67
carbohydrate had ceased.
The concentration of extracellular polysaccharide in the medium of
this c. a66~ culture increased from 6.5 to 43 mg/l between the 12th
and the 32nd day. During the last 13 days of the experiment this was
practically the only photosynthate produced by the alga.
Ignatiadesand Fogg (1973) also studied the influence of nutrients
(N, P and Si together) on the release of organic matter by phytoplankton. These authors were also applying "f" media in dilution series,
from "f" to "f"/60 and found that excretion of photosynthate increased
from 1% to 65% of photosynthetically fixed carbon as the nutrients were
diluted (constant N: P: Si); the main excretion took place after the
exponential phase. Since the authors worked with S. c.0-6.ta.tum rather
small quantities of material were released.
In cultures with 5 . 105
cells/ml which had been cultivated at the lowest nutrient levels (maxi-3 -1
mum excretion) a little more than 1 gem h
was produced. Although
these experiments demonstrate that nutrient stress favours production
of extracellular compounds by planktonic algae, the choice of the alga
and small quantities excreted as a consequense of this will not convince the ecologist that algal excretion is important in the energy
flow in the sea.
Myklestad and Haug kept reasonable track in their experiments with
the concentration of nutrient salts in the medium as the cultures developed, and they varied the nutrients independently. Excretion could
therefore to some extent be correlated with nutrient levels in the
medium.
In general, studies on excretion do not include determination
of the concentration of nutrients in the medium throughout the experiments. The frequently observed phenomenon of increased release of
organic matter in the stationary phase can therefore only be taken as
an indication of a causal relationship with nutrient stress in these
cases.
A recent exception is formed by the work of Soeder and Bolze (1981)
on the effects of sulphate deficiency on the release of dissolved
organic matter in cultures of Sc.enedeJ.>mM obliquM.
Sulphate deficiency
definitely stimulated this release.
The field studies involving natural phytoplankton can also provide
only indications and little solid information on the relationship be-
carbohydrate had ceased.
The concentration of extracellular polysaccharide in the medium of
this c. a66~ culture increased from 6.5 to 43 mg/l between the 12th
and the 32nd day. During the last 13 days of the experiment this was
practically the only photosynthate produced by the alga.
Ignatiadesand Fogg (1973) also studied the influence of nutrients
(N, P and Si together) on the release of organic matter by phytoplankton. These authors were also applying "f" media in dilution series,
from "f" to "f"/60 and found that excretion of photosynthate increased
from 1% to 65% of photosynthetically fixed carbon as the nutrients were
diluted (constant N: P: Si); the main excretion took place after the
exponential phase. Since the authors worked with S. c.0-6.ta.tum rather
small quantities of material were released.
In cultures with 5 . 105
cells/ml which had been cultivated at the lowest nutrient levels (maxi-3 -1
mum excretion) a little more than 1 gem h
was produced. Although
these experiments demonstrate that nutrient stress favours production
of extracellular compounds by planktonic algae, the choice of the alga
and small quantities excreted as a consequense of this will not convince the ecologist that algal excretion is important in the energy
flow in the sea.
Myklestad and Haug kept reasonable track in their experiments with
the concentration of nutrient salts in the medium as the cultures developed, and they varied the nutrients independently. Excretion could
therefore to some extent be correlated with nutrient levels in the
medium.
In general, studies on excretion do not include determination
of the concentration of nutrients in the medium throughout the experiments. The frequently observed phenomenon of increased release of
organic matter in the stationary phase can therefore only be taken as
an indication of a causal relationship with nutrient stress in these
cases.
A recent exception is formed by the work of Soeder and Bolze (1981)
on the effects of sulphate deficiency on the release of dissolved
organic matter in cultures of Sc.enedeJ.>mM obliquM.
Sulphate deficiency
definitely stimulated this release.
The field studies involving natural phytoplankton can also provide
only indications and little solid information on the relationship be-