St. Catherine University, St. Paul, MN.

Delorianne Sander

Hengill Watershed, Iceland 2012

Jill Welter, Mentor

Predicting the Effect of Climate Warming on Nitrogen Fixation and Its Ecological Consequences in Aquatic Ecosystems

.

.

Element Cycling

N P

CO2N2

Non nitrogen-fixing species Nitrogen-fixing speciesN≡N

•Cyanobacteria – organisms capable of accessing nitrogen gas in the atmosphere •Use enzyme nitrogenase •Allows access to N unavailable to other organisms (Marcarelli 2008)

Nitrogen (N2) Fixation

NH3

N≡N

Importance of N2 Fixation• Enzyme activity sensitive to temperature changes

(Campbell and Reece 2008)

• Changes in temperature may alter N input • When N is low, primary production may be reduced• C sink capacity may be diminished (Gruber and Galloway 2008)

(http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/enzymes/enzymes1.shtml)

Climate Warming

N P

CO2N2

CO2

CO2 CO2CO2

N2

N2 N2 N2

Prediction: N2 fixation increases as temperature increases

How will warming affect N2 fixation rates in freshwater ecosystems?

Research Question and Prediction

Hengill Watershed, Iceland

• Why Iceland?–Geothermal activity – streams 5-25°C–Proximity of study sites

• Study nitrogen-fixing algal species across a large temperature gradient

~5 ft 21°C18°C

Measuring N2 fixation• Measure indirectly

using analogous gas (ARA)

• Measure directly using 15N2 isotope

1 2

3 4

5 615N2

gasNitrogenase enzyme

N2 gasNitrogenase enzyme

Acetylenegas

Methods

• Pair 15N2 and ARA on same algal sample in situ• Run gas samples on gas

chromatograph• Dry algal samples – analyze for 15N

content on Isotope Ratio Mass Spectrometer

Relationship between N2 Fixation Rate (ARA Method) and Temperature

6 8 10 12 14 16 18 20 2205

1015202530354045

anabaena spp.nostoc spp. (golden)nostoc spp. (gray)nostoc spp. (pink)

Average Stream Temperature (°C)

Aver

age

ARA

Fixa

tion

Rate

(m

g N

/m2/

hr)

Anabaena spp.

Nostoc spp. (golden)

Nostoc spp. (gray)

Nostoc spp. (pink)

6 8 10 12 14 16 18 20 220

10203040506070

anabaena spp.

nostoc spp. (golden)

nostoc spp. (gray)

nostoc spp. (pink)

Average Stream Temperature (°C)

Aver

age

15N

Fix

ation

Rat

e (m

g N

/m2/

hr)

Anabaena spp.

Nostoc spp. (golden)

Nostoc spp. (gray)

Nostoc spp. (pink)

Relationship between N2 Fixation Rate (15N2 Isotope Method) and Temperature

So, why do the results differ between methods?

15N2 Isotope Enrichment Method

• 15N2 gas saturated water?

• Data suggest 15N2 underestimates N2 fixation (Mohr et al. 2010)

Conclusion• N2 fixation increases as temperature increases• Climate change predictions depend on

understanding N cycle• Further explore

methods to measure N2 fixation to refine predictions

N2

CO2CO2CO2

CO2CO2

Acknowledgements

Thank you to our collaborators from Montana State University, the University of Alabama, and the Veiðimálastofnun, as well as our collaborators here in Minnesota.

A special thank you to Dr. Jill Welter and Bayley Lawrence for their hard work on this project.

Most importantly, thank you to 3M, St. Catherine University, and individual donors for helping to make this possible!

Campbell NA, Reece JB, Urry LA, Cain ML, Wasserman SA, Minorsky PV, and Jackson RB. 2008. Biology Eighth Edition. San Francisco (CA): Pearson Benjamin Cummings.

Gruber N and Galloway JN. 2008. An earth-system perspective of the global nitrogen cycle. Nature 451(17):293-296.

Marcarelli A, Baker M, Wurtsburg W. 2008. Is in-stream N2 fixation an important N source for benthic communities and stream ecosystems? J-Nabs 27(1):186-211.

Mohr W, Großkopf T, Wallace DWR, Laroche J. Methodological underestimation of oceanic nitrogen fixation rates. Plos One [Internet]. cited Feb. 14, 2013] Available from www.plosone.com.

References