THE INTERACTION BETWEEN INORGANIC CARBON ACQUISITION AND LIGHT SUPPLY IN PALMARIA PALMATA (RHODOPHYTA)1

The dependence of the carbon concentrating mechanism of Palmaria palmata (L.) Kuntze on the growth light level was examined 1) to determine whether or not there is a threshold photon flux density (PFD) at which the inorganic carbon uptake mechanism can operate and 2) to attempt to quantify the relative energetic costs of acclimation to the two different limiting factors, PFD and dissolved inorganic carbon (DIC) concentration. Plants were grown at six PFDs: 5, 25, 50, 75, 95, and 125 μmol photons. m^?2.s^?1. Growth rates increased with increasing PFD from 5 to 50 μmol photons. m^?2. s^?1 and were light-saturated at 75, 95, and 125 μmol photons. m^?2. s^?1 Values of δ¹³C increased continuously with increasing growth PFD and did not saturate over the range of light levels tested. Time-resolved fluorescence characteristics indicated a progressive photoacclimation below 50 μmol photons. m^?2. s^?1. Analysis of chlorophyll fluorescence induction showed three levels of light use efficirncy associated with growth at 5 or 25, 50, and >75 μmol photons. m^?2. s^?1. The light-haruesting efficiency was inversely proportional to the effectiveness of DIC acquisition in plants grown at the six PFDs. These data were interpreted to indicate that there is a physiological tradeoff between photosynthetic efficiency and bicarbonate use in this species.     

ACCLIMATION OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT INTENSITY: COMPARISON BETWEEN ARTIFICIAL AND NATURAL LIGHT FIELDS

The acclimation of the photosynthetic apparatus of Palmaria palmata (L.) to light intensity was examined in the field and under laboratory conditions. Algae from 3 different shore levels and from laboratory cultures adapted to 6 different photon flux densities were compared. This was done on the basis of light doses, which were delivered by different light regimes in the field and in the laboratory. Laboratory samples were adjusted to constant photon flux densities between 7 and 569 μmol photons·m ^{? 2}·s ^{? 1} in a 16:8 light:dark photoperiod. Under field conditions the daily amplitudes reached up to approximately 2000 μmol photons·m ^{? 2}·s ^{? 1} within a natural daily light course. Over the course of 14 days the light doses resulting from those different regimes are similar for both treatments. An increasing growth rate per day with increasing light doses was observed in the laboratory. Growth was saturated at 113 mol photons·m ^{? 2}·14 d ^{? 1}. Light saturation points (E_k) of photosynthesis increased with increasing light doses for both field and laboratory samples, and all E_k values were significantly related to the growth light dose. A correlation between fresh weight‐related lutein content and growth light dose was found for laboratory samples only, whereas the lutein:chlorophyll a (chl a) ratio was strongly correlated with E_k for laboratory and field samples. The content of chl a and phycoerythrin (PE) per fresh weight decreased significantly with increasing light doses under field conditions. Simultaneously, the PE:chl a ratio increased, whereas this ratio was not influenced by laboratory treatments. The correspondence of E_k values for field and laboratory treatments indicated that they were affected mainly by light dose. However, the variability in pigmentation was mainly dependent on temporal variability in light intensity (the amplitude of variations in incident light).     

EFFECTS OF LIGHT AND TEMPERATURE ON NET PHOTOSYNTHESIS AND DARK RESPIRATION OF GAMETOPHYTES AND EMBRYONIC SPOROPHYTES OF MACROCYSTIS PYRIFERA1

The rates of net photosynthesis as a function of irradiance and temperature were determined for gametophytes and embryonic sporophytes of the kelp, Macrocystis pyrifera (L.) C. Ag. Gametophytes exhibited higher net photosynthetic rates based on oxygen and pH measurements than their derived embryonic sporophytes, but reached light saturation at comparable irradiance levels. The net photosynthesis of gametophytes reached a maximum of 66.4 mg O₂ g dry wt^?1 h^?1 (86.5 mg CO₂ g dry wt^?1 h^?1), a value approximately seven times the rate reported previously for the adult sporophyte blades. Gametophytes were light saturated at 70 μE m^?2 s^?1 and exhibited a significant decline in photosynthetic performance at irradiances 140 μE m^?1 s^?1. Embryonic sporophytes revealed a maximum photosynthetic capacity of 20.6 mg O₂ g dry wt^?1 h^?1 (25.3 mg CO₂ g dry wt^?1 h^?1), a rate about twice that reported for adult sporophyte blades. Embryonic sporophytes also became light saturated at 70 μE m^?2 s^?1, but unlike their parental gametophytes, failed to exhibit lesser photosynthetic rates at the highest irradiance levels studied; light compensation occurred at 2.8 μE m^?2 s^?1. Light-saturated net photosynthetic rates of gametophytes and embryonic sporophytes varied significantly with temperature. Gametophytes exhibited maximal photosynthesis at 15° to 20° C, whereas embryonic sporophytes maintained comparable rates between 10° and 20° C. Both gametophytes and embryonic sporophytes declined in photosynthetic capacity at 30° C. Dark respiration of gametophytes was uniform from 10° to 25° C, but increased six-fold at 30° C; the rates for embryonic sporophytes were comparable over the entire range of temperatures examined. The broader light and temperature tolerances of the embryonic sporophytes suggest that this stage in the life history of M. pyrifera is well suited for the subtidal benthic environment and for the conditions in the upper levels of the water column.     

TOXICITY OF BISULFITE TO PHOTOSYNTHESIS AND RESPIRATION1

Inhabition of photosynthesis in Chloroccoum sp by bisulfileion was the reciprocal of the light intensity curve. Respiration was least affected of the bisulfite after endogenous substrate was reduced by incubation in darkness. Maximum areduction in growth occurred with bisulfile treatment at or above optimal growth temperatures. Maximum phytotoxicity correlated with conditions resulting in maximum metabolic activity. The order of toxicity was –H₂SO₃HSO₃^?SO₃.     

EFFECT OF TEMPERATURE,LIGHT AND pH ON GROWTH,PHOTOSYNTHESIS AND RESPIRATION OF THE DINOFLAGELLATE PERIDINIUM CINCTUM FA. WESTII IN LABORATORY CULTURES1

Optimum light, temperature, and pH conditions for growth, photosynthetic, and respiratory activities of Peridinium cinctum fa. westii (Lemm.) Lef were investigated by using axenic clones in batch cultures. The results are discussed and compared with data from Lake Kinneret (Israel) where it produces heavy blooms in spring. Highest biomass development and growth rates occurred at ca. 23° C and ≥50 μE· m^?2·s¹ of fluorescent light with energy peaks at 440–575 and 665 nm. Photosynthetic oxygen release was more efficient in filtered light of blue (BG 12) and red (RG 2) than in green (VG 9) qualities. Photosynthetic oxygen production occurred at temperatures ranging from 5° to 32° C in white fluorescent light from 10 to 105 μE·m^?2·s^?1 with a gross maximum value of 1500 × 10^?12 g·cell^?1·h^?1 at the highest irradiance. The average respiration amounted to ca. 12% of the gross production and reached a maximum value of ca. 270·10^?12 g·cell^?1·h^?1 at 31° C. A comparison of photosynthetic and respiratory Q₁₀-values showed that in the upper temperature range the increase in gross production was only a third of the corresponding increase in respiration, although the gross production was at maximum. Short intermittent periods of dark (>7 min) before high light exposures from a halogen lamp greatly increased oxygen production. Depending on the physiological status of the alga, light saturation values were reached at 500–1000 μE·m^?2·s^?1 of halogen light with compensation points at 20–40 μE·m^?2·s^?1 and I_k-values at 100–200 μE·m^?2·s^?1. The corresponding values in fluorescent light in which it was cultured and adapted, were 25 to 75% lower indicating the ability of the alga to efficiently utilize varying light conditions, if the adaptation time is sufficient. Carbon fixation was most efficient at ca. pH 7, but the growth rates and biomass development were highest at pH 8.3.     

LIGHT DEPENDENCE OF GROWTH AND PHOTOSYNTHESIS IN PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

Phaeodactylum tricornutum Bohlin was maintained in exponential growth over a range of photon flux densities (PFD) from 7 to 230 μmol·m^?2s^?1. The chlorophyll a-specific light absorption coefficient, maximum quantum yield of photosynthesis, and C:N atom ratio were all independent of the PFD to which cells were acclimated. Carbon- and cell-specific, light-satuated, gross photosynthesis rates and dark respiration rates were largely independent of acclimation PFD. Decreases in the chlorophyll a-specific, gross photosynthesis rate and the carbon: chlorophyll ratio and increases of cell- or carbon-specific absorption coefficients were associated with an increase in cell chlorophyll a in cultures acclimated to low PFDs. The compensation PFD for growth was calculated to be 0.5 μmol·m^?2s^?1. The maintenance metabolic rate (2 × 10^?7s^?1), calculated on the basis of the compensation PFD, is an order of magnitude lower than the measured dark respiration rate(2.7 × 10^?6mol O₂·mol C^?1s^?1). Maintenance of high carbon-specific, light-saturated photosynthesis rates in cells acclimated to low PFDs may allow effective use of short exposures to high PFDs in a temporally variable light environment.     

RATES OF GROWTH,RESPIRATION AND PHOTOSYNTHESIS OF UNICELLULAR ALGAE AS RELATED TO CELL SIZE—A REVIEW1,2

The decline of growth rate with increasing species size of unicellular algae grown under uniform conditions is quantified by applying to published data the equation, growth, (cell · time)^?1= a (cell carbon)^b where a and b are coefficients. The degree of size-dependence might be highest under optimal conditions of growth where b is 0.75. Respiration rate is shown to decline with size in the same manner. It is postulated that gross photosynthesis and processes underlying growth are similarly size-dependent. Growth, efficiency (net over gross photosynthesis) cannot be shown to be size-dependent. Cell size, expressed as carbon, is proposed as a scaling factor in comparative algal physiology.     

LIGHT RESPIRATION IN SIX ESTUARINE PHYTOPLANKTON SPECIES: CONTRASTS UNDER PHOTOAUTOTROPHIC AND MIXOTROPHIC GROWTH CONDITIONS1

The photoresponse of respiration was examined in six estuarine phytoplankton species grown at a limiting irradiance in the presence or absence of glucose. Cellular respiration rates in the dark, at the growth irradiance, and at saturating light levels were higher in glucose-grown cultures than in photoautotrophic cultures. Glucose uptake also affected the relationship between incident irradiance and gross respiration rate, and the specific respiratory pathways that contributed to the respiration versus irradiance (R vs. I) response. The energy-conseming Mehler reaction contributed largely to light-stimulated respiration in photoautotrophic (energy-limited) cultures. However, the energy-dissipating alternative pathway was the predominant form of respiration measured in mixotrophic (energy-sufficient) cultures. Based on these and previous observations, a model of phytoplankton R us. I response is presented, in which the relative contributions of different light respiratory processes (cytochrome pathway respiration, alternative pathway respiration, chlororespiration, Mehler reaction) are dependent on the incident irradiance and cellular energy content.     

CHANGES IN NUTRIENT CONTENT OF PALMARIA PALMATA IN RESPONSE TO VARIABLE LIGHT AND UPWELLING IN NORTHERN SPAIN1

Light has been identified as one of the main factors affecting seaweed ecophysiology. We investigated the dependence of nutrient metabolism on sun and shade light conditions and whether episodes of upwelling of nutrient‐rich subsuperficial water could reduce the summer nutrient limitation driving physiological changes in Palmaria palmata (L.) Kuntze. We measured the major nutrient pools, photosynthetic pigments, and light curves, under sun and shade conditions during a summer period when one upwelling was recorded. The redundancy analysis (RDA) produced two clear groups: sun‐ and shade‐acclimated algae. Light was the major predictive factor. Sun‐acclimated algae exhibited higher carbon (C) and lower nitrogen (N) and phosphorus (P) content in association with the storage of floridoside (main C reserve) to benefit from higher irradiance (under nutrient limitation). Among N pools, N reserves (phycoerythrin, nitrate) were a lower proportion of the total N in sun‐acclimated algae, suggesting their degradation to fulfill the N demands of the cell. The orthophosphate content was also lower in sun‐acclimated algae, indicating its utilization as a nutrient reserve. In contrast, N within cell walls and membranes and chl a contributed to a similar proportion of the total N in sun‐ and shade‐acclimated algae, suggesting a response to sustain cell integrity. Transient high nutrient concentration due to the upwelling was unrelated to the nutrient content of the thallus. The storage of C as floridoside from high light exposure was shown to be the driving force for the metabolic adjustment of P. palmata at the end of summer before the onset of dormancy.     

ACCLIMATION OF THE PHOTOSYNTHETIC APPARATUS OF PALMARIA PALMATA (RHODOPHYTA) TO LIGHT QUALITIES THAT PREFERENTIALLY EXCITE PHOTOSYSTEM I OR PHOTOSYSTEM II1

Acclimation of the photosynthetic apparatus to light absorbed primarily by phycobilisomes (which transfer energy predominantly to photosystem II) or absorbed by chlorophyll a (mainly present in the antenna of photosystem I) was studied in the macroalga Palmaria palmata L. In addition, the influence of blue and yellow light, exciting chlorophyll a and phycobilisomes, respectively, ivas investigated. All results were compared to a white light control. Complementary chromatic adaptation in terms of an enhanced ratio of phycoerythrin to phycocyanin under green light conditions was observed. Red light (mainly absorbed by chlorophyll a) and green light (mainly absorbed by phycobilisomes) caused an increase of the antenna system, which was not preferentially excited. Yellow and blue light led to intermediate states comparable to each other and white light. Growth was reduced under all light qualities in comparison to white light, especially under conditions preferably exciting phycobilisomes (green light-adapted algae had a 58% lower growth rate compared to white light-adapted algae). Red and blue light-adapted algae showed maximal photosynthetic capacity with white light excitation and significantly lower values with green light excitation. In contrast, green and yellow light-adapted algae exhibited comparable photosynthetic capacities at all excitation wavelengths. Low-temperature fluorescence emission analysis showed an increase of photosystem II emission in red light-adapted algae and a decrease in green light-adapted algae. A small increase of photosystem I emission teas also found in green light-adapted algae, but this was much less than the photosystem II emission increase observed in red light-adapted algae (both compared to phycobilisome emission). Efficiency of energy transfer from phycobilisomes to photosystem II was higher in red than in green light-adapted algae. The opposite was found for the energy transfer efficiency from phycobilisomes to photosystem I. Zeaxanthin content increased in green and blue light-adapted algae compared to red, white, and yellow light-adapted algae. Results are discussed in comparison to published data on unicellular red algae and cyanobacteria.     

RATES OF PHOTOSYNTHESIS AND RESPIRATION OF THE MOSS BRYUM SANDBERGII AS INFLUENCED BY LIGHT INTENSITY AND TEMPERATURE

Using differential respirometry and air enriched to 3% CO₂ (v/v), the rates of photosynthesis and dark respiration of the moss Bryum sandbergii were measured as influenced by temperature and light intensity. The optimal temperature for net (apparent) photosynthesis was between 24 to 30 C; however, the photosynthesis/respiration ratio was about 11 to 27 between 4 to 24 C and dropped to lower values at 34 C., which indicates a wide temperature tolerance for this moss in short-term experiments. The maximum temperature for photosynthesis was about 41 C and the minimum was below –5 C. At 20 C light saturation was approached at 6.2 mw cm^–2 (ca. 700 ft-c) but not completely reached at 12 mw cm^-2; the light compensation point was estimated to be 0.4 mw cm^-2 (ca. 40 ft-c). At 4 C light saturation and the compensation point were at lower levels and apparently solarization occurred at 12 mw cm^-2. Light intensity had little or no apparent effect on dark respiration. However, respiration increased with temperature over various ranges extending from –5 to 39 C with temperature quotients of about 2.5 to 1.2. The significance of these characteristics is discussed with respect to the ecological relationships of the species.     

GROWTH AND PHOTOSYNTHESIS OF MARINE SYNECHOCOCCUS (CYANOPHYCEAE) UNDER IRON STRESS

Prokaryotic picoplankton such as Synechococcus are relatively abundant in putatively Fe-limited high-nutrient, low-chlorophyll (HNLC) regions of the oceans. The physiology of Synechococcus under Fe stress has been studied less than eukaryotic algae. Recent evidence suggests that although biomass and growth rates of Synechococcus are not typically Fe limited in situ, cells may still exhibit symptoms of Fe stress. We grew Synechococcus A2169 and WH7803 in laboratory batch cultures in the artificial medium Aquil and enriched natural seawater, at a series of Fe concentrations and Fe:macronutrient ratios, and with either nitrate or ammonium as the sole nitrogen source. Cell yields, and in some experiments exponential specific growth rate (μ), were more readily Fe limited in the Atlantic isolate WH7803 than in the equatorial Pacific isolate A2169. In both strains, final cell yields spanned about an order of magnitude and decreased continuously with Fe concentration from 900 to 3.6 nM (150 μM N, 10 μM P), whereas μ decreased much less and only at Fe concentrations below 90 nM. Synechococcus yield was controlled by both absolute Fe concentration and Fe:macronutrient ratio, but μ was determined primarily by absolute Fe concentration. Contrary to theoretical predictions, neither yield nor μ was higher in Fe-limited cells grown in ammonium compared to nitrate. Under severe Fe stress, cellular chlorophyll (Chl) content and light-saturated gross photosynthetic capacity (P^cell_m) decreased proportionately, and dark respiration (R^cell_d) increased, such that net P^cell_m was extremely low but gross P^Chl_m was unchanged. This is the first report of an absolute increase in R^cell_d under Fe stress in phytoplankton.     

PHOTOSYNTHESIS,DARK RESPIRATION AND DESICCATION RESISTANCE OF THE INTERTIDAL SEAWEEDS HESPEROPHYCUS HARVEYANUS AND PELVETIA FASTIGIATA F. GRACILIS12

Rates of net photosynthesis and dark respiration were determined under submersed and emerged conditions for Hesperophycus harveyanus S. & G. and Pelvetia fastigiata f. gracilis (Decne.) S. & G. Both species exhibited submersed photosynthesis-light relationships and dark respiration rates similar to those established for other closely related intertidal, fucoids. Maximal net photosynthesis of H. harveyanus (0.21 mmol O₂ g dry wt.^-1· h^-1; 0.18 mmol CO₂ g dry wt.^-1· h^-1) was similar to that of P. fastigiata f. gracilis (0.17 mmol. O₂ g dry wt.^-1· h^-1; 0.14 mmol CO₂ g dry wt. ^-1· h^-1). Light saturation occurred between 150 and 250 μE · m^-2· s^-1 for H. harveyanus and between 75 and 150 μE · m^-2· s^-1 for P. fastigiata f. gracilis; photon flux densities required for compensation were 6.4 and 9.2 μE · m^-2· s^-1, respectively. Photoinhibition was not observed for either species. The light-saturated, submersed net photosynthetic performances of both species varied significantly with temperature. Greatest photosynthetic rates were obtained at 23° C for H. harveyanus and at 18° C for P. fastigiata f. gracilis. Under emersed conditions, the maximal net photosynthetic rate and the photon flux densities required for saturation were greater for H. harveyanus (0.08 mmol CO₂ g dry wt.^-1· h^-1; 260 to 700 μE · m^-2· s^-1) than for P. fastigiata f. gracilis (0.02 mmol CO₂g dry wt.^-1· h^-1; 72 to 125 μE · m^-2· s^-1). However, for both species, emersed photosynthetic rates were much lower (14–44%) than those obtained under submersed conditions. Desiccation negatively influenced emersed photosynthesis, of both species, but H. harveyanus thalli contained more water when fully hydrated and lost water more slowly during dehydration, thus suggesting greater photosynthetic potential during field conditions of emersion.     

ACCLIMATION OF PHOTOSYNTHESIS AND PIGMENTS DURING AND AFTER SIX MONTHS OF DARKNESS IN PALMARIA DECIPIENS (RHODOPHYTA): A STUDY TO SIMULATE ANTARCTIC WINTER SEA ICE COVER1

The influence of seasonally fluctuating photoperiods on the photosynthetic apparatus of Palmaria decipiens (Reinsch) Ricker was studied in a year‐round culture experiment. The optimal quantum yield (F_v/F_m) and the maximal relative electron transport rate (ETR_max), measured by in vivo chl fluorescence and pigment content, were determined monthly. During darkness, an initial increase in pigment content was observed. After 3 months in darkness, ETR_max and F_v/F_m started to decrease considerably. After 4 months in darkness, degradation of the light‐harvesting antennae, the phycobilisomes, began, and 1 month later the light harvesting complex I and/or the reaction centers of PSII and/or PSI degraded. Pigment content and photosynthetic performance were at their minimum at the end of the 6‐month dark period. Within 24 h after re‐illumination, P. decipiens started to accumulate chl a and to photosynthesize. The phycobiliprotein accumulation began after a time lag of about 7 days. Palmaria decipiens reached ETR_max values comparable with the values before darkness 7 days after re‐illumination and maximal values after 30 days of re‐illumination. Over the summer, P. decipiens reduced its photosynthetic performance and pigment content, probably to avoid photodamage caused by excess light energy. The data show that P. decipiens is able to adapt to the short period of favorable light conditions and to the darkness experienced in the field.     

THE RELATIONSHIP BETWEEN PHOTOSYNTHESIS AND LIGHT FOR NATURAL ASSEMBLAGES OF COASTAL MARINE PHYTOPLANKTON1

In a series of 188 experiments on the, light-saturation curve for natural assemblages of phytoplankton at 3 stations in Nova Scotia coastal waters, it was found that both the initial slope (α) of the curve and the assimilation number (P_m^B) varied about 5-fold throughout the year. No differences could be detected between stations, but both α and P_m^B decreased with depth. The mean value of α for all the experiments was 0.21 mg C[mg Chl a]^?1· h^?1· W^?1· m² with a range from 0.03 to 0.63. An explanation is offered for the nonconstancy of a in terms of the effect of cell-size and shape on self-shading. An estimate is made from first principles of the physiological maximum-attainable value of α. This estimate corresponds, within the limits of experimental error, to the highest values of α observed in the study. It is deduced that on the average the phytoplankton were photosynthesizing at only 44% maximum capacity. The mean value of P_m^B for all experiments was 4.9 mg C[mg Chl a]^?1· h^?1, with a range from 0.73 to 24.8. In the matrix of partial correlation coefficients, α and P_m^B were positively correlated with each other; α was correlated with mean solar radiation averaged over the 3 days prior to the experiment, but uncorrelated with temperature; P_m^B was correlated strongly with temperature but uncorrelated with recent solar radiation. The results show that P_m^B could be estimated from α and temperature using an empirical multiple regression equation, independent of depth. It is suggested that α and P_m^B are both correlated with some other factor not measured in the study, perhaps the mean cell-size of the populations, or the nutrient status of the cells. The predictability of primary production is discussed in the light of this evidence.     

SHORT‐TERM EFFECTS OF ACETATE AND MICROAEROBIC CONDITIONS ON PHOTOSYNTHESIS AND RESPIRATION IN CHLORELLA SOROKINIANA GXNN 01 (CHLOROPHYTA)1

The culture of microalgae using organic carbon sources decreases the cost of operation in closed systems. The effect of carbon sources on microalgae is thus an interesting problem in not only theoretical research but also practical production. The short‐term effects of acetate and microaerobic conditions on the growth, photosynthesis, and respiration of the green microalga Chlorella sorokiniana I. Shihira & R.W. Krauss GXNN 01 were described after acetate addition to autotrophic cultures. As the acetate concentration increased, cells needed a longer lag phase to grow, and 243.8 mM acetate completely inhibited growth. Acetate addition induced an immediate response in photosynthesis and respiration. The activity of PS II and PS I were impaired and declined with different rates, and then recovered compared with autotrophic cells. Carbonic anhydrase and Rubisco activities were also inhibited at the beginning, and respiration was increased. We propose that ATP consumption for acetate assimilation results in surplus NADPH, and then accumulated reducing power over‐reduces inter‐photosystem components and raises the transthylakoid proton gradient, which redistributes energy between PS I and PS II, and leads to a decrease in the PS II/PS I ratio and O₂ evolution. An apparent cyclic electron flow was also observed, which may be mainly mediated by NAD(P)H dehydrogenase‐dependent pathway since NADPH was in excess. These observations pointed to an acclimation process after acetate addition, and suggested the interaction between photosynthesis and respiration involving ATP and reducing power.     

MICROENVIRONMENTAL CONTROL OF PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN AN EPILITHIC CYANOBACTERIAL BIOFILM1

The photosynthetic performance of an epilithic cyano-bacterial biofilm was studied in relation to the in situ light field by the use of combined microsensor measurements of O₂, photosynthesis, and spectral scalar irradiance. The high density of the dominant filamentous cyanobacteria (Oscillatoria sp.) embedded in a matrix of exopolymers and bacteria resulted in a photic zone of < 0.7 mm. At the biofilm surface, the prevailing irradiance and spectral composition were significantly different from the incident light. Multiple scattering led to an intensity maximum for photic light (400–700 nm) of ca. 120% of incident quantum irradiance at the biofilm surface. At the bottom of the euphotic zone in the biofilm, light was attenuated strongly to < 5–10% of the incident surface irradiance. Strong spectral signals from chlorophyll a (440 and 675 nm) and phycobilins (phycoerythrin 540–570 nm, phycocyanin 615–625 nm) were observed as distinct maxima in the scalar irradiance attenuation spectra in the upper 0.0–0.5 mm of the biofilm. The action spectrum for photosynthesis in the cyanobacterial layer revealed peak photosynthetic activity at absorption wavelengths of phycobilins, whereas only low photosynthesis rates were induced by light absorption of carotenoids (450–550 nm). Respiration rates in light- and dark-incubated biofilms were determined using simple flux calculations on measured O₂ concentration profiles and photosynthetic rates. A significantly higher areal O₂ consumption was found in illuminated biofilms than in dark-incubated biofilms. Although photorespiration accounted for part of the increase, the enhanced areal O₂ consumption of illuminated biofilms could also be ascribed to a deeper oxygen penetration in light as well as an enhanced volumetric O₂ respiration in and below the photic zone. Gross photosynthesis was largely unaffected by increasing flow velocities, whereas the O₂ flux out of the photic zone, that is, net photosynthesis, increased with flow velocity. Consequently, the amount of produced O₂ consumed within the biofilm decreased with increasing flow velocity. Our data indicated a close coupling of photosynthesis and respiration in biofilms, where the dissolved inorganic carbon requirement of the photo-synthetic population may largely be covered by the respiration of closely associated populations of heterotrophic bacteria consuming a significant part of the photosynthetically produced oxygen and organic carbon.     

PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN NATURAL BIOFILMS QUANTIFIED WITH OXYGEN MICROSENSORS1

Photosynthesis and respiration were analyzed in natural biofilms by use of O₂ microsensors. Depth profiles of gross photosynthesis were obtained from the rate of decrease in O₂ concentration during the first few seconds following extinction of light, and net photosynthesis of the photic zone was calculated from O₂ concentration gradients measured at steady state. Respiration within the photic zone was calculated as the difference between gross and net photosynthesis. Two types of biofilms were investigated: one dominated by diatoms, and one dominated by cyanobacteria. High O₂/CO₂ ratios caused increased respiration especially within the diatom biofilm, which could indicate that photorespiration was a dominant O₂-consuming process. The rate of respiration was constant within both biofilms during the first 4.6 s following extinction of light, even when respiration was stimulated by high O₂/CO₂ ratio. The assumption of a constant rate of respiration during the dark period is an essential one for the determination of gross photosynthetic activity by use of O₂ microsensors. We here present the first evidence to substantiate this assumption. The results strongly suggest that gross photosynthesis as measured by use of O₂ microsensors may include carbon equivalents that are subsequently lost through photorespiration. Computer modeling of photosynthesis profiles measured after 1.1, 1.6, and 2.6 s of dark incubation illustrated how the actual photosynthesis profile could have appeared if it had been possible to do the determination at time 0. Diffusion of O₂ during the up to 4.6-s long dark incubations did not affect gross photosynthetic rate when integrated over all depths, but the apparent vertical distribution of the photosynthetic activity was strongly affected.