RESULTS AND DISCUSSION
At 4m the mean cell volume, although generally increasing over the duration of the
experiment (Fig. 2a), had periods of no increase or, in the case of the unfiltered dialysis
bags, even a decrease in cell volume during the night. Also synchronous changes in the
specific growth rate of the whole community were noticable (Fig. 2b). Rapid increases
occurred in the mid afternoon or early evening which stabilized at night. This indicates
that there is a sequence of events during the growth of the bacterial community at 4m.
From morning to early evening (of the first day) there was an increase in mean cell
volume. During this period there was no significant change in the specific growth rate of
the whole population. Just after dusk the specific growth rate increased dramatically
while the mean cell size remained constant or even decreased. During the rest of the night
and the following day the specific growth rate remained at this elevated rate. By the late
afternoon another rapid increase in the specific growth rate occurred and the pattern of
events was repeated. It should be remembered that many people have found, using
microautoradiography (Hoppe, 1976), that not all the bacterial population is active - at
least not at the same time. These results indicate that a significant part of the population
was undergoing synchronous division, certainly sufficient to superimpose this periodic
step pattern of specific growth rates on the whole bacterial population.
At 60 m, in contrast to the surface waters, specific growth rates for the whole bacterial
community did not increase with time but rather had periods of rapid growth and periods
of little growth (Fig. 2c). During the daylight of the first day there was an increase in the
mean cell volume of the bacterial community (Fig. 2a) but no detectable increase in cell
numbers (Fig. 2c). During the night cell volumes decreased and stayed low for the next 24
hours when specific growth rates were high. In the last 12 hours mean cell volume
increased while change in specific growth rates decreased rapidly. This latter change in
mean cell volume, although small compared to that seen at 4 m, was significant and due to
a small shift in the percentage of bacteria in the size class “minibacteria” into “large cocci”
and a slightly larger shift in the percentage of bacteria in the size class “small rods” into
“large rods” (Turley & Lochte, 1986).
This relationship, seen at 4 m and 60 m, between specific growth rate and mean cell
volumes seems to indicate that cells increase in size before dividing (Krambeck 1984). It
must be remembered that the specific growth rate for the whole population is influenced
by the growth of the individual members of the community.
The specific growth rates for the 4 m whole bacterial community (Table 1 ) was over twice
as high as that at 60 m. Indeed, it was found that bacterial productivity was 4 times greater
at 4 m than at 60 m (Turley & Lochte, 1985). At 4 m both rods and cocci grew at the same
rate (Table 1). At 60 m, while the rods had a similar growth rate to those at 4 m, the cocci
were growing at a much slower rate. There is, therefore, an indication that there was a
difference in the growth rates of different members of the community.
At 4 m rods had higher specific growth rates during the day while more coccoid forms
divided at night (Fig. 3a). Therefore, part of each morphological group must have
exhibited synchronous cell division. It would seem that the periodic step pattern, seen in
the specific growth rates for the bacterial community as a whole (Fig. 2b), reflects the
combination of growth rates for both coccoid and rod forms. It is interesting that both
groups, the cocci and the rods, had doubling times of very close to one day (Table 1)
besides having diel periods when more cells divided (Fig. 3a). This may indicate that the
growth rates and even the times of most rapid growth are influenced by diurnal rythms of
other physical, chemical or biological variables around them. Indeed, diel variations of
biologically labile organic compounds have often been observed (Sieburth et al., 1977,
112
At 4m the mean cell volume, although generally increasing over the duration of the
experiment (Fig. 2a), had periods of no increase or, in the case of the unfiltered dialysis
bags, even a decrease in cell volume during the night. Also synchronous changes in the
specific growth rate of the whole community were noticable (Fig. 2b). Rapid increases
occurred in the mid afternoon or early evening which stabilized at night. This indicates
that there is a sequence of events during the growth of the bacterial community at 4m.
From morning to early evening (of the first day) there was an increase in mean cell
volume. During this period there was no significant change in the specific growth rate of
the whole population. Just after dusk the specific growth rate increased dramatically
while the mean cell size remained constant or even decreased. During the rest of the night
and the following day the specific growth rate remained at this elevated rate. By the late
afternoon another rapid increase in the specific growth rate occurred and the pattern of
events was repeated. It should be remembered that many people have found, using
microautoradiography (Hoppe, 1976), that not all the bacterial population is active - at
least not at the same time. These results indicate that a significant part of the population
was undergoing synchronous division, certainly sufficient to superimpose this periodic
step pattern of specific growth rates on the whole bacterial population.
At 60 m, in contrast to the surface waters, specific growth rates for the whole bacterial
community did not increase with time but rather had periods of rapid growth and periods
of little growth (Fig. 2c). During the daylight of the first day there was an increase in the
mean cell volume of the bacterial community (Fig. 2a) but no detectable increase in cell
numbers (Fig. 2c). During the night cell volumes decreased and stayed low for the next 24
hours when specific growth rates were high. In the last 12 hours mean cell volume
increased while change in specific growth rates decreased rapidly. This latter change in
mean cell volume, although small compared to that seen at 4 m, was significant and due to
a small shift in the percentage of bacteria in the size class “minibacteria” into “large cocci”
and a slightly larger shift in the percentage of bacteria in the size class “small rods” into
“large rods” (Turley & Lochte, 1986).
This relationship, seen at 4 m and 60 m, between specific growth rate and mean cell
volumes seems to indicate that cells increase in size before dividing (Krambeck 1984). It
must be remembered that the specific growth rate for the whole population is influenced
by the growth of the individual members of the community.
The specific growth rates for the 4 m whole bacterial community (Table 1 ) was over twice
as high as that at 60 m. Indeed, it was found that bacterial productivity was 4 times greater
at 4 m than at 60 m (Turley & Lochte, 1985). At 4 m both rods and cocci grew at the same
rate (Table 1). At 60 m, while the rods had a similar growth rate to those at 4 m, the cocci
were growing at a much slower rate. There is, therefore, an indication that there was a
difference in the growth rates of different members of the community.
At 4 m rods had higher specific growth rates during the day while more coccoid forms
divided at night (Fig. 3a). Therefore, part of each morphological group must have
exhibited synchronous cell division. It would seem that the periodic step pattern, seen in
the specific growth rates for the bacterial community as a whole (Fig. 2b), reflects the
combination of growth rates for both coccoid and rod forms. It is interesting that both
groups, the cocci and the rods, had doubling times of very close to one day (Table 1)
besides having diel periods when more cells divided (Fig. 3a). This may indicate that the
growth rates and even the times of most rapid growth are influenced by diurnal rythms of
other physical, chemical or biological variables around them. Indeed, diel variations of
biologically labile organic compounds have often been observed (Sieburth et al., 1977,
112