PLANKTON I N NITROOEN A N D PHOSPHORUS CYCLES
131
theless, renewed interest in Putter’s hypothesis has been stimulated by
a series of studies by Stephens and co-workers, summarized by
Stephens (1968), which have shown that a largc number of marine
animals, representing many different phyla, are able to remove organic
substances, including nitrogenous compounds such as amino acids, from
low concentrations in sea water. Most of the findings were made with
soft-bodied animals ; but McWhinnie and Johanneck (1 966) have
briefly described evidence consistent with the view that the Antarctic
euphausiid Euphausia tricccantha Holt and Tattersall can absorb
certain organic compounds directly from sea water.
Concerning nitrogenous substances, Corner and Cowey ( 1964) have
emphasized that the uptake of amino acids by zooplankton from sea
water would have t o take place against a high concentration gradient
and therefore involve considerable metabolic work. They drew attcntion to the presence of a relatively large concentration of free amino
acids in the tissues of Calanus finmarchicus, the quantity accounting for
1620% of the protein content of the animal (Cowey and Corner,
1963a). Furthcr evidence for the presence of high concentrations of
free amino acids in zooplankton has been found by Jeffries (1969)
using Acurtia clausi Giesbrecht and A . toma. The total amount of free
amino acids in CalanusJinmarchicus was 2.45 mg N/g wet weight ; and
assuming a wet weight : dry weight ratio of 6 : 1, this would mean that
the concentration of amino-acid N present in the tissue fluids was
roughly 3 mg/ml. So far, analyses of the levels of amino acids dissolved
in sea water have given values of only 2-1 6 pg amino acid11 (Chau and
Riley, 1966). Accordingly, these substances would have t o be transported against a concentration gradient of 10s-107 : I . However,
Stephens (1968) calculates that for an amino acid such as glycine the
work involved in moving against a high concentration gradient represents only a small fraction of the energy derivable by oxidation of the
molecule. He also estimates that the fraction of metabolic energy
required t o account for the accumulation of amino acids is not likely to
be more than 3-4% under natural conditions, and the process is thcrcfore “ energetically possible ”.
On the other hand, Johannes et ul. (1969) claim that the removal of
radioactively labelled organic compounds from solution by marine
invertebrates does not constitute proof of net uptake of these substances, for none of the studies made previously involved measurement
of the total release rates of these compounds by the animals. According
to Johannes et ul. (1969) there is, in fact, more likely t o be a net loss
than a net gain of organic substances, such as amino acids, by marine
invertebrates.
131
theless, renewed interest in Putter’s hypothesis has been stimulated by
a series of studies by Stephens and co-workers, summarized by
Stephens (1968), which have shown that a largc number of marine
animals, representing many different phyla, are able to remove organic
substances, including nitrogenous compounds such as amino acids, from
low concentrations in sea water. Most of the findings were made with
soft-bodied animals ; but McWhinnie and Johanneck (1 966) have
briefly described evidence consistent with the view that the Antarctic
euphausiid Euphausia tricccantha Holt and Tattersall can absorb
certain organic compounds directly from sea water.
Concerning nitrogenous substances, Corner and Cowey ( 1964) have
emphasized that the uptake of amino acids by zooplankton from sea
water would have t o take place against a high concentration gradient
and therefore involve considerable metabolic work. They drew attcntion to the presence of a relatively large concentration of free amino
acids in the tissues of Calanus finmarchicus, the quantity accounting for
1620% of the protein content of the animal (Cowey and Corner,
1963a). Furthcr evidence for the presence of high concentrations of
free amino acids in zooplankton has been found by Jeffries (1969)
using Acurtia clausi Giesbrecht and A . toma. The total amount of free
amino acids in CalanusJinmarchicus was 2.45 mg N/g wet weight ; and
assuming a wet weight : dry weight ratio of 6 : 1, this would mean that
the concentration of amino-acid N present in the tissue fluids was
roughly 3 mg/ml. So far, analyses of the levels of amino acids dissolved
in sea water have given values of only 2-1 6 pg amino acid11 (Chau and
Riley, 1966). Accordingly, these substances would have t o be transported against a concentration gradient of 10s-107 : I . However,
Stephens (1968) calculates that for an amino acid such as glycine the
work involved in moving against a high concentration gradient represents only a small fraction of the energy derivable by oxidation of the
molecule. He also estimates that the fraction of metabolic energy
required t o account for the accumulation of amino acids is not likely to
be more than 3-4% under natural conditions, and the process is thcrcfore “ energetically possible ”.
On the other hand, Johannes et ul. (1969) claim that the removal of
radioactively labelled organic compounds from solution by marine
invertebrates does not constitute proof of net uptake of these substances, for none of the studies made previously involved measurement
of the total release rates of these compounds by the animals. According
to Johannes et ul. (1969) there is, in fact, more likely t o be a net loss
than a net gain of organic substances, such as amino acids, by marine
invertebrates.