The relative availability of nitrogen and phosphorus in stream water has a strong influence on the quantity and stoichiometry of periphyton and microbial biomass. In addition, algae and bacteria differ in their nutrient storage capacity, leading us to hypothesize a link between storage capacity and N:P stoichiometry of uptake in response to nutrient additions. We investigated the effect of increasing nitrogen to phosphorus ratio on phosphate uptake in algae and bacteria cultures. Three treatments were used to control the colonization and growth of periphyton to produce biofilms dominated by either algae or bacteria on artificial substrates placed in Skunk Creek, a small, nitrogen-limited stream. Maracyn tablets (containing the antibiotic minocycline) were used to control the growth of bacteria, Algae Fix Pond Care was used to control the growth of algae, and diuron (a photosystem II inhibitor) was used to control the growth of both algae and cyanobacteria.  Because bacteria and cyanobacteria are single celled organisms with low capacity to store nutrients, we predicted they would be homeostatic in their nitrogen to phosphorus uptake ratio.  Therefore, phosphorus uptake was predicted to increase when N:P of nutrient supply was increased from 5 to 48:1.  In contrast, algae are more complex organisms and have the ability to store non-limiting nutrients in vacuoles. Under N limitation, we hypothesize that algal cells would contain significant PO₄ stores, which they would draw upon when N supply increased.  Therefore, algal phosphorus uptake from the water column was predicted to show little or no change when subjected to the high N:P treatment. 

Results/Conclusions

The results show that algal phosphorus uptake decreased when N:P was increased (13.01µg hr^-1L^-1 vs. 10.15µg hr^-1L^-1, respectively) and was significantly greater than the low and high treatments of either the diuron treatment (1.27µg hr^-1L^-1 vs 0.97µg hr^-1L^-1),the AlgaeFix treatment (0.51µg hr^-1L^-1 vs 0.97µg hr^-1L^-1), or the untreated controls (0.83µg hr^-1L^-1 vs 1.02µg hr^-1L^-1).  Increasing N:P of supply did not significantly increase phosphate uptake in the diuron treated cultures or the untreated controls, but did increase it in the AlgaeFix treatment.  These results show some support for the hypothesis and suggest that the mixed culture of cyanobacteria and bacteria exhibited evidence of homeostatis in N:P of uptake, while all other treatments exhibited a more plastic response.