ON AMMONIA FUTILE CYCLING IN A MARINE UNICELLULAR ALGA

Futile cycling of ammonia, involving passive release of NH3 and active transport of NH4+, is potentially a major energetic cost to a unicell ular organism. Nitrogen-starved cells of the marine diatom Phaeodactyl um tricornutum possess a sodium-dependent transport system for the amm onium (NH4+) analogue methylammonium (CH3NH3+). In nitrogen-replete ce lls and nitrogen-starved cells incubated in the absence of sodium the rate of methylamine uptake was low and increased as a linear function of increasing methylamine concentration. Cells incubated with 21 mu M ammonia or 500 mu M methylamine (which give the same concentration of uncharged base), had similar rates of uptake in both nitrogen-starved cells. In nitrogen-replete cells there was no inhibition of ammonia or methylamine uptake in the absence of sodium, but there was a marked i nhibition for both with nitrogen-starved cells. However, despite the a bolition of active CH3NH3+ and NH4+ uptake by nitrogen-starved cells i n the absence of sodium, these cells did not release ammonia. Moreover , neither urea or low pH (which decreased the rate of ammonia uptake) induced release of ammonia in the absence of sodium. In contrast, nitr ogen-replete cells released ammonia in darkness, with greater release occurring in the absence of sodium. Absence of sodium was as effective as the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) i n inhibiting methylamine uptake by nitrogen-starved cells. However, re lease of preaccumulated methylamine by nitrogen-starved cells only occ urred in the presence of CCCP; there was little release in the absence of sodium. These results suggest that futile cycling of ammonia acros s the plasma membrane of P. tricornutum is not quantitatively signific ant.