Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 160

Light

POM POM

Phytoplankton Mesozooplankton

DOM

POM

POM

Heterotrophic
bacteria

Microzooplankton

Figure 5.14 Schematic summary of the microbial loop, where heterotrophic bacteria transfer
dissolved organic material (DOM) back to the phytoplankton and to the larger
heterotrophs. Grey lines indicate pathways of organic material, the black arrow shows
the return of remineralised carbon and nitrogen back to the phytoplankton.

Bacteria (size ∼ 0.2-2 m m)
In the earlier discussion of nutrient sources and of regenerated production we already
met the heterotrophic bacteria as important recyclers of organic material. Part of
their recycling role involves mopping up components of organic material that would
otherwise never be re-accessible to larger organisms. Much of the organic material
generated by the phytoplankton ends up as dissolved organic matter (DOM), includ-
ing dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). DOM is
released into the water by a range of processes, including the breakdown of cells
(called cell lysis), leakage from the phytoplankton and inefficient feeding by larger
heterotrophs. The distinction between 'dissolved' and particulate organic material
(POM) in the water is operational, set by a filter pore size of 0.45 m. Such tiny

particles of organic material are not available to larger heterotrophs, so the bacteria
that can consume and recycle DOM play an important role in returning DOM back
to the rest of the ecosystem. This return pathway for DOM, referred to as the
microbial loop (Azam et al., 1983 ; Pomeroy et al., 2007 ) and summarised in
Fig. 5.14 , either supplies organic material into the heterotrophic food chain when
the bacteria are consumed by the microzooplankton, or accelerates the remineralisa-
tion of organic components which are then again available to the autotrophs.
A significant amount of the DOC taken up by the bacteria is released as dissolved

m