The role of the picoeukaryote Aureococcus anophagefferens in cycling of marine high–molecular weight dissolved organic nitrogen

Environmental evidence suggests that Aureococcus anophagefferens (Pelagophyceae), a eukaryotic picoplankton that blooms in coastal seawaters, can outcompete other organisms because of its ability to use abundant dissolved organic nitrogen (DON). To test this hypothesis, we isolated A. anophagefferens in axenic culture and monitored its growth on high–molecular weight (HMW) DON collected from sediment pore waters, a putative source for DON in bays where blooms occur. HMW DON originating from pore water had a substantially higher protein content than surface seawater DON. We found that A. anophagefferens could deplete 25–36% of the available nitrogen in cultures with HMW DON as the sole source of nitrogen and that this corresponded well with the protein fraction in pore-water HMW DON. High rates of cell surface peptide hydrolysis and no detectable N-acetyl polysaccharide hydrolysis, together with the high percentage of hydrolyzable amino acids compared to hydrolyzable aminosugars present in the HMW DON, pointed to the protein fraction as the more likely source of nitrogen used for growth. Whether or not nitrogen scavenging from protein is a common mechanism in phytoplankton is at present unknown but needs to be investigated. Intense blooms of Aureococcus anophagefferens, or brown tides, were first observed in 1985 at three separate locations along the northeast coast of the United States (Cosper et al. 1987; Sieburth et al. 1988; Olsen 1989). The simultaneous occurrence of brown tides in geographically isolated regions along the U.S. East Coast suggested that regional-scale, climatological forcing might play a role in triggering the blooms. Yet, attempts to correlate bloom initiation with rainfall (Cosper et al. 1990) or wind stress (Vieira and Chant 1993) have failed to provide a consistent explanation for the bloom. In a review of historical data, LaRoche et al. (1997) hypothesized that brown tide blooms are controlled by interannual variability in the relative supply of dissolved inorganic and organic nitrogen, determined in part by groundwater flow. The supply of inorganic nitrogen through groundwater is greater than any other external source of nitrogen to Long Island bays, and a decrease in groundwater input coupled with a seasonal build-up of DON 1 To whom correspondence should be addressed. Present address: Alfred-Wegener Institute for Polar and Marine Research, Postfach 120161, Bremerhaven D-27515, Germany.