214
incorporate these roles of bacteria into mechanistic models designed to understand variability in
the ocean's biogeochemical state.
Bacteria had been assigned the roles of remineralizers and particle-formers (via DOM catabolism
and assimilation). Recently, however, it has been found that bacteria play variable ecosystem
roles depending on the nutritional status of the environment. They can be remineralizers of plant
nutrients but also compete for nutrients with phytoplankton (Azam et al., 1983; Wheeler and
Kirchman, 1986; Tupas and Koike, 1991). Bacteria can be food for animals, but they also
compete with particle-feeding zooplankton for POM by solubilizing POM with hydrolytic
exoenzymes (Azam, 1984; Azam and Cho, 1987; Pomeroy and Wiebe, 1988). Obviously, these
"role reversals" pose difficulties in conceptualizing and modelling pelagic ecosystems. For
example, it would be a major uncertainty indeed if we did not know whether to treat bacteria
as a source or a sink for ammonium in attempting to understand biogeochemical dynamics of
an ecosystem where primary production was limited by the supply of regenerated nitrogen.
Likewise, whether bacteria enrich detritus with respect to nitrogen or whether they enzymatically
remove organic nitrogen from the particles will have quite different consequences for
biogeochemical dynamics of nitrogen and f-ratios.
How then do we accommodate such qualitative variability in the roles of bacteria into our
concepts of the pelagic ecosystems? We think that the problem is symptomatic of a broader
issue in the study of pelagic ecosystem dynamics. The traditional approach has been to focus on
discipline-specific themes and study them in isolation of other ecosystem components.
Consequently, our models lack the mechanistic complexity of "real world" interactions
considered at appropriate space and time scales. There has been progress in this respect in
studies of larger organisms and mesoscale patchiness of animals and their process-rates is now
generally accepted. Indeed, there has been a revolution in zooplankton ecology resulting from
the discovery that pelagic zooplankton are not just filtering machines, but have sophisticated
feeding behaviours including selectivity (Strickler, 1982). However, it remains
"business-as-usual" in the study of the ecology of microorganisms; our concepts are based on
the assumption that microbes and their nutrient molecules are randomly distributed in space.
incorporate these roles of bacteria into mechanistic models designed to understand variability in
the ocean's biogeochemical state.
Bacteria had been assigned the roles of remineralizers and particle-formers (via DOM catabolism
and assimilation). Recently, however, it has been found that bacteria play variable ecosystem
roles depending on the nutritional status of the environment. They can be remineralizers of plant
nutrients but also compete for nutrients with phytoplankton (Azam et al., 1983; Wheeler and
Kirchman, 1986; Tupas and Koike, 1991). Bacteria can be food for animals, but they also
compete with particle-feeding zooplankton for POM by solubilizing POM with hydrolytic
exoenzymes (Azam, 1984; Azam and Cho, 1987; Pomeroy and Wiebe, 1988). Obviously, these
"role reversals" pose difficulties in conceptualizing and modelling pelagic ecosystems. For
example, it would be a major uncertainty indeed if we did not know whether to treat bacteria
as a source or a sink for ammonium in attempting to understand biogeochemical dynamics of
an ecosystem where primary production was limited by the supply of regenerated nitrogen.
Likewise, whether bacteria enrich detritus with respect to nitrogen or whether they enzymatically
remove organic nitrogen from the particles will have quite different consequences for
biogeochemical dynamics of nitrogen and f-ratios.
How then do we accommodate such qualitative variability in the roles of bacteria into our
concepts of the pelagic ecosystems? We think that the problem is symptomatic of a broader
issue in the study of pelagic ecosystem dynamics. The traditional approach has been to focus on
discipline-specific themes and study them in isolation of other ecosystem components.
Consequently, our models lack the mechanistic complexity of "real world" interactions
considered at appropriate space and time scales. There has been progress in this respect in
studies of larger organisms and mesoscale patchiness of animals and their process-rates is now
generally accepted. Indeed, there has been a revolution in zooplankton ecology resulting from
the discovery that pelagic zooplankton are not just filtering machines, but have sophisticated
feeding behaviours including selectivity (Strickler, 1982). However, it remains
"business-as-usual" in the study of the ecology of microorganisms; our concepts are based on
the assumption that microbes and their nutrient molecules are randomly distributed in space.