Transcript Nutrient Dynamics

Nutrient Dynamics
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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 outcompete 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
de novo
synthesis
PO43-
PO4
PO43- 3PO4
3-
Nucleic
Acids
3Low
to
No
PO
34
High PO4 supply:
supply:
AP activity is
AP activityoris
repressed
expressed
inhibited at
high levels.
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
High
Low C:N ratio of organic substrates
Carbohydrates
Carbohydrates
N-replete
N-deplete
bacterium
Amino Acids
Amino Acids
Uptake
Regenerate
NH4+
NO3-
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 Nmetabolism?
• 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?
Denaturing Gradient Gel Electrophoresis (DGGE)
% chemical
denaturant
(-)
(-)
(-)
(-)
(+)
(+)
(+)
(+)
25 %
55 %
Lake Williams 16SrDNA DGGE
T0
C
+N
+P
+G
+G+P
+G+P+N
M
+P
+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
(n = 6)
bacteria (106 ml-1)
chlorophyll a (μg l-1)
2.2 ± 0.52
32.7 ± 10.4
4.3 ± 0.81
70.0 ± 21.4
TN:TP ratio (atom)
total N (μM)
total P (μM)
290 ± 36
220 ± 21
0.78 ± 0.14
17 ± 6.2
63 ± 18
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.