SEASONAL PATTERNS OF PHOTOSYNTHESIS AND LIGHT-INDEPENDENT CARBON FIXATION IN MARINE MACROPHYTES1

The contribution of light-independent carbon fixation (LICF) to the overall carbon gain and the seasonal patterns of maximum photosynthesis (P_max and LICF were characterized in a broad taxonomic range of macrophytes from Monterey Bay, California. P_max and LICF rates (nmol C.g filtered seawater^?1.min^?1) varied among species and taxonomic groups examined, and as a function of tissue type in the phaeophyte Laminaria setchellii Silva (Phaeophyceae). On average, P_max values were higher in the Rhodophyta, whereas LICF rates were greater in the Phaeophyceae. LICF rates were generally less than 5% of P_max in the marine macrophytes studied and, as a consequence, cannot fully compensate for respiratory carbon losses, which usually are greater than 10% of P_max. All species studied possessed the highest P_max and LICF rates when irradiance levels were highest and decreased during periods of low incident irradiance. Seasonal patterns of P_max and LICF in most of the macrophytes from the stenothermal environment of Monterey Bay were strongly correlated with photosynthetic photon flux rather than seawater temperature. The concomitant decrease of LICF and P_max rates in all species examined argues against LICF playing a major role in carbon acquisition under light-limiting conditions as suggested previously. Rather, the strong positive correlation of P_max and LICF indicates the direct coupling of photosynthate (e.g. 3-phosphoglyceric acid) generation with production of substrates for LICF reactions. Our results also suggest that LICF might be a useful indicator of photosynthetic metabolism in marine macrophytes.     

WATER COLUMN PRODUCTIVITY ATTRIBUTABLE TO DISPLACED BENTHIC DIATOMS IN WELL-MIXED SHALLOW ESTUARIES1

We attempted to determine the extent to which benthic diatoms contribute to water column primary productivity in shallow-water estuaries and to elucidate the primary mechanisms responsible for suspending the diatoms. A perliminary study conducted in Mugu Lagoon, California indicated that productivity of ocean water entering the lagoon during flood tides was often several orders of magnitude less than that of the same water mass about 3 h later. Benthic pennate diatoms displaced from the sediments into the water column accounted for the increase. A more detailed study was conducted in Barataria Estuary, Louisiana where, for one month, daily measurements were made of benthic and water column productivity and several other environmental variables. During the month, the relationship between water column and benthic primary productivity varied from strongly negative to weakly negative to positive. K-systems analysis indicated that factors comprised of wave height, meteorological tides, astronomical tides, and benthic productivity and standing crop accounted for the full range of variation in water column productivity. Benthic pennate diatoms, represented an average of 74% of the diatom taxa in water column samples. We conclude that the primary productivity of well-mixed shallow estuarine waters is often greatly aumented by displaced benthic algae.     

THE PATH OF CARBON IN PHOTOSYNTHESIS BY MARINE PHYTOPLANKTON12

Using cultures of a number of different marine algae (diatoms Skeletonema costatum (Grev.) Cleve and Phaeodactylum tricornutum Bohlin, chrysophyte Isochrysis galbana Parke, green flagellate Dunaliella tertiolecta Butcher, dinoflagellate Gonyaulax tamarensis Lebour) the short-term, pattern of ¹⁴CO₂ assimilation has been investigated. In all except D. tertiolecta the labelling of amino acids and intermediates of the tricarboxylic acid (Krebs) cycle was significantly heavier than that of sugar phosphates. Over periods of 30–120 s labelling in amino acids and Krebs cycle intermediates accounted for 41–95% of the ¹⁴C fixed (depending on the alga). Over shorter times (< 10 s) the pattern in the 2 diatoms showed significant labelling of C₄ acids (and related com-pounds) and little labelling of sugar phosphates. The reverse wits seen with D. tertiolecta. Also, in the 2 diatoms and in G. tamarensis significant inhibition of photosynthesis by oxygen could only be achieved with 100% oxygen; atmospheric levels having little effect. Parallel measurements of 2 carboxylating enzymes showed that ribulose-1,5-diphosphate carboxylase (RuDPCase) was significantly greater than phospho (enol)pyruvate carboxylase (PEPCase) activity only in the green flagellate. It is suggested that photosynthesis in marine diatoms depends on an active PEPCase utilizing bicarbonate as a substrate and that a less active RuDPCase utilizes CO₂. In D. tertiolecta the pattern more closely resembles that of a “Calvin (C₃)” plant. The dinoflagellate and the chrysophyte appeared to show a mixed C₃ and C₄ photosynthesis.     

THE ECOLOGICAL IMPLICATIONS OF GROWTH FORMS IN EPIBENTHIC DIATOMS 1

This paper evaluates the utilisation of space by epibenthic diatom cells, as a response to environmental variations. The aggregation pattern of five species of epibenthic diatoms was quantified and compared to provide evidence for the significance of cell motility as an adaptive mechanism for space occupation and monopoly. The epibenthic diatoms included (1) non-mobile colonial species forming either fan-shaped (Synedra tabulata (Ag)Kz.) or arborescent (Gomphonema kamtschaticum var. californicum Grun.) colonies; (2) slow-moving (Cocconeis costata Greg, and Amphora pusio Cl.), and (3) fast-moving (Navicula direct a (W. Sm.) Ra.) non-colonial species. The aggregation pattern of S. tabulata did not vary significantly among six different light intensities manipulated in nature. The major patterns of aggregation were identified using analysis of covariance and dummy-variable regression. Highly mobile N. directa are significantly less aggregated than the four other diatom species. Non-mobile and slow-moving species show a similar, highly aggregated pattern. The occurrence of two patterns of spatial dispersion indicates that growth forms bear far-ranging ecological implications with respect to colonization strategies, immigration, and possibly impact by grazers. An integrated model of growth form characteristics, biological properties, and ecological implications is presented for epibenthic diatoms.     

UTILIZATION OF DISSOLVED ORGANIC CARBON BY NATURAL POPULATIONS OF EPIBENTHIC SALT MARSH DIATOMS1

Natural populations of pennate diatoms collected from the mudflats of a Georgia salt marsh were cleansed by differential centrifugation and tested for their ability to assimilate and respire ¹⁴C-acetate, lactate and glucose at 1 μM concentrations. A correction was made for contaminating bacterial activity. Utilization rates were measured in the light and dark and in the presence of a second substrate. Mean incorporation rates were (in pmol ·μg chl a^?1· h^?1± SE): acetate, 117 ± 37; lactate, 53.7 ± 20.4; glucose 13.0 ± 5.8. Using a carbon/chlorophyll ratio of 53.2 and a generation time of 48 h we calculate these algae could obtain up to 1% of their carbon heterotrophically if several usable substrates were each available at 1 μM concentrations.     

DISTRIBUTION OF EDAPHIC DIATOMS IN A MISSISSIPPI SALT MARSH: A CANONICAL CORRELATION ANALYSIS1

A canonical correlation analysis was performed to examine the degree to which edaphic diatom distribution in Graveline Bay Marsh, Mississippi could be related to environmental factors. It was possible to collapse the complex data base into two interpretable, orthogonal dimensions and to identify five distinct diatom communities. Diatom distribution on the marsh was primarily regulated by elevation and height of the overstory spermatophyte canopy. Although redundancy in the species data given the environmental data was relatively low (23% for the first two canonical variables), similar values were obtained for studies of benthic and planktonic diatoms in Yaquina Estuary, Oregon. The present study represents the first application of multivariate statistics to the study of salt marsh diatoms and demonstrates its usefulness in an environment where gradients are difficult to perceive.     

SIZE DEPENDENCE OF GROWTH RATE,RESPIRATORY ELECTRON TRANSPORT SYSTEM ACTIVITY,AND CHEMICAL COMPOSITION IN MARINE DIATOMS IN THE LABORATORY1

The dependence of growth, electron transport system activity and chemical composition on the size of diatoms was examined during the exponential phase of growth. The six different marine centric species compared ranged in volume from 7.7 μm³ to 62 × 10⁵μm³. A size dependence was observed for growth, ¹⁴C uptake, respiration and the productivity index (¹⁴C/chl a). Although the size dependence of all parameters was similar, the results indicate that on a carbon basis, growth efficiency decreases with increasing size. The C/N and C/chl a ratios were not size dependent. The importance of the surface area to cell volume ratio, and the importance of carbon per unit volume in determining the observed size dependence are discussed.     

AMMONIUM STIMULATION OF DARK CARBON FIXATION AS AN INDICATOR OF NITROGEN DEFICIENCY IN PHYTOPLANKTON: POTENTIAL ERRORS CAUSED BY AMMONIUM-OXIDIZING BACTERIA1

Stimulation of dark fixation of carbon by NH₄⁺ is often used as an indicator of phytoplankton N deficiency. This assay is based on the influence of available NH₄⁺ on anaplerotic CO₂ fixation by algae. However, carbon fixation by chemoautotrophic NH₄⁺-oxidizing bacteria may also be stimulated by NH₄⁺ enrichment, a process that can mask the algal response in natural communities. NH₄⁺ addition enhanced dark carbon fixation up to 300%, relative to unamended controls, in organisms collected on a 0.7-μm retention filter in oligotrophic Flathead Lake, Montana, but the effect was not detectable in the presence of nitrapyrin, an inhibitor of NH₄⁺-oxidizing bacteria. Dark carbon fixation was enhanced with addition of NH₄⁺ in organisms retained on 2-μm filters (which should allow passage of most bacteria). NH₄⁺ stimulated dark carbon fixation in N-deficient axenic cultures of Chlamydomonas reinhardtii Dang but not in N-replete cultures in both the presence and absence of nitrapyrin. Application of nitrapyrin or size fractionation treatments, to separate the processes of dark carbon fixation by nitrifiers and phytoplankton, may improve the efficacy of assays using NH₄⁺ stimulation of dark carbon fixation to specifically indicate N deficiency in natural algal communities.     

PHOTOSYNTHETIC CARBON METABOLISM IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)1

The intertidal macroalga Enteromorpha compressa showed the ability to use HCO₃^? as an exogenous inorganic carbon (Ci) source for photosynthesis. However, although the natural seawater concentration of this carbon form was saturating, additional CO₂ above ambient Ci levels doubled net photosynthetic rates. Therefore, the productivity of this alga, when submerged, is likely to be limited by Ci. When plants were exposed to air, photosynthetic rates saturated at air-levels of CO₂ during mild desiccation. Based on carbon fixing enzyme activities and Ci pulsechase incorporation patterns, it was found that Enteromorpha is a C₃ plant. However, this alga did not show O₂ inhibited photosynthetic rates at natural seawater Ci conditions. It is suggested that such a C₄- like gas exchange response is due to the HCO₃^? utilization system concentrating CO₂ intracellularly, thus alleviating apparent photorespiration.     

BENTHIC METABOLISM ON A SHELTERED ROCKY SHORE: ROLE OF THE CANOPY IN THE CARBON BUDGET1

While the importance of canopy‐forming algae in structuring ecosystems is recognized, their role in the carbon budget is still not well understood. To our knowledge, no measurements of rocky shores primary production and respiration under emersion periods have been carried out in situ. A benthic chamber coupled to a CO₂‐infrared gas analyzer was used to measure gross primary production and respiration on the Ascophyllum nodosum (L.) Le Jol. zone of a sheltered rocky shore in Brittany, France. Over a year of monthly measurements on the zone with and without the A. nodosum canopy showed fairly high production and respiration values for the global community as well as carbon fluxes due to the canopy that largely dominated the benthic metabolism of the zone. The strong canopy respiration relative to the primary production also suggested a high metabolic activity by microscopic heterotrophs on the surface of the alga. Both the canopy and the understory annual primary production and respiration were under the control of light and temperature seasonal variations. Finally, the range of the amount of carbon produced on the A. nodosum zone during diurnal emersions was estimated. Additional measures accounting for the day–night cycles and seasonal light variations over an entire tidal cycle are, however, necessary to establish an annual carbon budget. Such measures using the benthic chamber together with complementary techniques would allow a better understanding of the functioning of sheltered rocky shores.     

CHLOROPHYLL PIGMENTS IN METHANOL EXTRACTS FROM TEN AXENIC CULTURED DIATOMS AND THREE GREEN ALGAE AS DETERMINED BY REVERSE PHASE HPLC WITH FLUOROMETRIC DETECTION 1

Chlorophyll pigments in 100% mrthanol extracts from ten axenic cultured diatoms and three green algae were examined using reverse phase HPLC with Jluoromelric detector and field desorption mass spectrometry to identify same unknown peaks tn HPLC chromatograms from natural phytoplankton samples. Characteristics of the HPLC chromatograms were the presence of chlorophyllide and two unknown peaks. Field desorption mass spectrometry of the oxidation products of authentic chlorophyll a suggested that the peaks were due to 10-hydroxy and 10-hydroxy lactone chlorophylls a and 10-methoxy and 10-methoxy lactone chlorophylls a fanned during each process of filtration, storage at -20°C and extraction in methanol. The present results indicated that a new type of chlorophyll c derivative may exist and, as far as we use filtration to collect phytoplankton and store the sample under air even at -20°C, the formation of oxidation products of chlorophyll pigments may he unavoidable.     

早春短命植物鸢尾蒜和准噶尔鸢尾蒜的光合途径

该研究以鸢尾蒜属两种早春短命植物准噶尔鸢尾蒜(Ixiolirion songaricum)和鸢尾蒜(Ixiolirion tataricum)为对象,通过解剖结构、光合参数、稳定碳同位素比值(δ¹³C)和光合关键酶活性分析二者的光合途径。结果发现：(1)显微结构和超微结构显示,两种短命植物的叶脉维管束鞘细胞1层、排列较紧密,鞘细胞内含有较多叶绿体且多离心分布,类似C₄花环结构。(2)准噶尔鸢尾蒜和鸢尾蒜最大净光合速率分别为14.81和15.04 μmol·m^-2·s^-1;两者的CO₂补偿点较低,分别为3.57和2.54 μmol·mol^-1,δ¹³C分别为-25.36±0.55‰和-25.76±1.38‰,光合酶PEPC/Rubisco比值分为别0.244和0.322。(3)最大净光合速率、δ¹³C和PEPC/Rubisco比值均说明两种植物为C₃光合途径,但二者均具有类似C₄的维管束鞘结构、较低的CO₂补偿点和暗呼吸速率,并具有高于部分C₃和C₃ C₄中间型植物的PEPC/Rubisco比值,表明两种短命植物的光合途径并非典型的C₃途径,而是C₃ C₄中间型。     

RELATIONSHIPS BETWEEN LITTORAL DIATOMS AND THEIR CHEMICAL ENVIRONMENT IN NORTHEASTERN GERMAN LAKES AND RIVERS1

We explored statistical relationships between the composition of littoral diatom assemblages and 21 chemical and physical environmental variables in 69 lakes and 15 river sites in the lowland of northeastern Germany. Canonical correspondence analysis with single treatment and with forward selection of environmental variables was used to detect 11 important ecological variables (dissolved inorganic carbon [DIC], Na ⁺ , total phosphorus [TP], dissolved organic carbon [DOC], total nitrogen [TN], pH, oxygen saturation, dissolved iron, SO₄^{2 ?} , NH₄ ⁺ , soluble reactive silicium) and maximum water depth or Ca^{2 +} or soluble reactive phosphorus that most independently explain major proportions of the total diatom variance among the habitats. Monte Carlo permutation tests showed that each contributed a significant additional proportion (P < 0.05) of the variance in species composition. Together, these 11 most important environmental variables explained 34% of the total variance in species composition among the sites and captured 73% of the explained variance from the full 21 parameters model. Weighted‐averaging regression and calibration of 304 indicator taxa with tolerance down‐weighting and classic deshrinking was used to develop transfer functions between littoral diatoms and DIC, pH, TP, TN, and Cl ^? . The DOC:TP ratio was introduced and a weighted‐averaging model was developed to infer allochthonous DOC effects in freshwater ecosystems. This diatom‐DOC/TP model was significant (P < 0.001) and explained 7.6% of the total diatom variance among the sites, surpassing the inferential power of the diatom‐TP‐transfer function (7.3% explained variance). The root‐mean‐square errors of prediction of the models were estimated by jack‐knifing and were comparable with published data sets from surface sediment diatom samples. The data set of littoral diatoms and environmental variables allows use of the diatom‐environmental transfer functions in biomonitoring and paleolimnological approaches across a broad array of natural water resources (such as floodplains, flushed lakes, estuaries, shallow lakes) in the central European lowland ecoregion.     

MODULATION OF PHOTOSYNTHETIC CARBON ASSIMILATION IN SELENASTRUM CAPRICORNUTUM (CHLOROPHYCEAE) BY cAMP: AN ELECTROGENIC MECHANISM?1

Laboratory batch cultures of the unicellular alga Selenastrum capricornutum Printz (Chlorophyceae) were used as a model system to test the hypothesis that the regulatory molecule cAMP may alter algal photo synthetic carbon assimilation (PCA) rates via an electrogenic mechanism. In one series of experiments, cells were resuspended in cAMP-free media subsequently augmented with 0.5 nmol·L^?1to 0.1 μmol·L^?1of added cAMP, concentrations paralleling dissolved cAMP levels in lake water and culture filtrates but insufficient to result in short-term increases in intracellular cAMP content. cAMP-treated cells were moderately and transiently hypopolarized concomitant with a reduction in PCA capacity. Media and intracellular pH and quinacrine fluorescence quenching rates were also altered, with similar effects on PCA, but specific effects appeared to be associated with diel cycles. Higher concentrations of added cAMP (10 μmol·L^?1to 10 mmol·L^?1) further hypopolarized the transmembrane potential, greatly enhanced PCA rates, acidified the cell interior, and enhanced quinacrine quenching rates. With the exception of cGMP, no nucleotide derivative examined elicited the same effects. Inhibitors of plasmalemma ATPase and NADH oxidase activity also hypopolarized the membrane potential but inhibited PCA rates. Collectively, the data support the cAMP-electrogenic thesis, but the mechanism, although resembling features of H⁺/ ion-cotrans-port, may in fact be more complex.     

EFFECTS OF NH4+ ASSIMILATION ON DARK CARBON FIXATION AND β-1,3-GLUCAN METABOLISM IN THE MARINE DIATOM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)

The effects of NH₄⁺ assimilation on dark carbon fixation and β-1,3-glucan metabolism in the N-limited marine diatom Skeletonema costatum (Grev.) Cleve (Bacillariophyceae) were investigated by chemical analysis of cell components and incorporation of ¹⁴C-bicarbonate. The diatom was grown in pH-regulated batch cultures with a 14:10 h LD cycle until N depletion. The cells were then incubated in the dark with ¹⁴C-bicarbonate, but without a source of N for 2 h, then in the dark with 63 μmol·L⁻¹ NH₄⁺ for 3 h. Without N, the cellular concentration of free amino acids was almost constant (∼4.5 fmol·cell⁻¹). Added NH₄⁺ was assimilated at a rate of 12 fmol·cell⁻¹·h⁻¹, and the cellular amino acid pool increased rapidly (doubled in <1 h, tripled in <3 h). The glutamine level increased steeply (45× within 3 h), and the Gln/ Glu ratio increased from 0.1 to 2.4 within 3 h. The rate of dark C fixation during N depletion was only 1.0 fmol·cell⁻¹·h⁻¹. The addition of NH₄⁺ strongly stimulated dark C fixation, leading to an assimilation rate of 4.0 fmol·cell⁻¹·h⁻¹, corresponding to a molar C/N uptake ratio of 0.33. Biochemical fractionation of organic ¹⁴C showed no significant ¹⁴C fixation into amino acids during N depletion, but during the first 1–2 h of NH₄⁺ assimilation, amino acids were rapidly radiolabeled, accounting for virtually all net ¹⁴C fixation. These results indicate that anaplerotic β-carboxylation is activated during NH₄⁺ assimilation to provide C₄ intermediates for amino acid biosynthesis. The level of cellular β-1,3-d-glucan was constant (16.5 pg·cell⁻¹) during N depletion, but NH₄⁺ assimilation activated a mobilization of 28% of the reserve glucan within 3 h. The results indicate that β-1,3-glucan in diatoms is the ultimate substrate for β-carboxylation, providing precursors for amino acid biosynthesis in addition to energy from respiration.     

IMPACT OF ULTRAVIOLET-B RADIATION ON PHOTOSYSTEM II ACTIVITY AND ITS RELATIONSHIP TO THE INHIBITION OF CARBON FIXATION RATES FOR ANTARCTIC ICE ALGAE COMMUNITIES1

One goal of the Icecolors 1993 study was to determine whether or not photosystem II (PSII) was a major target site for photoinhibition by ultraviolet-B radiation (Q_UVB, 280–320 nm) in natural communities. Second, the degree to which Q_UVBinhibition of PSII could account for Q_UVBeffects on whole cell rates of carbon fixation in phytoplankton was assessed. On 1 October, 1993, at Palmer Station (Antarctica), dense samples of a frazil ice algal community were collected and maintained outdoors in the presence or a bsence of Q_UVBand l or ultraviolet-A (Q_UVB, 320–400 nm) radiation. Samples were then collected at intervals over the day to track the time course of UV inhibition of primary production. The ice algae were assessed for changes in pigment composition and rates of carbon fixation. The quantum yield of PSII (Ø_IIc°) was measured by P ulse A mplitude M odulated fluorometry. Over the day, Ø_IIc° declined due to increasing time-integrated dose exposure of Q_UVB. The Q_UVB-driven inhibition of Ø_IIc° increased from 4% in the early morning hours to a maximum of 23% at the end of the day. The Q_UVB photoinhibition of PSII quantum yield did not recover by 6 h after sunset. In contrast, photoinhibition by Q_UVA and photosynthetically available radiation (Q_PAR, 400–700 nm) recovered during the late afternoon. Flourescence-based estimates of carbon fixation rates were linearly correlated (P =0.002, r²=0.45) with measured carbon fixation. Fluorescence overestimated the observed Q_UVB inhibition in measured carbon fixation rates by 8% in the morning hours; however, the discrepancy increased during the afternoon. Therefore, researchers should be cautious in using fluorescence measurements to infer ultraviolet inhibition for rates of carbon fixation until there is a greater understanding of the coupling of carbon metabolism to PSII activity for natural populations. Despite these current limitations, fluorescence-based technologies represent powerful tools for studying the impact of the ozone hole on natural populations on spatial/ temporal scales not possible using conventional productivity techniques.     

BIOCHEMICAL TAXONOMY OF MARINE PHYTOPLANKTON BY ELECTROPHORESIS OF ENZYMES. II. LOSS OF HETEROZYGOSITY IN CLONAL CULTURES OF THE CENTRIC DIATOMS SKELETONEMA COSTATUM AND THALASSIOSIRA PSEUDONANA1, 2

An electrophoretic survey of 12 new isolates of Thalassiosira pseudonana Hasle & Heimdal and 25 new isolates of Skeletonema costatum (Grev.) Cleve revealed several heterozygote genotypes at malate dehydrogenase (MDH) and phosphohexose isomerase (PHI) loci. The new clones were maintained in culture for 6 mo and then reasayed at these two loci. All MDH heterozygotes and halj of the PHI heterozygotes had become homozygous. This resulted in a collection of clones that are largely homozygous but that are samples of polymorphic species. The physlogical implications of this loss of heterozygosity in clonal cultures has not been analyzed. Hawever, any change in a clone that is the result of culturing conditions reduces the usefulness of that clone as a laboratory test organism for ecological correlations.     

DNA:ATP RATIOS IN MARINE MICROALGAE AND BACTERIA: IMPLICATIONS FOR GROWTH RATE ESTIMATES BASED ON RATES OF DNA SYNTHESIS1

DNA: ATP and carbon: DNA (C:DNA) ratios were measured in a total of 14 species of marine microalgae and bacteria. Comparison of several DNA assay methods with results obtained with cultures uniformly labeled with ³³P indicated that by far the most accurate results were obtained using diaminobenzoic acid (DABA) or diphen-ylamine, with DABA having the highest precision. Both the Hoechst and DAPI methods seriously underestimated DNA concentrations in algal cultures. Average DNA: ATP ratios in the algal and bacterial cultures were I7 and 34 by weight, respectively, with almost all values lying in the range of 10–40. DNA: ATP ratios in the microalgae showed no correlation with growth conditions but varied by about a factor of 3 among species. C:DNA ratios for individual species of microalgae and bacteria ranged from 21 to 155 by weight and averaged 50 for the microalgae and bacteria taken together. Growth rates of microalgal species grown in cyclostats were estimated to within 8% of dilution rates when calculated from the uptake of ³H-adenine and the DNA: ATP ratio of the species. Use of the ³H-adenine method for estimating microalgal growth rates in the field may thus be a useful tool for investigating the physiology of microalgae in nature.     

CARBON UPTAKE IN A MARINE DIATOM DURING ACUTE EXPOSURE TO ULTRAVIOLET B RADIATION: RELATIVE IMPORTANCE OF DAMAGE AND REPAIR1

Experiments on a marine diatom, Thalassiosira pseudonana (Hustedt) clone 3H, demonstrate that under moderate photon flux densities (75 μmol quanta·m^?2·s^?1) of visible light the inhibition of photosynthesis by supplemental ultraviolet (UV) radiation (UV-B: 280–320 nm) is well described as a hyperbolic function of UV-B irradiance for time scales of 0.5–4 h. Results are consistent with predictions of a recently developed model of photosynthesis under the influence of UV and visible irradiance. Although net destruction of chlorophyll occurs during a 4-h exposure to UV-B, and the effect is a function of exposure, the principal effect of UV-B is a decrease in chlorophyll-specific photosynthetic rate. The dependence of photoinhibition on dosage rate, rather than cumulative dose, and the hyperbolic shape of the relationship are consistent with net photoinhibition being an equilibrium between damage and repair. The ratio of damage to repair is estimated by a mathematical analysis of the inhibition of photosynthesis during exposures to UV-B. A nitrate-limited culture was much more sensitive to UV-B than were the nutrient-replete cultures, but the kinetics of photoinhibition were similar. The analysis suggests that the nutrient-limited culture was more sensitive than the nutrient-replete cultures because repair or turnover of critical proteins associated with photosynthesis is inhibited. An inhibitor of chloroplast protein synthesis was used to suppress repair processes. Photoinhibition by UV-B was enhanced, and inhibition was a function of cumulative dose, as would be expected if damage were not countered by repair. The fundamental importance of repair processes should be considered in the design of field experiments and models of UV-B effects in the environment, especially in the context of vertical mixing. Repair processes must also be considered whenever biological weighting functions are developed.     

PHOTOSYNTHETIC CAPACITY AND LUXURY UPTAKE OF CARBON DURING PHOSPHATE LIMITATION IN PEDIASTRUM DUPLEX (CHLOROPHYCEAE)1

When cells of Pediastrum duplex Meyen experience phosphorus depletion, their capacity for carbon fixation declines, but sizes and carbon contents of the cells increase several-fold, an apparent instance of “luxury uptake” of carbon. Maximum rates of uptake of phosphate increase during the same period, and are consequently correlated with the enlarged surface area of the cells. Thus the disadvantage of increased cell volume, which may accelerate sinking speed in nature, is offset by the increased capacity of the cells for nutrient uptake.