.
High values occur toward the peak or termination of blooms under nutrient-limited
conditions (Wetz & Wheeler 2007). This is believed to result from continued
production of carbohydrates, for which nitrogen and phosphorus supplies need not be
limiting. Indeed, when release of DOM is a high fraction of primary production (e.g.
Biddanda & Benner 1997), more of it is carbohydrate, both small molecules and
polymers. An overall average is toward the lower end of the range, about 13% (Baines
& Pace 1991). Polymeric exudates have been termed transparent extracellular
polymers or TEP (Alldredge et al. 1993). Some DOM release may be the inevitable
result of an organic-matter concentration gradient of roughly a million-fold across the
cell membrane.