Phosphorus is commonly a limiting nutrient in freshwater, largely due to seasonal availability and rapid uptake. Despite limiting conditions, bacterial growth is continual and turnover relatively rapid. Past studies have shown that bacteria with hydrophobic cell surfaces grow faster on phosphorus from phospholipids than on inorganic phosphate in vitro, suggesting that phospholipids may be a source of phosphorous sustaining growth in a portion of the bacterial community. Our objective was to assess potential contribution of phospholipid-P to an aquatic microbial community. We sampled two habitats (n=10) in Thompson Lake, IL; subsurface water (~10 cm below surface) and air-water interface (AWI; collected with fiberglass screen), an environment with an accumulation of organic matter. Two approaches were used in this assessment. First we determined abundance of hydrophobic bacteria by fluorescently labeling all cells with DAPI and co-labeling hydrophobic cells with lipophilic dye, DiO. Second we cultured bacteria able to grow on phospholipid-P. Water samples were spread plated onto minimal media containing the phospholipid phosphatidylethanolamine as the only source of phosphorus. Up to 150 colonies were randomly selected from each sample and transferred to a second phospholipid plate to enumerate colonies able to produce clearings (assumed to indicate phospholipid hydrolysis) in the agar.

Results/Conclusions

Hydrophobic bacteria composed 15.0% ± 0.02 of total bacteria in both habitats. The proportion of colonies able to hydrolyze phospholipid-P was nearly five times higher in air-water interface (30.5% ± 2.2) than subsurface (6.8% ± 2.3) samples (paired t-test, p<0.001). Sequencing results (16S rRNA; 40 isolates) showed that isolates able to use phosphatidylethanolamine were distributed across the beta- and gamma-proteobacteria. These results suggest that a substantial fraction of aquatic microorganisms can have hydrophobic cell surfaces, a trait linked to phospholipid harvesting. Clearing isolate quantification indicates that the adaptation for phospholipid-P use is more prevalent at the air-water interface, making the AWI an essential habitat to consider when re-thinking phosphorus cycling in lakes. The observation that the percent of hydrophobic cells is similar between habitats while the ability to lyse phospholipid-P is not suggests that cell surface hydrophobicity may not be directly indicative of ability to use phospholipids. Further characterization of bacteria able to use phospholipid-P will enable us to test the extent to which phosphorus derived from membrane lipids provides a sustainable source of phosphate for aquatic microbial communities.