Nutrient source effects on growth rate-irradiance relationships in the Texas brown tide Aureoumbra iagunensis
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The Texas brown tide is a dense, persistent bloom of the small (4-5 [Greek small letter mu]m diameter) pelagophyte alga Aureoumbra lagunensis. The first A. lagunensis bloom persisted in the Laguna Madre of Texas without interruption from 1990-1997. A. lagunensis reached bloom densities once again beginning in the spring of 2005. Although the original Texas brown tide bloom persisted for 8 years, only limited information on the growth characteristics of the causative organism was gathered. The purpose of this study was to investigate the effect that dissolved organic nitrogen (DON) availability and reduced photosynthetically available radiation (PAR) have on the growth of A. lagunensis. Such interactions seem critical for blooms of the morphologically and genetically similar east coast brown tide species Aureococcus anophagefferens. We tested the ability of A. lagunensis strain TBA-2 to utilize DIN (NO2 - or NH4+) and DON (urea or glutamate) as the sole source of nitrogen under 8 light intensities ranging from limiting to saturating irradiances using a light gradient table. The experiments demonstrated significantly higher measured TBA-2 growth on NH4 + (1.0 div day-1) compared to urea (0.8 div day-1), glutamate (0.54 div day-1) and NO2 - (0.48 div day-1). These findings differ from A. anophagefferens studies that noted no significant variation in maximum growth when cultured on DIN (NO3 -) or DON (urea). Growth rate vs. irradiance curves revealed that the tested forms of organic nitrogen do not enhance growth of TBA-2 at any irradiance above that observed for NH4+-grown TBA-2. Furthermore, growth efficiency [Greek small letter alpha] at low irradiances did not vary significantly between nitrogen treatments. Ancillary experiments demonstrated TBA-2 growth on dissolved organic phosphorous (DOP) when supplied with NH4+ and urea with maximum growth rates of 1.37 and 0.46 div day-1 respectively. TBA-2 growth on urea and organic phosphorous was significantly lower than when inorganic phosphorous was the sole source of P. We hypothesize that A. lagunensis may utilize ambient DON and DOP to continue to grow and persist in the Laguna Madre when inorganic forms become environmentally limiting.