DISCUSSION
The factors and dynamics generating the harmful algae blooms (HABs) in marine ecosystem still remains as an open question (Smayda, 2002). Using a biochemical approach, we analysed the enzymatic activities of a dinoflagellate bloom in low inorganic nutrients waters
An important result of our study is that GS activity (absolute values) was associated with high autotrophic biomass (up to 1000 µg Lâ€"œ1) and with low external nitrate and ammonium concentrations. Fan et al. (Fan et al., 2003) indicated that the dinoflagellate bloom-forming species, Prorocentrum minimum, correlates with low NO3â€"œ and high NH4+ or urea concentrations. Our finding could have ecological implications for the dinoflagellate bloom in southern Chile. GS activity correlates with the use of ammonium as an external nitrogen source and is an important enzyme that converts glutamate and ammonium into amino acids in marine algae (Syrett, 1981). Thus, the observed pattern of increasing phytoplankton biomass and GS activity levels associated with a dinoflagellate bloom (r2 = 0.57, P < 0.05; Fig. 3a) may imply that GS is an indicator of dissolved ammonium utilization. There is strong experimental evidence indicating that G-S activity is higher where the ammonium concentration is low (Falkowski and Rivkin, 1976), as well as the fact that the GS activity in several marine phytoplankton species exhibited a high affinity for ammonium levels (Bressler and Ahmed, 1984). We suggest that phytoplankton single species are able to efficiently utilize low NH4+ levels mediated by GS reaction in coastal waters of limited nitrogen sources.
The differences in NR and GS activities observed during the G. cf. chlorophorum bloom agreed with the view that eukaryotic picoplankton and dinoflagellates have been associated with high and/or frequent addition of ammonium or dissolved organic nitrogen (i.e. urea) (Wilkerson and Grunseich, 1990; Glibert and Terlizzi, 1999; Kudela and Cochlan, 2000; Berg et al., 2002; Dyhrman and Anderson, 2003; Fan et al., 2003; Lomas, 2004). There is some experimental evidence that N-deficient algal cells (low ambient nitrogen concentrations) have higher GS activity than those grown on high levels of external nitrate (Chaetoceros affinis; Slawyk and Rodier, 1986, Phaeodactylum tricornutum; Slawyk and Rodier, 1988). However, little is known about the expression of different enzymatic activities and related ecophysiological responses of the bloom-forming algal species under different sources of nitrogen (NO3â€"œ, NH4+ and urea). In field conditions, Harrison (Harrison, 1973) found that NR activity was associated with algal biomass and low nitrate concentrations in typically nitrate-rich waters during a Peridinium triquetrum bloom. Dyhrman and Anderson (Dyhrman and Anderson, 2003), found a weak inverse relationship between urease activity and dissolved inorganic nitrogen concentration in field population of the toxic dinoflagellate Alexandrium fundyense in Gulf of Maine. In our study, an inverse relationship was found between NR and GS (logâ€"œlog scale; r2 = â€"œ0.57, P > 0.05; Fig. 3b) during the dinoflagellate bloom. A possible explanation for this inverse relationship during the bloom is an apparently inhibitory effect of ammonium (internal/external) on NR activity in phytoplankton (Berges, 1997). Both, experimental and field studies, demonstrate that the relative contribution of the fluxes of dissolved organic nitrogen and dissolved inorganic nitrogen may significantly alter community properties as species composition and biomass, as well as biochemical pathways of phytoplankton