1. 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)?

2. 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)

3. 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.

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

5. Phosphorus Supply

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

7. 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

8. 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.

9. 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.

10. 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

11. 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).

12. 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?

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

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

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

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

17. 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.

18. 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 = (sewage-P)
(Susquehanna River Basin Commission, 2001)

19. 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

20. 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.