(^) Thirteen percent of primary production (on average) is not enough DOM generation
to cover bacterial production and respiration, so there must be additional sources
(Williams, P.J.L. 1981). The bulk of the remaining requirement probably comes from
losses to the water during feeding by grazing protists and larger zooplankton. Strom et
al. (1997) showed that 16 to 37% of carbon from ingested phytoplankton quite rapidly
shows up as DOC. Protist digestion is not complete, with the remains in food vacuoles
returned to the water. Copepods, euphausiids, and other mesozooplankton break cells
during ingestion and digestion, with some immediate loss near the mouth. Additional
DOC leaches from zooplankton fecal pellets and some is present in excretory
products. Since phytoplankton growth is nearly balanced most of the time by grazing,
the transfers to DOM during grazing are probably sufficient to fill out the budget of
DOM production. Balance implies that ∼100% of primary production is eaten daily.
Thus, grazer generation of DOC is 16–37% of primary production. The sum with
direct release of 13% is 29–40%. Additional DOC release can come from viral lysis
of both bacteria (“recycled DOC”) and phytoplankton. Still-further production will
come from losses when bacteria are eaten by protists, a recycling of DOC (from DOC
to bacteria, back to DOC). Given the uncertainties of all constituent numbers in this
calculation, a rough balance is implied. No substantial amounts need be supplied by
rivers, for example, to sustain the balance. In coastal systems, primary production
often exceeds the bacterial carbon demand, but, in oligotrophic systems, bacterial
carbon demand equals or exceeds the local rate of primary production (Duarte &
Regaudie-de-Gioux 2009).
(^) Approximately 50% of bacterial production (using DOC) takes place in the
epipelagic zone of the water column. The other half of heterotrophic production and
most archaeal production occurs in the mesopelagic and bathypelagic zones (the dark
ocean). In the dark ocean, most bacterial production is supported by sinking or
suspended particulate organic carbon (POC) rather than direct export of DOC from
the epipelagic zone (Aristegui et al. 2009). In these environments, a large percentage
of the total number of bacteria can be found attached to particles. Utilization of POC
is primarily via extracellular enzymatic activity that releases bioavailable DOM (the
primary substrate for heterotrophic prokaryotes) from particles. Meso- and
bathypelagic bacteria have a larger genome size and higher per-cell respiratory
activity than those in the euphotic zone and, like bathypelagic archaea, they have a
gene repertoire indicative of a predominantly surface-attached mode of life (Robinson
et al. 2010). Exohydrolases release dissolved material faster than it can be absorbed
by the attached bacteria, thereby also supplying DOM for free-living prokaryotes
(Aristegui et al. 2009).
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