THE EFFECTS OF TEMPERATURE ON ACETYLENE REDUCTION BY MATS OF BLUE-GREEN ALGAE IN SUB TROPICAL GRASSLAND

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1 New Phytol (1977) 78, 4-4. THE EFFECTS OF TEMPERATURE ON ACETYLENE REDUCTION BY MATS OF BLUE-GREEN ALGAE IN SUB TROPICAL GRASSLAND BY K.JONES* Department of Botany, University of Pretoria, Pretoria 0002, South Africa {Received 12 July 1976) SUMMARY Temperature is the main factor governing the rate of acetylene reduction by mats of bluegreen algae in sub-tropical kilcuyu lawns. High midday temperatures ( C at soil surface) inhibit acetylene reduction and low temperatures prior to dawn reduce the rate of nocturnal acetylene reduction. Data obtained from laboratory studies on algal mats and on Nostoc isolated from the mats are in agreement with those from in situ measurements confirming an acetylene reduction of - C with inhibition above C. INTRODUCTION The indications from in situ studies are that, in the absence of dessication, temperature is the main factor controlling the rate of acetylene reduction by mats of blue-green algae in subtropical ldkuyu lawns (Jones, 1977a; Jones, 1977b). The highest rates of diurnal acetylene reduction are found in the afternoon when both soil temperature and light intensity are below the levels at midday when acetylene reduction is low. Light intensity does not appear to contribute to this midday drop in acetylene reduction (Jones, 1977c) hence there is the need for an investigation into the effects of temperature on acetylene reduction by the algal mats. MATERIALS AND METHODS 77?^ algae and measurement of acetylene reduction Algal mats used in theseexperiments are from the same site as those used in previous studies (Jones, 1977a). They are found amongst kikuyu grass in lawns around Pretoria and are comprised almost entirely of Nostoc sp. A species of Nostoc isolated from the algal mats was also used (Jones, 1977c). Acetylene reduction was measured as described previously (Jones, 1977c). Measurements of temperature Measurements of temperature in the field were made using a Sekunden Thermometer probe. The internal temperature of serum bottles was measured by inserting the probe through a rubber stopper. * Present address: Department of Biological Sciences, University of Lancaster, Lancaster LAI 4YQ, England. 4

2 4 K. JONES RESULTS The results for the effect of temperature on acetylene reduction by algal mats and by Nostoc (isolated from the mats) are shown in Table 1. There was little acetylene reduction below 10 C or above C. The optimum temperature for acetylene reduction was C for the algal mats and -3O''C fox Nostoc in culture. Although amounts of acetylene reduction were lower in the dark the pattern relating to temperature was similar for the algal mats in the dark and in the light. Table 1. The effect of temperature on acetylene reduction by mats of blue green algae in the light and the dark and by a Nostoc sp. (isolated from the mats) in the light Temperature ('C) Algal mats (Mg Light CM^ cm Dark Acetylene reduction ~'h-') Nostoc (Mg CjH^ mg dry wt) Light AlgaJ mats or 1-ml suspensions of Nostoc were placed in serum bottles and incubated outside in the sunlight in a series of waterbaths. After min equilibration they were acetylated and analysed for ethylene production after 60 min. Dark incubations were carried out in water baths in the laboratory with the serum bottles wrapped in aluminium foil. Table 2. Comparison of temperatures at the soil surface on a sunny day Time of day (hours) (5 min. cloud) Temperature ( C) Bare soil Algal mats Serum bottle Air Grass Temperatures were measured with a Sekunden Thermometer Probe The data obtained from these and in situ experiments prompted further investigation of the temperature occurring in the field. Temperature measurements were taken of the following during a hot, still, sunny day (10 March 1976): air 5 cm above ground, in amongst the grass, the surface of algal mats, the surface of adjacent bare soil, inside the serum bottles used for acetylene incubations. The results are presented in Table 2 and show that temperature in amongst the grass was slightly higher than the air temperature but was not as high as that at the soil surface. The bare soil heated up more quickly and attained a higher tem-

3 Effect of temperature on acetylene reduction 4 perature than adjacent algal mats. The temperature inside the serum bottles was higher than the algal mats in the morning but lower during midday and in the afternoon. The highest temperature (51 C) was measured at the bare soil surface at noon. DISCUSSION The effect of temperature on acetylene reduction by blue-green algal mats from kikuyu lawns is similar in the light and in the dark. Data obtained from laboratory experiments fit in well with those from in situ studies. Indeed if all other variables are ignored and acetylene reduction in the field is plotted against the soil surface temperature a remarkably similar pattern (Fig. 1) to that from laboratory studies is obtained. Temperatures above C in the ' a- CJ> c c o 10 3 QJ 10 Temperature ( 0) at the soil surface Fig. 1. Results for in situ acetylene reduction by mats of blue-green algae plotted against soil surface temperature. {In situ measurements of acetylene carried out as described by Jones, 1977a). field inhibit acetylene reduction just as they do in the laboratory, maximum rates were found in the field during the afternoon when soil surface temperatures were near the optimum shown by laboratory experiments. Hence, it would seem that temperature is the main controlling factor for acetylene reduction by active algal mats in hot climes just as it is where low temperatures predominate (Eglund and Meyerson, 1974). Dark acetylene reduction is also temperature dependent and the observed predawn drop in acetylene reduction in the field can be correlated with the drop in temperature from 17 to 12 C. Laboratory studies have shown that only low rates of dark acetylene reduction occur below 15 C. Henriksson (1971), working with temperate soils, found that temperature changes had little effect on nitrogen fixation in the dark. The alga in culture showed a similar response to a range of temperature but the optimum for acetylene reduction was lower than that for the algal mats. The optimum for acetylene reduction is similar to that obtained for some blue-green algae isolated from temperate soils (Jones and Stewart, 1969; Jones, 1974) but the alga from the sub-tropical habitat is active at higher temperatures and less active at lower temperatures than those from temperate soils. The data obtained from temperature measurements show that the soil surface easily reached temperatures known to be inhibitory to acetylene reduction by the algae. On the day that the temperatures were measured the temperature of the algal mats would have inhibited

4 4 K. JONES acetylene reduction for several hours. At all times the temperature within the grass was lower than that of the algal mat. Thus not only may the grass protect the algae from dessication (Jones, 1977b) it may protect the algae from excessive temperatures. The temperature within the serum bottles is higher than that of the algal mats in the morning and may cause high results for in situ studies. At midday and in the afternoon the temperatures in the bottles were lower than^ those of the algal mats and it is possible that some of the effects of temperature may be underestimated in field measurements taken during the hotter part ofthe day. Prolonged incubation in the serum bottles (over 2 h) in direct sunlight had deleterious effects on the algal mats as they became moribund and were unable to reduce acetylene when the temperatures fell. Algal mats collected from the field, which had been subjected to similar conditions (except that they were not in bottles), were able to resume acetylene reduction at the lower temperature. Therefore, in ecosystems where the temperature of the soil surface becomes hot, long-term exposures to acetylene such as those employed with temperate soils (Paul Myers and Rice, 1971; Jones, 1974) are not possible. ACKNOWLEDGMENTS The author is grateful for the help from members ofthe Botany Department ofthe University of Pretoria and in particular to Professor N. Grobbelaar for his interest and assistance. A postdoctoral fellowship from the CSIR is gratefully acknowledged. REFERENCES ALLEN, M. B. & ARNON, D. 1. (1955). Studies on nitrogen-fixing blue-green algae. 1. Growth and nitrogen fixation by Anabaena cylindrica Lemm. PL Physiol., Lancaster,, 6. EGLUND, B. & MEYERSON, H. (1974). In situ measurement of nitrogen fixation at low temperatures. Oikos,,283. HENRIKSSON, E. (1971). Algal nitrogen fixation in temperate regions. In: Biological Nitrogen Fixation in Natural and Agricultural Habitats (Ed. by T. A. Lie and E. G. Mulder), pp. 415^19. Plant and Soil Special Volume. JONES, K. (1974). Nitrogen fixation in a salt marsh./. Ecol., 62, 553. JONES, K. (1977a). Acetylene reduction by mats of blue-green algae in sub-tropical grassland. New Phytol. 78, JONES, K. (1977b). The effects of moisture on acetylene reduction by mats of blue-green algae in subtropical grassland. Ann. Bot. (In press). JONES, K. (1977c). The effects of light intensity on acetylene reduction by mats of blue-green algae in sub-tropical grassland. New Phytol. 78, JONES, K. & STEWART, W. D. P. (1969). Nitrogen turnover in marine and brackish habitats. III. The production of extracellular nitrogen by Calothrix scopulorum. J. Mar. Biol. Ass. U.K., 49,475. PAUL, E. A., MYERS, R. J. K. & RICE, W. A. (1971). Nitrogen fixation in grassland and associated cultivated ecosystems. In: Biological Nitrogen Fixation in Natural and Agricultural Habitats (Ed. by T. A. Lie and E. G. Mulder), pp Plant and Soil Special Volume. STEWART, W. D. P. (1971). Physiological Studies on nitrogen-fixing blue-green algae. In: Biological Nitrogen Fixation in Natural and Agricultural Habitats (Ed. by T. A. Lie and E. G. Mulder), pp Plant and Soil Special Volume.

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