Photosynthetic Rates and Photoinhibition of Surf Diatoms in Fluctuating Light

Cells of the surf diatom, Anaulus birostratus are exposed to a fluctuating light environment as a result of their being brought to the water surface by attachment to bubbles and their return to the water column by passing wave bores. Fluctuating light did not alter the rate of photosynthesis in Anaulus birostratus, either on a short (10 minute) or long (4 hour) time scale. The photosynthetic rate of an algal cell moving up and down in the water column should be calculated as the mean of the photosynthetic rate at the surface and that at the bottom rather than as the photosynthetic rate at the mean irradiance received. Photoinhibition was found to be entirely quantum dose dependent with regard to the time required for photoinhibition to occur. Photoinhibition levels were the same after exposure to constant or fluctuating light fields. This has certain implications for the estimation of surf-zone primary production. Three daily periods of photosynthesis can be identified as: 1) photoinduction phase, 2) Pmax phase, and 3) photoinhibition phase. Fluctuating irradiance has no effect on the photoinduction or Pmax phases. In the photoinhibition phase, absorbed quanta rather than maximum irradiance received should be calculated in order to determine the time when photoinhibition will occur. T „ _ , ^ during its determination. If the time-scale of exposure Introduction * · jj * · ι * ^ *· ι r*u to irradiance does not simulate the time scales of the In the estimation of phytoplankton primary producfluctuations of the light environment, it is not likely tion consideration must be given to fluctuations in that the photosynthetic rate measured will simulate the incident irradiance as well as the total quantity of the time scales of photosynthetic responses to irradiirradiance (Gallegos et al. 1980). In natural systems ance (Marra 1978 a). Regularly fluctuating irradiance there are several factors which cause variations in has been shown to alter the rate of photosynthesis of irradiance. Irregular variations are caused by surface marine algae (Savidge 1986). Enhancement of phowave movement (Walsh and Legendre 1983), cloud tosynthesis to values as high as 180% of the control cover (Marra and Heinemann 1982) and the vertical have occasionally been recorded (Jewson and Wood movement of phytoplankton (Falkowski and Wirick 1975, Marra 1978b). 1981). Regular variations, are due to the circulatory _ t. . n u · ju * t-u·*· ' * . . „ ι /o -j -ΙΠΟΖΛ The elimination of high irradiance photoinhibition is motion within internal waves (Savidge 1986). ' ' , 4 , , , ff . f ~ ^ · ' one of the most commonly reported effects of flucThe functional relationship between irradiance and tuating irradiance (Harris 1978). This is because phophotosynthesis, the light saturation curve, forms the toinhibition is a time-dependent process and relatively basis of most models of phytoplankton production short exposures to high irradiance do not depress (Platt et al. 1977). Because of the semi-empirical basis photosynthesis to the extent that it does when phyof the photosynthesis-light relationship, its accuracy toplankton are kept in full sunlight for several hours in estimating primary production depends on how (Harris 1978, Marra 1978b). The elimination of phoclosely conditions in the natural system are simulated toinhibition by periodic movement away from inhibBotanica Marina / Vol. 31 / 1988 / Fasc. 5 Copyright © 1988 Walter de Gruyter · Berlin · New York 412 Campbell et a/.: Surf diatoms in fluctuating light itory irradiances causes depth-integrated primary production to be enhanced. Nevertheless, the enhancement effect of fluctuating light is not entirely accounted for by the elimination of photoinhibition (Yoder and Bishop 1985). The reason for the additional enhancement is not clear. Savidge (1986) proposed that it could be due to reduced dark respiration and possibly reduced light respiration as well. The influence of fluctuating light is. also not reflected in the daily rate of growth (Marra 1978 a, Falkowski and Wirick 1981). When fluctuating light enhanced or decreased photosynthetic rates, it did not influence growth rates. Cosper (1982 a, b) suggested that growth and photosynthesis became uncoupled under fluctuating irradiance, resulting in decreased growth despite higher photosynthetic rates. In the surf-zone of the Sundays river beach, the dominant phytoplankter, Anaulus birostratus (Grun.) Grun. possesses a mechanism which enables the cells to adhere to wave entrained bubbles. As a result they float to the surface between wave bores (Sloff et al. 1984). With each passing wave, the cells are tumbled into the water column and rise back into the surface foam again after the wave has passed. This vertical movement exposes the cells to a more or less regularly fluctuating irradiance. When determining the productivity of these organisms, it is necessary to integrate the rates on the basis of both incident and fluctuating irradiance. It is also necessary to know how fluctuating irradiance influences the rate of photosynthesis, in order to estimate annual surf-zone primary production. The aim of this study was to investigate the photosynthetic responses of A. birostratus to fluctuating irradiances which simulate those induced by the passing of wave bores. Material and Methods Water samples containing between 0.5 and 1 mg Chi a I" were collected from the surf-zone of our sandy beach study site in Algoa Bay (34°02'S; 25°45Έ to 33°46'S; 26°27Έ). The diatom A. birostratus is the dominant phytoplankter (99%) in surface foam (Sloff 1984). Near-monospecific samples were collected by scooping the surface foam into a collection vessel. After collection, the algae were stirred continually until used. All measurements were done within 36 hours after collection. When Anaulus birostratus cells are kept in the dark continually, they do not divide. If re-exposed to light, normal division is re-established. For this reason, the cells were held at a low irradiance (20 μπιοί m~ s" photosynthetic photon flux density) which is just above the compensation point (Campbell and Bate 1987) in order to reduce the utilization of nutrients during the pre-experimental period. Light was provided in two ways, a Phillips 77485 light bulb was fitted to a light projector to illuminate the cuvette in which the rate of photosynthesis was determined. Secondly, samples were incubated in front of Phillips 500 W Thotoflood' lamps. The rate of photosynthesis (as oxygen evolved) was measured using the system described by Campbell et al. (1985). The oxygen evolved was converted to carbon uptake using the photosynthetic quotient of 1.2 (Campbell and Bate 1987). Fluctuating light was produced by filters attached to a rotating circular frame placed in front of the light source. The filters consisted of black, 1 mm mesh shade sheeting and were arranged in such a way that they gave a fluctuating bright/dim sequence of equal duration. Different intensities were produced using different numbers of sheeting layers. The frame was secured to a variable speed motor which provided frequencies of between 1 and 15 second exposure of each irradiance. Measurements were made using frames which provided 1200, 540, 300, 150 and 15 μπιοί mr s~' (PPFD), by attenuating an incident irradiance of 2000 μπιοί m"" s" (equivalent to full sunlight). Each of the fluctuating irradiance treatments comprised the alternating of one of these irradiances with 2000 μπιοί m~ s" in equal exposure times (eg. 1200/2000 μπιοί m~ s" and 540/2000 μπιοί m~ s", etc.). The maximum irradiance value (2000 μπιοί m~ s") was obtained by exposing the sample to constant irradiance (2000/ 2000 μπιοί m~ s") after a 10 minute exposure to the fluctuating irradiance of 1200/2000 μπιοί m" s". The controls consisted of measurements done at constant irradiances of 2000, 1200, 5400, 300, 150 or 15 μπιοί m"~ s" without prior exposure to fluctuating irradiance. Exposure periods of 1, 2, 5, 8, 12 and 15 seconds were used, where, for example, the 1 second field consisted of 1 second exposure to maximum irradiance followed by 1 second exposure to the minimum irradiance. To determine whether fluctuating light had an influence on photoinhibition, samples were incubated for 4 hours in front of the light source with fluctuating irradiance provided using the frames as described before. Irradiances used were 500/100; 820/170; 900/ 180; 1300/200 and 1700/320 μπιοί nr s" (PPFD). It was not possible to provide irradiances above 1700 μπιοί m" s"" with the Thotoflood' lamps. During all incubations, the pH of the sample was controlled by a pH-stat (Fig. 1). The pH-stat consisted of an autotitrator (Metrohm) set to mainstain the pH at 8.0. The pH electrode was submerged in the sample Botanica Marina / Vol. 31 / 1988 / Fasc. 5 Campbell et al.: Surf diatoms in fluctuating light 413