PLANKTON I N NITROQEN AND PHOSPHORUS CYCLES
121
K t values; and (3) that oceanic species usually have low K: values,
which would be in accord with their need to assimilate nutrients at the
low concentrations present away from the land. This is also in agreement with the low values obtained for the natural populations observed
by MacIsaac and Dugdale (1969). Surprisingly, euryhaline species such
as Dunaliella tertiolecta and Momchrysis lutheri usually had low K:
values also, even though their natural habitat would normally be
expected to have high levels of nutrients.
V. UPTAKE OF PHOSPHORUS COMPOUNDS BY PHYTOPLANKTON
A. Inorganic forms of phosphorus
Probably because phosphate is less often a limiting nutrient, its
uptake by marine phytoplankton has not been studied in as much
detail as that of nitrate. Phytoplankton in culture media where the
phosphate concentration is limiting continue to grow long after the
nutrient has been stripped from solution, albeit at a gradually reducing
rate (Ketchum, 1939b; Goldberg et al., 1951 ; Kuenzler and Ketchum,
1962). It thus appears that cells are able to accumulate phosphate in
excess of their immediate requirements, and to utilize this store at times
of nutrient depletion in the external medium. Ketchum (1939b) showed
that Phueodmtylum tricornutum can replenish this reserve by taking up
phosphate even in the dark. The amount of the nutrient absorbed by
the cells in this way (between 1.1 and 8-6 x lo-* pg-atoms PO:--P/cell)
increased with the length of time for which the cells had utilized their
phosphate reserve, and a constant cellular concentration independent
of the phosphate level in the medium was finally attained.
The uptake of phosphate by depleted cells is a very rapid process.
Kuenzler and Ketchum (1962), for instance, found that in 12 hours an
increase from 4 to 66 x
pg-atoms PO:--P/cell could take place in
illuminated cultures of Phaeodaetylum tricornutum from media with
initial concentrations of 30 or 80 pg-atoms PO:--P/I.
During the
ensuing growth of the cultures, phosphate was completely removed
from the media and there was a gradual diminution in the cellular
concentration. Goldberg et al. (1951) showed that the phosphate
content of exponentially growing cells of Asterionella japonica was a
linear function of the level in the culture medium. However, up to
50% of the phosphate absorbed appeared to be only loosely bound into
the cells, as it was readily exchangeable.
B. Organic forms of phosphorus
Organic phosphate esters may also provide a source of phosphorus
for phytoplankton (Chu, 1946; Harvey, 1953). Kuenzler and Perras
121
K t values; and (3) that oceanic species usually have low K: values,
which would be in accord with their need to assimilate nutrients at the
low concentrations present away from the land. This is also in agreement with the low values obtained for the natural populations observed
by MacIsaac and Dugdale (1969). Surprisingly, euryhaline species such
as Dunaliella tertiolecta and Momchrysis lutheri usually had low K:
values also, even though their natural habitat would normally be
expected to have high levels of nutrients.
V. UPTAKE OF PHOSPHORUS COMPOUNDS BY PHYTOPLANKTON
A. Inorganic forms of phosphorus
Probably because phosphate is less often a limiting nutrient, its
uptake by marine phytoplankton has not been studied in as much
detail as that of nitrate. Phytoplankton in culture media where the
phosphate concentration is limiting continue to grow long after the
nutrient has been stripped from solution, albeit at a gradually reducing
rate (Ketchum, 1939b; Goldberg et al., 1951 ; Kuenzler and Ketchum,
1962). It thus appears that cells are able to accumulate phosphate in
excess of their immediate requirements, and to utilize this store at times
of nutrient depletion in the external medium. Ketchum (1939b) showed
that Phueodmtylum tricornutum can replenish this reserve by taking up
phosphate even in the dark. The amount of the nutrient absorbed by
the cells in this way (between 1.1 and 8-6 x lo-* pg-atoms PO:--P/cell)
increased with the length of time for which the cells had utilized their
phosphate reserve, and a constant cellular concentration independent
of the phosphate level in the medium was finally attained.
The uptake of phosphate by depleted cells is a very rapid process.
Kuenzler and Ketchum (1962), for instance, found that in 12 hours an
increase from 4 to 66 x
pg-atoms PO:--P/cell could take place in
illuminated cultures of Phaeodaetylum tricornutum from media with
initial concentrations of 30 or 80 pg-atoms PO:--P/I.
During the
ensuing growth of the cultures, phosphate was completely removed
from the media and there was a gradual diminution in the cellular
concentration. Goldberg et al. (1951) showed that the phosphate
content of exponentially growing cells of Asterionella japonica was a
linear function of the level in the culture medium. However, up to
50% of the phosphate absorbed appeared to be only loosely bound into
the cells, as it was readily exchangeable.
B. Organic forms of phosphorus
Organic phosphate esters may also provide a source of phosphorus
for phytoplankton (Chu, 1946; Harvey, 1953). Kuenzler and Perras