Author Institution: The Franz Theodore Stone Institute of Hydrobiology, Pun-in-Bay, Ohi

McMillan, Gladys L.

Verduin, Jacob

The Ohio Journal of Science

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PHOTOSYNTHESIS OF NATURAL COMMUNITIES DOMINATEDBY CLADOPHORA GLOMERATA AND ULOTHRIX ZONATAGLADYS L. McMILLAN1 AND JACOB VERDUINThe Franz Theodore Stone Institute of Hydrobiology, Put-in-Bay, OhioThe littoral zone in western Lake Erie is populated by attached filamentousalgal communities dominated in summer by Cladophora glomerata (L.) Kutz.,and in winter by Ulothrix zonata (Weber & Mohr) Kutz. During 1950-51 thesecommunities were studied to determine their photosynthetic activity under near-natural conditions. Graphs showing the relationship between photosynthesis andlight intensity are reproduced and discussed below. These researches were carriedout by the senior author at the Stone Institute of Hydrobiology in partial fulfill-ment of the requirements for the M. S. degree.METHODSThe method used to measure photosynthesis was similar to that used by Verduin(1951) in studies of phytoplankton photosynthesis. Algae were collected fromtheir natural habitat and studied shortly after collection. Small amounts ofalgae (1-2 gm) were squeezed manually to remove most of the water, and weighedon a triple-beam balance, having a sensitivity of 0.01 gm. These samples wereintroduced into glass bottles of 125 ml capacity containing lake water of knownpH and total alkalinity (average, 90 ppm, as CaCO3). Two bottles, blackenedto exclude light, were treated similarly to provide an estimate of respiration duringthe test period (0.5-1.0 hr). The bottles were suspended at one-half meter intervalsbetween 0-3 meters, to subject them to a variety of natural light intensities.Experiments were always performed at about 1:00 P.M. when light was near itsmaximum intensity for the day. Tests were also made on cloudy days, so surfacelight intensities varied considerably from one test to another. At the end of thetest period the pH of the water in each bottle was determined using a BeekmanpH meter. The micromoles of CO2 absorbed per gm of algae per hr was estimatedby reading from a graph relating pH change to CO2 change (Verduin, 1951, fig. 1.)In the experiments described below final pH values higher than 9.5 were avoidedbecause the lake pH seldom rises above 9.0, so higher experimental values wouldrepresent an unnecessary departure from natural conditions.Light intensities were measured at the depth of each bottle, using a Westonphotronic cell enclosed in a water-tight case.Temperature measurements were made during each experiment using anordinary mercury thermometer, calibrated in tenths of a degree C.Estimates of ash-free dry weight of samples were made by drying in an oven,weighing, igniting in a furnace, and reweighing. For these weights a chainomaticbalance was used.RESULTSA wide range of variability was observed among tests carried out on differentdays. Figure 1 shows the results of 5 tests made between February 14 and 28,1951, using samples from the Ulothrix community. The photosynthetic ratesare graphed against scamps of current generated by the photronic cell. The rateswere calculated using the original pH of the water in the bottles, rather than thexMrs. Theodore Ripper, Coraopolis, Pennsylvania.THE OHIO JOURNAL OF SCIENCE 53(6): 373, November, 1953.374 GLADYS L. MCMILLAN AND JACOB VERDUIN Vol. LIIIfinal pH in the black bottles, as zero point. Thus the points below the zero linerepresent conditions under which respiration exceeded photosynthesis, and thepoints on the ordinate represent respiration rates in the black bottles. This figureshows a rather wide scatter of data points, but it is significant that the scatteris not purely random i.e., the points for any single test lie on a fairly smooth curve.The relative position of the curve with respect to the ordinate is the most variablefeature, and is primarily caused by variable respiration rates observed fromday to day. The causes of the day to day variation have not been studied. Similarvariation was observed in Cladophora samples; consequently a single test doesnot represent a very good estimate of the average photosynthetic rate undernatural conditions, but the mean of several tests provides a more reliable estimate.FIGURE 1. Apparent photosynthesis Oumoles CO2 absorbed per gm wet wt per hr) of aUlothrix community, graphed against light intensity (^amps Weston photronic cell, X 10~3).The broken lines indicate separate experiments carried out between February 14 and 28, 1951.The solid line represents the average of the five tests shown.Figure 1 also shows that inhibition of photosynthesis was frequently observedin the surface bottles, but that such inhibition was not associated with a particularintensity of light, but rather with the surface position of the bottle. Thus onFebruary 23 inhibition was observed at a light intensity of only 3200 juamps,although this same, and higher, light intensities, when encountered at a depthof one-half meter on other days, promoted maximal photosynthetic activity.This phenomenon was observed in other tests not shown in figure 1. It suggeststhat some quality of surface light, rather than its intensity, is responsible for suchinhibition. It may be that certain wave-lengths, rapidly absorbed by waterlayers, are responsible. Tests with Cladophora samples never showed reducedrates at the surface.The solid line in figure 1 represents the mean obtained from the 5 tests shown.The average respiratory rate amounted to about 20% of the total photosyntheticrate at optimal light intensity. The compensation point was reached, on theaverage, at a light intensity of about 100 /^ amps. In the data presented belowall curves are means of several tests made on different days.No. 6 PHOTOSYNTHESIS OF ALGAL COMMUNITIES 375Figure 2 shows four curves: two for Cladophora, one obtained during the Autumnof 1950, the other during the Spring of 1951; two for Ulothrix, one obtained duringFebruary and March, one during April, 1951. The shape of these curves reflectsthe adaptation of these littoral communities to bright light intensity; they show arelatively low efficiency at dim light intensities. The Ulothrix communities,however, showed higher relative efficiency at dim light than did Cladophora. Forexample, in figure 2, Ulolhrix absorbed one-third as much CO2 at 500 stamps asat 4000. Cladophora used only one-fourth as much CO2 at 500 /xamps as at 4000.This characteristic probably represents an adaptation by Ulothrix to the dimmerlight of the winter months.4 0-LIGHTFIGURE 2. Comparison of photosynthesis (/imoles CO2 absorbed per gm wet wt per hr)vs. light (jLiamps Weston X 10~3) curves in Cladophora and Ulothrix communities. A = Clado-phora, Autumn, 1950. B = Cladophora, Spring, 1951. C = Ulothrix, April, 1951. D = Ulo-thrix, January-March, 1951. The broken line represents the curve published by Manning et al.for a species of Cladophora.Manning, Juday and Wolf (1938) have published a graph of photosynthesisv. light intensity for an unidentified species of Cladophora, showing data from asingle experiment. The relative curve shape obtained by them is indicated bythe broken line in figure 2. It differs radically from the curves of the presentreport, although their samples also were collected from an intensely illuminatedhabitat in shallow water. Plants showing such a curve would thrive at muchdimmer light than would the littoral community of Lake Erie. No abrupt increasein photosynthesis in the region of maximal light intensity was ever observed inthe Lake Erie material.Since the photosynthetic rates appearing in figure 2 are based on similar quanti-ties of plant matter, a comparison of the absolute rates exhibited by the differentcommunities may be significant. The most obvious environmental difference376 GLADYS L. MCMILLAN AND JACOB VERDUIN Vol. L1IIbetween the communities is that of water temperature. The mean temperaturefor the autumn tests of Cladophora was 18° C. for the spring tests 16°. The meantemperature for the winter tests of Ulothrix was 2°, for the April tests 7°. Sincephotosynthesis under optimal light shows a temperature coefficient of approxi-mately 2 in laboratory experiments (Meyer and Anderson, 1952, p 355) it is in-teresting to test the hypothesis that the same relationship controls the photo-synthetic rates, at optimal light intensity, of two different communities occupyingidentical space habitats but subjected to different environmental temperatures.4 0-3 0-X 2 0too/ 0I 0 I 5 2'0TEMPERATUREFIGURE 3. The temperature coefficient of photosynthesis at optimal light in naturalcommunities. Temperatures below 10° represent Ulothrix, above 15° represent Cladophora.The broken lines indicate the slope required for a Qio of 2. A = jumoles CO2 absorbed pergm ash-free dry wt per hr, X 10"1. Observed Qio about 1.7. B = jumoles CO2 absorbedper gm wet wt per hr. Observed Qio about 2.1.The hypothesis is tested in figure 3. Here the photosynthetic rates shown at4000 Mamps in figure 2 are plotted (logarithmic ordinate) against temperature indegrees C (linear scale). The solid line (B) was drawn, by inspection, throughthe data points. The broken line represents the slope required for a Qio of 2.It is evident that the points lie reasonably close to a straight line whose slopeindicates a temperature coefficient of approximately 2. The data were recalculatedas photosynthetic rate per gm ash-free dry wt (line A). On this basis the apparentNo. 6 PHOTOSYNTHESIS OF ALGAL COMMUNITIES 377Qio is approximately 1.7. Figure 3 shows that the differences observed betweenthe photosynthetic rates of Cladophora and Ulothrix communities are largelyexplained by the difference in environmental temperature. It should be pointedout that the temperature coefficient of photosynthesis, as determined in laboratoryexperiments, refers to photosynthetic rates exhibited by a single species subjected todifferent temperatures, with all other factors supplied in abundance. It is note-worthy that the relationship seems to be approximately valid for different speciesstudied under near-natural conditions at the temperatures of their natural environ-ment. Moreover, the supplies of CO2 and other necessary materials, are probablynot present in abundance in the natural environment, although they would beapproximately the same for both communities in the experiments discussed above.This situation lends support to Baule's (1918) concept of limiting factors, whichhe has expressed in the form of a general equation (Verduin, 1953). Baule contendsthat all of the factors influencing a process are operative at all times, the degreeof limitation being inversely, and exponentially, proportional to the relativeabundance of each factor. Thus, although CO2 supplies in Lake Erie water arepresent in less than optimal concentrations, yet a powerful influence of the tempera-ture factor is apparent. Furthermore, the curves in figure 2 remain separate inthe region of suboptimal light intensity, suggesting that the temperature factor isstill pronounced although both light and CO2 are supplied at suboptimal intensities.SUMMARYPhotosynthesis studies of littoral Cladophora and Ulothrix communities undernear-natural conditions showed rates at optimal light of 33 //moles CO2 absorbedper gm (wet) hr for Cladophora at temperatures of about 17° C, and 12 jumolesCO2 for Ulothrix at about 4°. The photosynthesis vs. light curves showed re-latively inefficient utilization of low light intensities, differing markedly from acurve for Cladophora in the literature. The temperature coefficient estimatedfrom these data was approximately 2. The data seem to support the Baule-Mitscherlich theory of limiting factors.LITERATURE CITEDBaule, B. 1918. Zu Mitscherlich's Gesetz der physiologischen Beziehungen. Landw. Jahrb.,51:363-385.Manning, W. M., C. Juday, and Michael Wolf. 1938. Photosynthesis of aquatic plants atdifferent depths in Trout Lake, Wisconsin. Wise. Acad. Sci., Arts and Lett., 31: 377-410.Meyer, B. S., and D. B. Anderson. 1952. Plant Physiology. 2nd ed. D. Van Nostrand Co.,New York.Verduin, J. 1951. Photosynthesis in naturally reared aquatic communities. Plant Physiol.,26: 45-49.. 1953. The Baule-Mitscherlich limiting factor equation. Science 117 (3041): 392.

Photosynthesis of Natural Communities Dominated by Cladophora Glomerata and Ulothrix Zonata

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Photosynthesis of Natural Communities Dominated by Cladophora Glomerata and Ulothrix Zonata

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