Abstract
Two marine microflagellates, isolated from the North Pacific Central Gyre, and tentatively identified as Platymonas sp. and Mantoniella sp., were examined for uptake of 14C-methylamine, a non-metabolizable analogue of ammonium. Half-saturation constants (Km) for ammonium transport, estimated from competitive inhibition of methylamine transport by ammonium, were 50 nM for Platymonas sp. and 2.9 nM for Mantoniella sp. These values are the lowest so far reported, indicating that the transport systems are well adapted to extremely low ammonium environments. The ratio of the Km for methylamine to the Km for ammonium was 17 for Platymonas sp. and 41 for Mantoniella sp. The observed ratios fell within the relatively narrow range reported for various organisms, including bacteria, fungi and phytoplankton (25.0±15.5, \(\bar x\)± SD, n=9), although the reported Kmvalues for both ammonium and methylamine vary over a thousand-fold range. The use of 14C-methylamine may therefore be a promising technique for studying the kinetics of ammonium transport by marine microorganisms, particularly those in oligotrophic environments.
Published Version
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