THE EFFECTS OF LIGHT INTENSITY ON ACETYLENE REDUCTION BY BLUE-GREEN ALGAL MATS IN SUB TROPICAL GRASSLAND

New Phytol. (1977) 78, 427-431. THE EFFECTS OF LIGHT INTENSITY ON ACETYLENE REDUCTION BY BLUE-GREEN ALGAL MATS IN SUB TROPICAL GRASSLAND BY K. JONES* Department of Botany, University of Pretoria, Pretoria 0002, South Africa [Received] 2 July 1976) SUMMARY High light intensities such as those found during the middle of the day in southern Africa are not responsible for inhibition of acetylene reduction by mats of blue-green algae in kikuyu lawns. Similar results were obtained with uni-algal cultures of Nostoc isolated from the algal mats. Short term experiments (3 h) with both cultures and algal mats showed a direct relationship between acetylene reduction and light intensities of up to 323 lux but no significant increase in acetylene reduction for light intensities above this. The amount of acetylene reduction by algae in the dark is determined by the light intensity received prior to darkness. INTRODUCTION Much work has been concerned with the relationship between photosynthesis and nitrogen fixation by blue-green algae (see reveiws by Stewart, 1973; Fogg et al., 1973; Fogg, 1974; Stewart, 1974). Most of the literature relating to the affects of light intensity on nitrogen fixation is for aquatic habitats and the results show that there is a dependence on light for nitrogen fixation and that high light intensities may be inhibitory (Fogg, 1974). Stewart (1974) summarizes the information for the effects of light intensity on terrestrial blue-green algae and states that there is a general but not always a very direct correlation between light intensity and nitrogen fixation in natural ecosystems and that light intensities at the soil surface may be excessive. In situ studies with mats of blue-green algae found in kikuyu lawns have shown that acetylene reduction is lower during the middle ofthe day than at other times (Jones, 1977a). The work in this paper is an investigation of the possible role of high light intensities in causing this midday fall in acetylene reduction. MATERIALS AND METHODS The algae The algal mats used in these experiments come from the same site as those used in other investigations (Jones, 1977a; Jones, 1977b), and are located in lawns of kikuyu grass around Pretoria. Uni-algal cultures of Nostoc sp. were isolated from the algal mats and grown routinely in * Present address; Department of Biological Sciences, University of Lancaster, Lancaster LAI 4YQ, England. 427

428 K. JONES liquid medium free of combined-nitrogen (Allen and Aaron, 1955) at room temperature and at 431 lux. Samples of intact algal mats (the equivalent of 1 cm~^ or suspensions of Nostoc cultures were placed in serum bottles (volume 6 cc). When experiments with algal cultures required pre-incubation the serum bottles were left unstoppered to prevent depletion of carbon dioxide (Jones, 1977a). They were stoppered prior to acetylation. Acetylene (1 ml) was added via the rubber stopper and the acetylated samples incubated at various light intensities for 3 h. Gas samples (1 ml) were removed from the serum bottles and analysed for ethylene content using a Beckman GC-5 gas chromatograph fitted with a 3-m stainless steel column (internal diameter 3 mm) packed with Poropak R (Waters Assoc. Inc.). The column temperature was 60 C and the flow rate of the helium carrier gas was 30 ml min"^ A flame ionization detector was used. Samples were incubated in a phytotron for light intensities ranging from 81 to 1938 lux and were placed outside for higher light intensities. Values of 8611, 4306 and 2153 lux were obtained by covering samples in direct sunlight with one, two or three layers of Saron cloth. Eight intensities up to 17222 lux were measured using a Western Instruments Inc. Newark, N.J. U.S.A. light meter. RESUETS by algal mats in high light intensity at normal soil surface temperatures was compared with that by algal mats at a similar high light intensity but at a lower temperature. The results (Table 1) show that acetylene reduction occurred in the cooled algal mats but not in the warmer ones. Algal mats and Nostoc isolated from the mats were tested for acetylene reduction in a range of light intensities. The results (Table 2) show a correlation between acetylene reduction and low light intensities (below 323 lux) but little additional activity for the rise in light intensity from 323 to full sunlight (17222 Table 1. The effect of incubating blue-green algae at high light intensity at normal soil surface temperature (40 C) and at 35 C Temperature ( C) (MgC,H,cm-'3h-') 40 0.2 35 45.3 Algal mats were placed in serum bottles, acetylated, and incubated outside at light intensity 17222 lux. Half were left at the soil surface and others were placed in shallow water baths at 35 C.

Effect of light intensity on acetylene reduction 429 Table 2. The effect of a range of light intensities on Nostoc sp. when in culture and in algal mats Oux) Dark 81 161 323 646 969 1292 2153 4306 8611 17222 Algal mats (jugcjh^ cm'^ 3 3.0 13.2 22.5 37.2 33.9 37.2 43.4 45.6 42.9 48.6 41.7 Algal culture h^') (MgCjH^mgdry wt"* 3h-') 17.3 90.0 103.5 109.3 114.3 106.8 117.0 107.4 107.3 103.4 Algal mats or 1-ml suspensions of algae were placed outside for light intensities above 1938 lux and in a phytotron for lower light intensities. Table 3, The effect of a range of light intensity on acetylene reduction by Nostoc sp. after preincubation for twenty four hours at those same light intensities (lux) Dark 108 323 646 969 1938 (jugc^h^mgdry wf' 3 h"') 0 42.7 107.0 239.5 265.0 285.0 1-ml suspensions of Nostoc were incubated in unstoppered serum bottles for 24 h. The bottles were stoppered, acetylated, and replaced in the light intensity from whence they came. Table 4. 77?^ effect of a range of light intensity on subsequent acetylene reduction in the dark by mats of blue-green algae (lux) Dark 81 161 323 969 (MgCjH^cm^^3 h"') 0.2 1.4 3.9 6.2 14.4 Algai mats were placed in various light intensities in a phytotron. After 24 h they were acetylated and placed in the dark for a further 3 h at the same temperature.

430 K. JONES A slightly different response to the effects of light intensity was obtained when Nostoc cultures were incubated in a range of light intensities for twenty four hours and then tested for acetylene reduction in those same light intensities. The results (Table 3) show a steady increase in acetylene reduction up to 646 lux and a slower increase up to 1938 lux. Algal mats which had been incubated for twenty four hours in a range of light intensities were placed in the dark and tested for acetylene reduction. The results (Table 4) show that algal mats incubated at the higher light intensities reduced acetylene in considerably greater quantities than those previously incubated at low light densities. DISCUSSION by blue-green algal mats in kikuyu lawns is not inhibited by high light intensities at the soil surface. The midday drop in acetylene reduction observed during in situ studies (Jones, 1977a) is due to excessive temperature. by the algal mats is proportional to light intensity only below 323 lux. Above this figure there is no significant increase in acetylene reduction for increases in light intensity. These results are somewhat similar to those of Henrikssson (1971) and Eglund and Meyerson (1974) who could find little correlation between high light intensities and nitrogen fixation by blue-green algae in temperate soils. The ability to fix nitrogen at low light intensities may be an adaptation to shade conditions found beneath the kikuyu grass. The behaviour of Nostoc in culture in its response to light intensity is similar to that of the algal mats collected fresh from the field. These results, however, are for short-term experiments and differ from those obtained in longer term growth experiments when it was found that algae in culture were inhibited or discoloured at light intensities normally pertaining in the habitat from which they were isolated (Allison, Hoover and Morris, 1937; Jones and Stewart, 1969; Fogg et al., 1973; Jones, 1974). by the algal mats in the dark is related to the light intensity prior to darkness. Similar results for aquatic blue-green algae have been obtained by Home and Fogg (1970). The range of light intensities which permit nitrogen fixation is narrower than that for photosynthesis (Fogg and Than Tun, 1960; Stewart, 1965; Pattnaik, 1966; Jones and Stewart, 1969) hence photosynthesis at higher light intensities may outstrip nitrogen fixation thereby leaving a greater supply of energy/carbon skeletons for nitrogen fixation in the dark. This is to some extent confirmed by the results in Table 3 where algae precincubated at a range of light intensities showed a broader response to light intensity than that obtained in short term experiments. They suggest that there is a lag in the production of nitrogenase in response to higlier rates of photosynthesis. ACK NO WLEDGMENTS The author is indebted for assistance from members of the Department of Botany at the University of Pretoria and in particular to Professor N. Grobbelaar. A postdoctoral fellowship from the CSIR is gratefully acknowledged. REFERENCES ALLEN, M. B. & ARNON, D. 1. (1955). Growth and nitrogen fixation hy Anabaena cylindrica. Plant Physiol, 30(Suppl), 366.

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