An Integrated Limnology, Microbiology & Chemistry

Home , Lake stratification

An IntegratedLimnology, Microbiology & ChemistryExercise For TeachingSummer Lake Stratification, Nutrient Consumption )-4H & ChemicalThermodynamics

H- 0 FRA N K M. DUNNIVANT

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M 1 ost chemistryand biologyteachers will reactionsis introduced,with standardGibbs free energy agree that students have a "disconnect"between these (corrected for lake water conditions) conclusivelyshow- Downloaded from http://online.ucpress.edu/abt/article-pdf/68/7/424/53822/4452031.pdf by guest on 02 October 2021 two disciplines. This likely results from our categoriza- ing why the reactions occur in the given order,in every tion of the topics into two classes or two separateyears of eutrophic stratified lake, every summer. The following study. This articleprovides one example of how the two exercise takes from one to three 50-minute lecture peri- disciplines can be related in an environmentalapplica- ods, depending on the number of questions from the tion that many students have first-handexperience with: students and how group problem-solving sessions are diving into a stratifiedwater body. The stratification(a conducted. physical process) is explained first,and then the chemistry that results from the stratificationis considered. Introductory Limnology Material integration Two examples of this biology-chemistry Instructors that are unfamiliar with limnology 2000) are at Florida Gulf Coast University (Barreto, should review textbooks such as Lampertand Sommer An inte- and Wellesley College (Wolfson et al., 1998). (1997) and Wetzel and Likens (2000). First, Figure 1 laboratoriesis described by grated set of biochemistry is used to illustrate the stratificationof a lake during A few additional integrated Bevilacqua et al. (2002). summer and the creation of a thermocline (a rapid exercises address chemistry at introductory laboratory decrease in water temperatureat a specific depth). In 1994), inte- the biology-chemistryinterface (Meinwald, early summer, the surface of the lake heats up relative rartionof scientific instrumentation in the classroom (Izydore et al., 1983), and chemical kinetics for the biology student Figure1. Sketchof a lakeafter summer heating and thermal stratification of the water.The tem- (Pederson, 1974). The peratureversus depth plot is shownon the left. introductory exercise for teaching limnology and boundary thermodynamics pre- v Atmosphere/water sented here seeks to add Epilimnion to this body of integrated I,s\ ------Thermcdine--- exercises. The exercise Hypolimnion can be used in limnology, Sediment/waterboundar ecology, microbiology,or chemistryclasses. Lecture Temperature material first addresses the limnology of a ther- to the underlying water, and the depth of this heated during the mally stratifiedlake as stratificationdevelops layer (the epilimnion) moves downward as the summer (and beginning of summer. As the summer progresses progresses. Figure 2 shows why the warmerwater stays lake continues), microorganismpopula- heating of the on the surface of the lake-water is more dense at the lake shift from using oxygen tions in the bottom of the temperaturesfound in the hypolimnion because water acceptor for oxidizing glucose, as the terminal electron here is constantlycooled by the surroundingearth. If the dioxide. to using nitrate,then sulfate, and finally carbon lake is oligotropic(contains low concentrationsof nutri- equations for these Next, the coupling of the chemical ents and readilyoxidizable organic matter), the chemical gradients remain essentially constant from the top to FRANKM. DUNNIVANTis Associate Professorin the Chemistry the bottom of the lake throughout the summer and the Department at Whitman College, Walla Walla, WA 99362; transition through terminal electron acceptors given e-mail: [email protected]. below does not occur. However, if sufficient organic

424 THEAMERICAN BIOLOGY TEACHER, VOLUME 68,NO. 7, SEPTEMBER2006 matteris present to consume all of the dissolved oxygen in Figure2. Waterdensity as a functionof temperature. the hypolimnion (as in a eutrophic lake), a chemical gradient, or chemocline, will exist, approximatelycorresponding to the thermocline.This chemocline representsa change in Density as a Function of Temnerature chemical composition with change in water depth. Hypolimnion Terminal Electron Acceptors

(TEAs) E The thermocline and chemoclines, in Figures 3 and 4, are shown at mid-depth in the lake to keep the diagrams C :X simple and to allow for labeling of the figures. In reality, a) the thermocline initially forms near the surface of the lake and proceeds downward as summer heating progresses. This is illustrated in Figures 3a and 3b. All of the arrows Epilimnion in Figure 3 represent the progression of time through the l l l l l I summer. Two transitionsare occurringin the lake. First, as 0 5 10 15 20 25 30 35 just noted, the depth of thermocline is increasing towards Temp (C) Downloaded from http://online.ucpress.edu/abt/article-pdf/68/7/424/53822/4452031.pdf by guest on 02 October 2021 the bottom of the lake. In the sDringit is initiallycool but slowly heats as summer approaches.The volume and depth Figure3. Illustrationof the progressionoftemperature and TEA concentrations as a func- of the epilimnion increases during the tionof timeduring summer heating.The arrows indicate the direction of timeand concen- summer as more and more heat is input to the system. Second, certain oxidized trationgradient. chemicals (referredto as terminal elec- Surface (a) (b) (c) tron acceptorsbelow) are being reduced (d) (e) (consumed) in the following order in the hypolimnion: first, dissolved oxy- Epilimnion ~~~~~~Aerobic: H gen (0,), then nitrate (NO3-), followed a) by sulfate (SO2)), and finally carbon Thermocline Beginning i dioxide (CO2). Examples of the transi- of summer Chemocline tions of two TEAsare shown in Figures -) 3c-3e. Figure3c shows the consumption a) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ii of 02. At the beginning of the summer, Hypolimnion E Aaoi l dissolved is in oxygen usually present Edosummer decreasing E~ I uniformlyhigh concentrationsthrough- Temp Temp [02] [NH4 out the lake. As the thermoclineis estab- [NO13-] lished, the dissolved oxygen in the hypolimnion cannot be replenished by the atmosphere, and its concentra- F gure4. Latesummer temperature and chemical profiles of a stratifiedeutrophic lake. tion slowly decreases due to microbial surface NH4? NO3 HS S04 2 CH4 C02 respiration.When most or all of the Hypolimnion Anaerobic:aeroic: dissolved oxygen is consumed, a new set of microorganismsbegin oxidizing Epilimnion Aerobic: HighE~ organic matter using nitrate. Figure

3d shows this profile. The concentra- TThermocline Chemocline tion of nitrate starts off uniform in the lake, but then the nitrate starts to be consumed in the hypolimnion as Hypolimnion a TEA.The concentration gradient is Anaerobic:II illustrated by the arrow in Figure 3d. LowEHII

As nitrate is consumed, ammonium is Temp 02 [NO3]1 [SO42-] [CO]2 produced in the hypolimnion (Figure 3e). This coupling of oxidized and Progression during summer beating reduced forms of TEAsand the trends TerminalElectron Acceptor (TCE)consumption order shown in Figures 3d and 3e occur for all of the TEAs. ent, carbon dioxide reducers (methanogens) oxidize the After the nitrate in the hypolimnion is consumed, organicmatter and produce CH,. As each terminalelectron sulfate-reducingmicrobes produce H,S by the reduction of acceptor is consumed during the summer, the oxidation- SO- (Figure 4), and finally, if organic matter is still pres- reduction potential (EH,or reducing state) of the lake water

LIMNOLOGY,MICROBIOLOGY &CHEMISTRY EXERCISE 425 decreases. A summary of the results of Figure5. Balancedhalf reactions for glucose and common terminal electron acceptors and the TEA trends are shown in Figure 4. associatedAG values. Gibbs free energy values were taken from Schwarzenbach etal. (1 993).* Dissolved iron (III) can also be reduced to iron (II) by microbes during the oxi- BALANCEDHALF REACTION dation of organic matter. AG0H20 - C61206 + + 24H+ + 24e 984.0 The Chemistry (glucose) 6H120 6CO2 ...... kJ/mol 02 + 4H+ + 4e -e 2H2O...... -313.2 The students are asked why the ter- - minal electron acceptors progress from NO3- + 1OH++ 8e NH4+ + 3H2O...... -277.6 0. to N03-, to S042-, and finally to CO2. SO42- + 9H+ + 8e- - HS- + 4H20...... 170.4 To answer this question, we turn to thermodynamics,and Figure5 is distributed CO2 + 8H+ + 8e- - CH4+ 2H2O...... 188.0 to the students. It should be noted that the AG values on this figure have been corrected for conditions present in natu- Figure6. Combinedhalf reactions showing energy yields for different electron acceptors, ral water (temperature, ionic strength, andthus indicating which microorganisms yieldthe most energy per mole of glucose.*

and typical concentrations of the termi- Downloaded from http://online.ucpress.edu/abt/article-pdf/68/7/424/53822/4452031.pdf by guest on 02 October 2021 nal electron acceptors) (Schwarzenbach, COMBINEDBALANCED REACTIONS Net AG0H20 et al., 1993). As is explained to the students, for simplificationpurposes we 602 + C6H12O6(glucose) -e 6H20 + 6CO2...... -2863 kJ/mol use glucose as our general organic mat- -, ter. (Actually, any organic compound 3NO3- +C6H1-206 (glucose) + 6H+ 3NH4++ 6CO2 + 3H20.. -1817kJ/mol could be used, as long as we used the - 3S042-+ C6-1206 (glucose)+ 3H+ 3HS-+ 6H20 + 6CO2...... -473 kJ/mol same organic compound for all reactions and had a standard AG value for 3C2 + C6H11206 (glucose) -e 3CH4+ 6CO2...... 420. kJ/mol that compound.) Students are divided into groups and asked to calculate the *AGOH20represents the AGO, corrected for typical lake temperature, ionic strength, and AG values that result from combining reactantand product concentrations. the glucose reaction with a certain terminal electron acceptor (one terminal electron acceptor is assigned to each group). The results are summarized on

; ,,, ,$ V-X~s . Ft; * ',t.*a ..no the chalkboard (Figure 6), and the students are asked to Reptiles and Amphibians draw conclusions based on the calculated AG values and the order of use of the terminalelectron acceptors.Figure 6 of East Africa shows that oxygen users yield substantiallymore energy as STEPHEN SPAWLS, KIM HOWELL & ROBERT C. DREWES obtained by nitrate users and between six and seven times This lightweight and portable the energy yielded by the sulfate and carbon dioxide users. guide covers the 15o reptiles We next discuss, based on the AG calculations, the relative REMJL4ES and 8o amphibians you are efficiency or difficulty of the organisms' lives, the natural most likely to encounter selection mechanisms involved, and microbialevolution. AND)AMP1111 ANS across the five countries of East Africa-Kenya, Tanzania, () i\>%tm RIC1\ oF Uganda, Rwanda, and Classroom Interactions & Burundi. Here you will find a corresponding wealth of e Difficulties secretive yet often unwittingly No quantitativeassessment of this lecture exercise has . w_ conspicuous tortoises, lizards, :* Stcpel)l, Sp)a.ls * illi N1. lI (mull w w # been undertaken, but I do have several observations and . RobeKrt C. I)rc% crocodiles,snakes, and frogs. Reptilesand Amphibiansof suggestions for using this exercise. First, I should note that East Africaoffers concise and . I have used this exercise to integrate biology and ecology accessible, identification-ori- into the chemistryclassroom, but have not done the reverse, ented text, color photographs, > since I am a chemistry professor and do not teach biology. and color distribution maps 4 ! However,I see the importanceof incorporatingquantitative for each species aspects into the biology classroom, which is why I am pre- 240 pages. 230 color plates. 230 maps. 5 x 7 1/2. senting the materialin this journal. While it is true that most PrincetonPocket Guides of the students taking general chemistry are future biology Paper $24.95 0-691-12884-7 DueOctober N majors, my observations are from a chemistry perspective. Not available fromPrinceton in the Commonwealth - (exceptCanada) and the EuropeanUnion I have used this exercise for six years, in classes ranging from 35 to 80 students, and I find no differencein student Princeton University Press understandingof the materialin the two class size extremes. i 800-777-4726 . Read excerpts online at www.pup.princeton.edu I do have more difficultyhandling more questions from the - R *; -E <- **- ,-i,ir' K, largerclasses, but this is true of any materialthat I cover in

426 THEAMERICAN BIOLOGY TEACHER, VOLUME 68, NO.7, SEPTEMBER2006 these large classes. Largerclass sizes also make group activi- an easier time integratingbiological and chemical concepts, ties more difficult, such as the calculation of AG for all of although, as noted above, this does not come easily.Students the combined half reactions. In large classes, I simply have certainlyenjoy this integrationand list the exercise as one of the students work with their neighbor and then ask specific the more memorable events of the semester. Exercises such student pairs to write their results on the chalkboard. as this help beginning students bridge the gap between ecol- BeforeI start the exercise, I explain to the class that my ogy, microbiology,and chemistry, and hopefully allow stu- goal is to integratebiology and chemistry.I begin by discuss- dents to approachscience as an interdisciplinarysubject. ing the human (animal) respiration process and then the diurnal plant photosynthesis/respirationprocess through a Acknowledgments question-and-answerformat. Next I remind the students of I would like to thank my spring generalchemistry class- what it's like to dive into a lake or pond in earlysummer and es of 2002 and 2003 for help and feedback in developing ask if they can explain why the observed temperaturegradi- this exercise.I am indebted to SamanthaSaalfield (Whitman ent is present. Next, I remind them of the smell of swamps College, Class of 2004) for suggestions and editing of the or anaerobicmud and ask, although I do not expect accurate manuscript. replies, "Whyor how does this happen?"Finally, I give the lecture as presented in this article. The first and most difficult "apparent"problem to References help students overcome is that we are discussing biology Barreto,J.C. (2000). Journalof ChemicalEducation, 77(12), 1548. Downloaded from http://online.ucpress.edu/abt/article-pdf/68/7/424/53822/4452031.pdf by guest on 02 October 2021 in chemistry class. Even though we have now had years of Bevilacqua,V.L.H., Powers,J.L., Vogelien, D.L., Rascati, Rj., Hall, M., integrated high school classes, students still seem to learn Diehl, K., Tran,C., Swapan,Sj. & Chabayta,R. (2002). Journal by discipline and use differentlobes of their brain to process of ChemicalEducation, 79(11), 1311-1312. this information. I observe pencils drop to the desk, and I can almost see a differentset of gears in their brains start to Izydore, R.A.,Jones, C.R., Townes, M.M. & Harriet, A. (1983). Journalof ChemicalEducation, 60(12), 1065-1067. initializewhen I bring up the topic of biology. This may stem from the fact that I teach a three-weekunit on the environ- Lampert,W. & Sommer, U. (1997). Limnoecology:The Ecologyof ment at the end of my second semester general chemistry Lakesand Streams.New York,NY: Oxford UniversityPress. class, after the students have been programmed to learn Meinwald,J. (1994). Journalof ChemicalEducation, 71(6), 506. chemistry in my class for a year. In this three-weeksection, Pederson,D.M. I try to integrate biology, physics, geology, and chemistry. (1974). Journalof ChemicalEducation, 51(4), 268. Comments from student evaluations rate lectures in this Schwarzenbach,R.P., Gschwend, P.M. & Imboden, D.M. (1993). section of the class as the most popular,important, and inte- EnvironmentalOrganic Chemistry, Table 12.16. New York,NY: grative lectures of the year. But nonetheless, I feel a distinct John Wiley & Sons Inc. difficultyin co-processingchemistry, biology, and other dis- Wetzel, R.G.& Likens,G.E. (2000). LimnologyAnalysis, 3rd Edition. ciplines in a single classroom setting. I would be interested New York,NY: Springer. to learn if the same is observed in a biology setting when you Wolfson, AJ., Hall, M.L.& Allen, M.M.(1998). Journalof Chemical introduce chemical thermodynamicsin the form of terminal Education,75(6), 737-739. electron acceptors. Perhapsthe most difficult topic for the students to grasp is the movement of the thermocline (and chemocline) I downward during the summer, so in ttVst$ Mtiuqutrquc recent years I have put more emphasis on this part of the lecture and asked more questions during the presenta- tion of this material to note important points and check comprehension. The ),_t Attend a full day of ASM-Sponsored Sessions concept of terminal electron acceptors at NABT2006, Friday,October 13 is the next most difficult, and I usually __ review this at the end of the presenta- tion, using an evolutionary exercise (the microbe or organism that yields Learnhow to incorporate ASM's the most energy per mole of glucose weekly video podcast series has more spare time to lie in the sun or Intimate Strangers:Unseen Lifeon Earth watch television). This review, from a into your curriculum different perspective, seems to signifi- cantly aid in the student understand- ing of energy flow. I strongly feel that students com- For more info: AMERICAN d os as SOCIETYFOR pleting this introductory exercise have [email protected] MICROBIOLOGY

LIMNOLOGY,MICROBIOLOGY &CHEMISTRY EXERCISE 427