Nutrient Dynamics Nutrient Uptake and Growth Models Nitrogen Assimilation & Preference Phosphorus Nutrient Limitation Assays Nutrient Regeneration How are rates of uptake and regeneration often measured (plankton versus benthic)?

Nutrient Uptake and Growth Models Uptake rate vs substrate concentration in environment (Michaelis-Menten model). Growth rate vs substrate concentration intracellular (Droop model). Growth rate vs substrate concentration in environment (Monod model)

Nutrient Competition Large algal cells may perform luxury uptake and storage (e.g., diatoms) Small algal cells out-compete at lower concentrations. Bacteria can do both for phosphate; they compete with phytoplankton.

Nitrogen “Preference” Phytoplankton: NH4+ > NO3- ≈ (urea) Bacteria: aa> NH4+ > NO3- ≈ (urea) N2-fixation last (most E)

Phosphorus Supply

Nutrient Limitation Liebig's law of the minimum. Cellular elemental balance as a index. Environmental elemental balance. Enzyme expression as an index.

Alkaline Phosphatase (AP) Activity of Aquatic Bacteria Indicates P-Bioavailability Low to No PO43- supply: AP activity is expressed at high levels. High PO43- supply: AP activity is repressed or inhibited de novo synthesis Nucleic Acids PO43- PO43- PO43- PO43-    Phospholipids Dissolved Organic Phosphorus (DOP) = energetically costly

Nutrient Regeneration Microbial food web dominates regeneration. Bacteria important when organic matter consumed is C:N < 10 or C:P < 60. Often U = R. R > U; concentration increases. R < U; concentration decreases.

How is uptake and regeneration measured in the field? Net effects (difference in U and R). Incubation with 15N labeled compounds: 0.3663% of 15N +14N as 15N (add < 10%) Uptake is what accumulates in particles. Regeneration is by “isotope dilution” of DIN. Whole system budgets: Upstream addition of a conservative tracer. Again use 15N added directly to the ecosystem.

Control of role in N-cycling Carbohydrates Amino Acids NH4+ N-deplete bacterium Uptake High C:N ratio of organic substrates Low C:N ratio of organic substrates Regenerate Carbohydrates NH4+ NO3- N-replete bacterium Amino Acids

Response to C & N supply: GDH Regulation: Expression and activation at low C:NLDOM ratio Repression and inactivation at high C:NLDOM ratio GS Regulation: Reverse of GDH. GDH:GS activity ratio (Hoch et al., 2006).

Assess the bioavailability of N and P in freshwater bacterioplankton. Does GS & GDH activity respond to amendments of C and N in lake bacterioplankton cultures? Does P supply (assessed by AP activity) affect N-metabolism? Are results influenced by community composition?

Amendment Experiments < 0.8 μm filtrate is inoculum and media. Amend replicates with NH4+, PO43- & glucose. Monitor parameters initially and after 24 h.

Lake Bacteria Amendment Experiments Expected +N response. Unexpected glucose response (need P). +P repressed AP. Increasing GS activity requires +P; DIN uptake increased.

Did the community change Did the community change? Denaturing Gradient Gel Electrophoresis (DGGE) % chemical denaturant 25 % 55 % (-) (+) (-) (+) (-) (+) (-) (+)

Lake Williams 16SrDNA DGGE +P +G+P Minor richness increase after 24 h in +P and +P+G treatments.

Relationships among N-metabolism and that of P and C. More N-replete bacteria are more P-limited. More N-replete bacteria have less efficient growth. More P-replete bacteria have more efficient growth.

Lake Sites: Contrasting TN:TP ratio Both lakes: Lower Susquehanna River Basin Piedmont region Eutrophic Lake Williams: East Branch Codorus Watershed ≈ 80% agriculture land use TN:TP = 286 Lake Pinchot: Conawego Watershed ≈ 40% agriculture landuse TN:TP = 17.1 (sewage-P) (Susquehanna River Basin Commission, 2001)

Lakes of Contrasting TN:TP Ratio Parameter Summer 2006 Lake Williams (n = 6) Lake Pinchot bacteria (106 ml-1) 2.2 ± 0.52 4.3 ± 0.81 chlorophyll a (μg l-1) 32.7 ± 10.4 70.0 ± 21.4 TN:TP ratio (atom) 290 ± 36 17 ± 6.2 total N (μM) 220 ± 21 63 ± 18 total P (μM) 0.78 ± 0.14 3.7 ± 0.47 bacterial AP (nmol h-1 μg protein-1) 5.4 ± 1.2 0.73 ± 0.38 bacterial GDHT:GS 560 ± 110 72 ± 23

Watershed Summary Low GDHT:GS due to greater supply of labile organic-C and PO43-; DIN uptake. Low GDHT:GS suggest N-replete bacteria that regenerate NH4+. Bacterial community composition does not appear to greatly influence enzyme activity. TN:TP ratio of lake ecosystems influences bacterial nutrient dynamics (sewage effect). Similar results with periphyton (“rock slime”) communities in streams.