CHANGES IN INTRACELLULAR NITROGEN POOLS AND FEEDBACK CONTROLS ON NITROGEN UPTAKE IN CHAETOMORPHA-LINUM (CHLOROPHYTA)

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH4+ and NO3-) rates were determined for Chaetomorpha linum (Muller) Kutzing grown sequentially under nutrient-saturating and nutrient-limiting conditions. The size of individual pools in N-sufficient algae could be ranked as residual organic N (RON) comprised mainly of amino acids and amino compounds > protein N > NO3- > NH4+ > chlorophyll N. When the external N supply wa s removed, growth rates remained high and individual N pools were depl eted at exponential rates that reflected both dilution of exiting pool s by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the pr otein pool increased throughout the N depletion period and thus did no t serve a storage function. RON was the largest storage reserve nitrat e was the second largest, but more temporary, storage pool that was de pleted within 10 days. Upon N resupply, the RON pool increased 3 x fas ter than either the inorganic or protein pools, suggesting that protei n synthesis was the rate-limiting step in N assimilation and caused a buildup of intermediate storage compounds. Maximum uptake rates for bo th NH4+ and NO3- varied inversely with macroalgal N status and appeare d to be controlled by changes in small intracellular N pools. Uptake o f NO,- showed an initial lag phase, but the initial uptake of NH4+ was enhanced and tons present only when the intracellular NH4+ pool was d epleted in the absence of an external N supply. A strong negative corr elation between the RON pool size and maximum assimilation between the RON pool size and maximum assimilation uptake rates for both NH4+ and NO3- suggested a feedback control on assimilation uptake by the build up and depletion of organic compounds. Enhanced uptake and the accumul ation of N as simple organic compounds or nitrate both provide a tempo rary mechanism to buffer against the asynchrony of N supply and demand in C. linum.