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PHYTOPLANKTON-BACTERIA INTERACTIONS
The ''purpose /I of phytoplankton exudation
Bacteria-phytoplankton interactions have been a major theme in marine microbiology for
decades. There is a large volume of data on the flow of organic matter via exudation by
phytoplankton and there is a rich literature on the subject which has been widely debated and
reviewed (e.g. Bjcf>msen, 1988; Williams, 1990). However, despite extensive research there
continues to be a lack of consensus on the magnitude and particularly on the ecological and
evolutionary significance of DOM exudation by phytoplankton. Bjcf>msen (1988) has argued that
phytoplankton exudation is a "property tax", the price phytoplankton must pay for maintaining
high metabolite pools which inexorably tend to leak out into seawater along an extremely high
downward concentration gradient. Williams (1990) argued that exudation has evolved to put a
substantial part of the photosynthate into the dissolved phase in order to reduce N loss through
vertical flux. We have hypothesized that phytoplankton exudation has evolved as a mechanism
to establish mutualism with bacteria (Azam and Ammerman, 1984; Azam and Cho, 1987);
phytoplankton provide reduced carbon for bacterial metabolism while bacteria produce
remineralized nutrients for phytoplankton. Cell surface mucus exudation may also serve to keep
bacteria from attacking the phytoplankton cell. The foregoing sampling of ideas illustrates the
divergence of views and the lack of a generally accepted theory regarding the "purpose" of
exudation by phytoplankton. We will further explore our explanation to see if it may serve as
a basis for a theory of phytoplankton-bacteria interaction.
Phytoplankton-bacteria mutualism
Bratback and Thingstad (1985), from observations on P-limited mixed bacteria-phytoplankton
chemostats, discovered a paradoxical behaviour of phytoplankton. As P declined, bacteria
became strong competitors with phytoplankton, but at that time the phytoplankton secreted
carbohydrate which encouraged bacteria to further deplete P, sequestering most of it (Fig 1a).
A natural analog of this interaction in N-limited waters is shown in Fig. 2a. In highly
oligotrophic waters, bacterial biomass can actually be greater than phytoplankton biomass
PHYTOPLANKTON-BACTERIA INTERACTIONS
The ''purpose /I of phytoplankton exudation
Bacteria-phytoplankton interactions have been a major theme in marine microbiology for
decades. There is a large volume of data on the flow of organic matter via exudation by
phytoplankton and there is a rich literature on the subject which has been widely debated and
reviewed (e.g. Bjcf>msen, 1988; Williams, 1990). However, despite extensive research there
continues to be a lack of consensus on the magnitude and particularly on the ecological and
evolutionary significance of DOM exudation by phytoplankton. Bjcf>msen (1988) has argued that
phytoplankton exudation is a "property tax", the price phytoplankton must pay for maintaining
high metabolite pools which inexorably tend to leak out into seawater along an extremely high
downward concentration gradient. Williams (1990) argued that exudation has evolved to put a
substantial part of the photosynthate into the dissolved phase in order to reduce N loss through
vertical flux. We have hypothesized that phytoplankton exudation has evolved as a mechanism
to establish mutualism with bacteria (Azam and Ammerman, 1984; Azam and Cho, 1987);
phytoplankton provide reduced carbon for bacterial metabolism while bacteria produce
remineralized nutrients for phytoplankton. Cell surface mucus exudation may also serve to keep
bacteria from attacking the phytoplankton cell. The foregoing sampling of ideas illustrates the
divergence of views and the lack of a generally accepted theory regarding the "purpose" of
exudation by phytoplankton. We will further explore our explanation to see if it may serve as
a basis for a theory of phytoplankton-bacteria interaction.
Phytoplankton-bacteria mutualism
Bratback and Thingstad (1985), from observations on P-limited mixed bacteria-phytoplankton
chemostats, discovered a paradoxical behaviour of phytoplankton. As P declined, bacteria
became strong competitors with phytoplankton, but at that time the phytoplankton secreted
carbohydrate which encouraged bacteria to further deplete P, sequestering most of it (Fig 1a).
A natural analog of this interaction in N-limited waters is shown in Fig. 2a. In highly
oligotrophic waters, bacterial biomass can actually be greater than phytoplankton biomass