The availability of refractory dissolved organic nitrogen (rDON) in wastewater effluent to eutrophying organisms is an important engineering question. Model diazotrophic and non-diazotrophic algae were cultured in a tertiary effluent from a local nitrifying-denitrifying facility, and
the extent to which DON was utilized as a nitrogen source was investigated. Cultures were incubated for approximately one month. Changes in the molecular weight distribution of DON were also examined. Selenastrum capricornutum was found to use 26% of DON present. When microbes
filtered from a local surface water body were present, the fraction of DON used by S. capricornutum rose to 68%. Anabaena variabilis fixed nitrogen, and no net DON utilization could be detected. Introducing microbes from a surface water source was found to not have any
observable effect on A. variabilis DON utilization. In the absence of algae, the surface water microbes did not use a statistically significant fraction of the DON present. However, S. capricornutum and A. variabilis were both observed to alter the molecular weight distribution
of DON present during the culturing period. Algae were observed to utilize a higher molecular weight fraction (30kDa to 100kDa) of DON.
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