Physiological responses of Aureoumbra lagunensis and Synechococcus sp. to nitrogen addition in a mesocosm experiment

Aureoumbra lagunensis is the causative organism of the Texas brown tide and is notable because it dominated the Laguna Madre ecosystem from 1990 to 1997. This species is unusual because it has the highest known critical nitrogen to phosphorus ratio (N:P) for any microalgae ranging from 115 to 260, far higher than the 16N:1P Redfield ratio. Because of its high N:P ratio, Aureoumbra should be expected to respond to N additions that would not stimulate the growth of competitors having the Redfield ratio. To evaluate this prediction, a mesocosm experiment was performed in the Laguna Madre, a South Texas coastal lagoon, in which a mixed Aureoumbra–Synechococcus (a cyanobacterium) community was enclosed in 12 mesocosms and subjected to nitrogen addition (6 controls, 6 added ammonium) for 16 days. After day 4, added nitrogen did not significantly increase Aureoumbra specific growth rate but the alga retained dominance throughout the experiment (64–75% of total cell biovolume). In control mesocosms, Aureoumbra became less abundant during the first 4 days of the experiment but rebounded by the end of the experiment and was dominant over Synechococcus. Despite the lack of a strong positive growth response, Aureoumbra did respond physiologically to N addition. By the end of the experiment, the average N:P ratio of the Aureoumbra-dominated community was 86 in the N+ treatment and 41 in the control, indicating that the alga became less N-limited in the N+ treatment. The average C:N ratio was 6.6 in the N+ treatment (8.6 in the control) and suggests that the alga was not N-limited, however, C:N ratio may not be a good indicator of nitrogen limitation since this alga can produce significant quantities of carbon-containing extracellular polysaccharides, depending on growth conditions. Both Aureoumbra cellular chlorophyll fluorescence and cell size increased in response to added N, indicating a reduction in N limitation. It appeared that the N additions were not large and/or frequent enough to stimulate Aureoumbra growth. The main competitor, the unicellular cyanobacterium Synechococcus, responded positively to the nitrogen addition by increased specific growth rate. Unlike Aureoumbra, no significant effect on Synechococcus cellular pigment fluorescence or cell size was noted. Literature data suggest that Synechococcus, like Aureoumbra, may have a critical N:P ratio much higher than 16:1, which could explain its response.