Mulholland, M. R. Chesapeake Biological Lab, University of Maryland Center for Environmental Science,
Capone, D. R. Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science,

It is generally held that the cyanobacterium, Trichodesmium, relies entirely upon N2 fixation for their N requirement. However, N-based growth rate estimates in natural populations are generally substantially lower than those based on C, suggesting utilization of other N sources. Moreover, Trichodesmium releases combined N (as DON) and in blooms this can result, after regeneration, in elevated ambient NH4+ levels. We examined the capacity of Trichodesmium to assimilate various forms of combined N (NH4+, NO3-, urea, and glutamate) at tracer levels. Our results indicate that in laboratory cultures of Trichodesmium IMS101 and NIBB1067 between 70 and 85% of the daily nitrogen uptake in cultures can be accounted for by combined N uptake (primarily NH4+). While the assimilated NH4+ ultimately derives from N2 fixation, NH4+ uptake can represent a substantial portion of N utilization. Rates of NH4+ uptake paralleled the concentration of NH4+ in the media. N2 fixation was restricted to the light period and accounted for the balance of total daily uptake. Both Trichodesmium NIBB1067 and field populations had a high affinity for NH4+. Carbon incorporation is also being examined over a diel cycle in cultures of Trichodesmium IMS101 to determine whether measured N utilization satisfied the N demand estimated from rates of C fixation. Our results suggest that in nature, Trichodesmium populations do utilize ambient pools of NH4+, possibly for net growth.
Day: Wednesday, Feb. 3
Time: 10:45 - 11:00am
Location: Hilton of Santa Fe
Code: SS38WE1045H