Analysis of ammonium transporter and urease gene expression in Aureoumbra lagunensis

Abstract

The pelagophyte Aureoumbra lagunensis D. A. Stockw., DeYoe, Hargraves et P. W. Johnson, causes ecosystem disruptive algal blooms and is responsible for the longest lasting harmful algal bloom ever recorded (1989–1997). Because of Aureoumbra's small size and its inability to use nitrate, it has been hypothesized that its ability to use ammonium and organic nitrogen, especially at low concentrations, contributed to the unusual persistence of this bloom. This study examined the expression of genes involved in organic and inorganic nitrogen assimilation to determine whether transcript levels were indicative of nitrogen source use and/or sufficiency in this organism. Large volume batch cultures of Aureoumbra were grown with either ammonium or urea as a nitrogen source. Physiological characteristics (C:N, chlorophyll a cell −1, and F v/ F m) were monitored throughout the growth period, and the expression of the Amt-s1, Amt-s2 and Urec genes was assayed at early-, mid- and late-exponential phases. The transcript levels of only one gene, Amt-s1, appeared to change in response to changing nitrogen concentration, and only by ammonium. The ability to grow and maintain the efficiency of PSII in the absence of external nitrogen suggests that Aureoumbra is well-adapted to low-nutrient environments. The results of this study contribute to our understanding of how algae in general cope with low nutrient availability and should ultimately help to define the dynamics of these HAB events.