Nutrient-limited growth of Aureoumbra lagunensis (Pelagophyceae), with implications for its capability to outgrow other phytoplankton species in phosphate-limited environments

Growth and chemical compositional characteristics of the brown tide-forming alga, Aureoumbra lagunensis Stockwell, DeYoe, Hargraves et Johnson, were studied through a series of nitrogen-limited and phosphate-limited continuous cultures over a range of growth rates. The specific growth rate of A. lagunensis was hyperbolically related to the cell quota of the limiting nutrient in ammonium-limited cultures. In phosphate-limited cultures, the relationship was best described by a straight line. The N cell quota of A. lagunensis ranges from about 20 fmol at zero growth rate under N-limited conditions to a high of roughly 85 fmol under N-replete conditions. Similarly, the P cell quota of A. lagunensis ranges from about 0.15 fmol at zero growth rate under P-limited conditions to a high of 2 fmol under P-replete conditions. Aureoumbra lagunensis has a very high N:P critical ratio (> 100). The high N:P critical ratio, as well as the organism's apparent ability to use forms of phosphorus other than phosphate under severe phosphate deficiency, may partially explain its success in P-limited environments, such as the Laguna Madre. In addition, a uniqe quadratic relationship between the productivity index (PI) and growth rate was discovered. Such a relationship supports an earlier argument that PI may not be a good indicator of nutritional status.