AMMONIUM SURGE UPTAKE AND INHIBITION OF NITRATE UPTAKE BY SMALL AND LARGE CELL-SIZED PSEUDO-NITZSCHIA SPECIES FROM THE PACIFIC NORTHWEST

Regina L. Radan¹, Maureen E. Auro¹, and William P. Cochlan¹

¹Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA, 94920-1205

Despite the importance of nitrogen in maintaining blooms of toxigenic diatoms and their production of the neurotoxic amino acid, domoic acid, the nitrogenous nutrition of Pseudo-nitzschia species is still poorly known, and is primarily limited to the larger cell-sized species. Quantification of the physiological capacity for nitrogen uptake by Pseudo-nitzschia species under both N-replete and N-deplete conditions is a necessary prerequisite for an understanding of the success of Pseudo-nitzschia relative to other phytoplankton in coastal systems, and may help to identify the proximate causes of toxin production by these diatoms. The nitrogen uptake capabilities of three Pseudo-nitzschia species, isolated from the Pacific Northwest and maintained in unialgal cultures, are presented here: the transient elevated ‘surge’ uptake rates of ammonium by P. cf. delicatissima and P. multiseries, and the ammonium inhibition of nitrate uptake by P. cf. cuspidata and P. multiseries. Elevated ammonium uptake rates were determined in duplicate, N-starved cultures of the two species (cultures depleted of external nitrate for ~ 2 generations) using both the accumulation of N-15 labeled ammonium into the cells and the disappearance of ammonium from the medium at varying intervals (5-30 min) over a six-hour period. Both the large and small cell-sized species demonstrated a capacity for transient ‘surge’ uptake in the first minutes following ammonium enrichment, but P. cf. cuspidata exhibited much faster rates than P. multiseries. However within 0.5-1.0 h, ammonium specific uptake rates decreased and stabilized to values sufficient to support the pre-conditioned growth rates observed for both species prior to N starvation. During multi-day experiments, duplicate nitrogen-replete cultures of P. cf. cuspidata demonstrated complete inhibition (suppression) of nitrate uptake by elevated ammonium concentrations. Only after the ammonium concentrations were decreased by the cells to below the ‘threshold’ concentration (< ~5 µM) did these diatoms begin their utilization of nitrate. These laboratory results will be discussed with respect to the relative surface area per unit cell volume of the large and small cell-sized species, and the possible ecological consequences of simultaneous uptake of nitrate and ammonium at the ambient N concentrations normally found in the coastal waters of the Pacific Northwest.