Bacterial growth limited by phosphorus

Recent efforts to regulate phosphorus runoff may be long overdue. In a study of bacterial growth in a pristine coastal salt marsh in South Carolina, researchers found that phosphorus, not nitrogen, was the limiting nutrient. Moreover, as the levels of phosphorus dropped, microbial heterotrophs produced more N₂O, suggesting that limiting phosphorus could reduce excess nitrogen in these marshes.

Extensive earlier research has shown that nitrogen is the limiting nutrient in coastal areas, including salt marshes. This research confirms those findings for plants (macrophytes). However, when P. V. Sundareshwar, J. T. Morris, and their colleagues at the University of South Carolina and Coastal Carolina University added nitrogen or phosphorus to unfertilized marsh plots, bacterial growth was much higher with the latter nutrient. In fact, overall bacterial growth was limited by phosphorus and secondarily by labile carbon.

These results, say the authors, show that ecosystems cannot be managed just by regulating a single nutrient such as nitrogen. For example, although hypoxia in temperate coastal waters can be linked to high levels of nitrogen, phosphorus enrichment of “black-water” rivers has been shown to increase biological oxygen demand.

Moreover, phosphorus’s relationship with nitrogen proved to be complex. As expected, plants grew best when both nitrogen and phosphorus were present. Phosphorus is known to stimulate nitrogen fixation in temperate coastal water columns and with legumes and cyanobacteria.

However, in the marsh sediments, the nutrient reduced heterotrophic nitrogen fixation unless more carbon (in the form of glucose) was added, suggesting that phosphorus loading can limit carbon for microbial processes. On the other hand, limiting phosphorus increased the loss of nitrogen through the production of N₂O, probably by a denitrification pathway. Overall, the researchers report that adding both nitrogen and phosphorus increased soil respiration and carbon turnover, which has important ramifications for climate change models factoring in carbon fixation, storage, and release. (Science 2003, 299, 563–565)