223
Phosphorus remineralization and its availability to phytoplankton may also involve
bacteria-phytoplankton interactions as discussed in relation to the paradox of Bratback and
Thingstad (1985). Ammerman and Azam (1985; manuscript) have shown that bacterial
5'-nucleotidase plays a major role in P remineralization. DNA and RNA, which are substantial
pools of DOP, are hydrolysed to nucleotides on bacterial cell surface by surface-bound nucleases
(Paul et al., 1988). The nucleotides are hydrolysed by the (periplasmic?) 5' -Nase to liberate Pi
(so in contrast with N remineralization, the remineralization of P is extracytoplasmic). A
variable fraction of the liberated Pi is taken up by the bacterium. Interestingly, the
hydrolysis-uptake coupling decreases with increasing Pi concentration in the environment
(Fig. 3b) following isotope dilution curves. Essentially, at high environmental levels of Pi, the
Pi-permease molecules tend to be occupied by the Pi diffusing into the periplasm from the
environment, thus reducing the uptake of periplasmically produced Pi. As the environmental Pi
level decreases, the hydrolysis-uptake coupling becomes tighter. This means that bacteria become
competitors at low environmental Pi, competing for the environmental Pi but not releasing much
Pi produced periplasmically at the expense of DOP. At high environmental Pi, on the other
hand, because of loose hydrolysis-uptake coupling, bacteria become remineralizers, releasing a
larger fraction of the remineralized Pi into the environment. Once a high net level of Pi is
generated in the bacterium's microenvironment (e.g. by predation on bacteria by protozoa;
Fig. lc) then the hydrolysis-uptake coupling becomes loose and the release of remineralized Pi
sustains the high Pi level in the microenvironment. Therefore, the system is "autocatalytic" in
a kinetic sense. This mechanism explains, at biochemical level, how bacteria can move smoothly
between being strong competitors or being efficient net remineralizers of P.
Creation of nutrient rich microzones
It is noteworthy in this connection that phytoplankton grow quite rapidly (- 1 d- 1 ) in waters with
unmeasurable (::;;; few Nm) N (McCarthy and Goldman, 1979; Laws et al. 1984). Several
explanations to account for this observation have been advanced (e.g. exposure to plumes of
zooplankton excretion, McCarthy and Goldman, 1979; and association of phytoplankton with
marine aggregates, also known as the "Spinning Wheel Hypothesis", Goldman, 1984). While
these hypotheses seek to create nutrient-rich patches via environmental variables, our hypothesis
Phosphorus remineralization and its availability to phytoplankton may also involve
bacteria-phytoplankton interactions as discussed in relation to the paradox of Bratback and
Thingstad (1985). Ammerman and Azam (1985; manuscript) have shown that bacterial
5'-nucleotidase plays a major role in P remineralization. DNA and RNA, which are substantial
pools of DOP, are hydrolysed to nucleotides on bacterial cell surface by surface-bound nucleases
(Paul et al., 1988). The nucleotides are hydrolysed by the (periplasmic?) 5' -Nase to liberate Pi
(so in contrast with N remineralization, the remineralization of P is extracytoplasmic). A
variable fraction of the liberated Pi is taken up by the bacterium. Interestingly, the
hydrolysis-uptake coupling decreases with increasing Pi concentration in the environment
(Fig. 3b) following isotope dilution curves. Essentially, at high environmental levels of Pi, the
Pi-permease molecules tend to be occupied by the Pi diffusing into the periplasm from the
environment, thus reducing the uptake of periplasmically produced Pi. As the environmental Pi
level decreases, the hydrolysis-uptake coupling becomes tighter. This means that bacteria become
competitors at low environmental Pi, competing for the environmental Pi but not releasing much
Pi produced periplasmically at the expense of DOP. At high environmental Pi, on the other
hand, because of loose hydrolysis-uptake coupling, bacteria become remineralizers, releasing a
larger fraction of the remineralized Pi into the environment. Once a high net level of Pi is
generated in the bacterium's microenvironment (e.g. by predation on bacteria by protozoa;
Fig. lc) then the hydrolysis-uptake coupling becomes loose and the release of remineralized Pi
sustains the high Pi level in the microenvironment. Therefore, the system is "autocatalytic" in
a kinetic sense. This mechanism explains, at biochemical level, how bacteria can move smoothly
between being strong competitors or being efficient net remineralizers of P.
Creation of nutrient rich microzones
It is noteworthy in this connection that phytoplankton grow quite rapidly (- 1 d- 1 ) in waters with
unmeasurable (::;;; few Nm) N (McCarthy and Goldman, 1979; Laws et al. 1984). Several
explanations to account for this observation have been advanced (e.g. exposure to plumes of
zooplankton excretion, McCarthy and Goldman, 1979; and association of phytoplankton with
marine aggregates, also known as the "Spinning Wheel Hypothesis", Goldman, 1984). While
these hypotheses seek to create nutrient-rich patches via environmental variables, our hypothesis