THE PRODUCTION O F MARINE PLANKTON
139
copper, zinc, cobalt, and molybdenum are usually considered as such
limiting trace elements. With the more crude culture media such as are
used for mass cultures of marine algae, there are usually sufficient concentrations of these elements in the chemicals added, or in such
materials as sterilized soil extract added to the cultures, to provide
sufficient trace substances. However, where precise artiflcial media for
the culture of marine algae are listed (e.g. Provasoli et al., 1957; Provasoli, 1963) small quantities of these elements must be added to the
medium.
Although culture experiments demonstrate that these elements are
essential to the growth of phytoplankton, our knowledge of the concentrations of these elements in the sea, and even more of the spatial
and temporal variations, is so limited that we cannot say whether they
are ever limiting in nature. The position is corn plicated by the fact that
some of these elements such as manganese and iron occur to a remarkable extent as particulate matter varying in size from colloidal aggregates to particles that may be retained by normal filtration. Indeed the
amount of true ionic iron which can exist in sea water is extremely
small, Algal cells can make use of some particidate forms, for example,
of iron, and therefore the question of whether such elements ever become
limiting is even more difficult.
Silicon might be considered as a possible limiting nutrient in so far as
it is essential to the growth of diatoms and to silicoflagellates. Normally
silica is present in relatively considerable quantities, though even in the
Antarctic where very large amounts exist, Ha& has reported the presence of exceptionally thin-shelled diatoms which may reflect a lack of
silica over a short period of time. Silica, however, appears to be rapidly
regenerated in sea water, and it is extremely doubtful whether it can
ever be regarded as seriously limiting production. The possibility still
exists that with iron, and possibly with manganese, aggregation of
particles may occur in the upper layers of an ocean so that the element
might sink and be lost to the euphotic zone. If' this depletion occurs at
all, it will take place in nutrient-poor waters, especially in open oceans
far from land. The experiments of Menzel, Hulburt, and Ryther (1962)
in which Sargasso Sea water was enriched with nitrate, phosphate, and
iron may be of interest here. In short-term experiments iron as well as
nitrate and phosphate appeared to be essential for growth of phytoplankton, but to some extent the result varied with the species of algae
present. This raises the very interesting possibility that not only may
phytoplankton algae differ in their minimum nitrate and phosphate
requirements, but they may show differences with regard to their requirements for trace elements. For example, it has been suggested that
Skeletonem has fairly high iron requirements, and indeed this mainly
139
copper, zinc, cobalt, and molybdenum are usually considered as such
limiting trace elements. With the more crude culture media such as are
used for mass cultures of marine algae, there are usually sufficient concentrations of these elements in the chemicals added, or in such
materials as sterilized soil extract added to the cultures, to provide
sufficient trace substances. However, where precise artiflcial media for
the culture of marine algae are listed (e.g. Provasoli et al., 1957; Provasoli, 1963) small quantities of these elements must be added to the
medium.
Although culture experiments demonstrate that these elements are
essential to the growth of phytoplankton, our knowledge of the concentrations of these elements in the sea, and even more of the spatial
and temporal variations, is so limited that we cannot say whether they
are ever limiting in nature. The position is corn plicated by the fact that
some of these elements such as manganese and iron occur to a remarkable extent as particulate matter varying in size from colloidal aggregates to particles that may be retained by normal filtration. Indeed the
amount of true ionic iron which can exist in sea water is extremely
small, Algal cells can make use of some particidate forms, for example,
of iron, and therefore the question of whether such elements ever become
limiting is even more difficult.
Silicon might be considered as a possible limiting nutrient in so far as
it is essential to the growth of diatoms and to silicoflagellates. Normally
silica is present in relatively considerable quantities, though even in the
Antarctic where very large amounts exist, Ha& has reported the presence of exceptionally thin-shelled diatoms which may reflect a lack of
silica over a short period of time. Silica, however, appears to be rapidly
regenerated in sea water, and it is extremely doubtful whether it can
ever be regarded as seriously limiting production. The possibility still
exists that with iron, and possibly with manganese, aggregation of
particles may occur in the upper layers of an ocean so that the element
might sink and be lost to the euphotic zone. If' this depletion occurs at
all, it will take place in nutrient-poor waters, especially in open oceans
far from land. The experiments of Menzel, Hulburt, and Ryther (1962)
in which Sargasso Sea water was enriched with nitrate, phosphate, and
iron may be of interest here. In short-term experiments iron as well as
nitrate and phosphate appeared to be essential for growth of phytoplankton, but to some extent the result varied with the species of algae
present. This raises the very interesting possibility that not only may
phytoplankton algae differ in their minimum nitrate and phosphate
requirements, but they may show differences with regard to their requirements for trace elements. For example, it has been suggested that
Skeletonem has fairly high iron requirements, and indeed this mainly