Competition for Urea among Estuarine Microorganisms

Phytoplankton were responsible for the major part of the urea decomposition in the Savannah—Wilmington—Ogeechee estuaries and adjacent coastal waters in Georgia. This is an exception to the general rule that bacteria are favored of algae in the competition for dissolved organic compounds. Filtration of estuary water through a 20—μ mesh filter did not significantly change the concentration of urea—decomposing bacteria; there was, however, a significant change in phytoplankton cell concentrations, chlorophyll a, and urea decomposition rates. Thus, any differences in the decomposition of urea in filtered and unfiltered aliquots could be attributed to the phytoplankters removed by filtration. Average cell concentrations and chlorophyll a removed by filtration was 15% and 39% of the totals, respectively. However, the removal of urea—decomposing activity average 53% of the total. This indicates that the phytoplankters removed by filtration, mainly diatoms with large cross—sectional areas such as Rhizosolenia, Chaetoceros, Asterionella, Thalassiosira, etc., were responsible for proportionally higher amounts of urea decomposition than would be indicated on a purely cell—number basis. Urea concentrations ranged from 0.6 to 8.9 μM, with the highest values (average 3.41 μM) being found in the more polluted Savannah estuary. Lower values, generally 1—2 μM, were observed in the Wilmington and Ogeeche estuaries. Fresh water generally had slower urea decomposition rates (average 6.2 μmmoles liter — 1 hr — 1 ) than brackish waters (average 19.6 μmmoles liter — 1 hr — 1 ). Turnover times for urea, calculated from these average values, was 25 days for fresh water and 4 days for brackish water.