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protozoa may not change the concentration-dependent competitive advantage of bacteria over
phytoplankton since the released ammonium will again diffuse to low levels where bacteria have
the kinetic edge. So, the paradox of Bratbak and Thingstad persists.
A solution for the phytoplankton cell is to attract bacteria in high abundance in its
microenvironment (Azam and Cho, 1987). Intense remineralization (directly by bacteria and due
to predation by protozoa on bacteria clustered around a phytoplankton cell) could sustain high
ammonium levels in the microenvironment thus switching the kinetic advantage over to
phytoplankton. However, Jackson (1989) and Mitchell et al. (1985) have argued that exudate
diffusion and rapid sinking of phytoplankton prohibit the formation of bacterial clusters. This
problem may be solved by phytoplankton exuding surface-bound (rather than diffusible) exudates
such as polysaccharides. We suggest that the polysaccharide layer serves two purposes: it attracts
bacteria but also keeps them at "arm's length".
Phytoplankton are known to exude cell surface-bound polysaccharides and the exudation
increases under nutrient limitation (Mykelstad, 1977). The polysaccharide layer may become
enriched with DON and DOP by scavenging protein and other DON and DOP pools from
seawater (Azam and Cho, 1987). So, the phytoplankter may, simply by exuding cell-surface
polysaccharide, surround itself with a bacterial "culture medium" rich in energy, nitrogen and
phosphorus. Bacteria have cell-surface hydrolytic enzymes, such as proteases, polysaccharidases
and nucleases, which can hydrolyse the polymers on the phytoplankton surface. Since bacterial
exohydrolases are surface-bound, physical contact of bacteria with phytoplankton is required for
hydrolysis. However, the phytoplankton remains essentially bacteria-free because polymer
hydrolysis cuts bacteria loose from the point of attachment; bacteria attacking the phytoplankter
thus find themselves on a mucus treadmill.
Variable roles of bacteria in phytoplankton microenvironments
Our hypothetical scenario can explain how bacteria may be both remineralizers of and
competitors for ammonium (Fig. 3a.). Bacteria are net remineralizers when their environment
is rich in organic nitrogen and energy sources (Wheeler and Kirchman, 1986; Tupas and Koike,
protozoa may not change the concentration-dependent competitive advantage of bacteria over
phytoplankton since the released ammonium will again diffuse to low levels where bacteria have
the kinetic edge. So, the paradox of Bratbak and Thingstad persists.
A solution for the phytoplankton cell is to attract bacteria in high abundance in its
microenvironment (Azam and Cho, 1987). Intense remineralization (directly by bacteria and due
to predation by protozoa on bacteria clustered around a phytoplankton cell) could sustain high
ammonium levels in the microenvironment thus switching the kinetic advantage over to
phytoplankton. However, Jackson (1989) and Mitchell et al. (1985) have argued that exudate
diffusion and rapid sinking of phytoplankton prohibit the formation of bacterial clusters. This
problem may be solved by phytoplankton exuding surface-bound (rather than diffusible) exudates
such as polysaccharides. We suggest that the polysaccharide layer serves two purposes: it attracts
bacteria but also keeps them at "arm's length".
Phytoplankton are known to exude cell surface-bound polysaccharides and the exudation
increases under nutrient limitation (Mykelstad, 1977). The polysaccharide layer may become
enriched with DON and DOP by scavenging protein and other DON and DOP pools from
seawater (Azam and Cho, 1987). So, the phytoplankter may, simply by exuding cell-surface
polysaccharide, surround itself with a bacterial "culture medium" rich in energy, nitrogen and
phosphorus. Bacteria have cell-surface hydrolytic enzymes, such as proteases, polysaccharidases
and nucleases, which can hydrolyse the polymers on the phytoplankton surface. Since bacterial
exohydrolases are surface-bound, physical contact of bacteria with phytoplankton is required for
hydrolysis. However, the phytoplankton remains essentially bacteria-free because polymer
hydrolysis cuts bacteria loose from the point of attachment; bacteria attacking the phytoplankter
thus find themselves on a mucus treadmill.
Variable roles of bacteria in phytoplankton microenvironments
Our hypothetical scenario can explain how bacteria may be both remineralizers of and
competitors for ammonium (Fig. 3a.). Bacteria are net remineralizers when their environment
is rich in organic nitrogen and energy sources (Wheeler and Kirchman, 1986; Tupas and Koike,