LISTENING TO PHYTOPLANKTON: MEASURING BIOMASS AND PHOTOSYNTHESIS BY PHOTOACOUSTICS

A simple technique based on photoacoustic measurements allowed us to determine the biomass, as well as the efficiency of photosynthesis, for different taxa of phytoplankton in situ. The method yields rapid, direct results compared to tedious standard measurements of ¹⁴C fixation and oxygen evolution or compared to indirect results from measurement of fluorescence. The procedure has a high potential for following the effects of environmental parameters such as irradiance, nutrient status, and pollution on phytoplankton communities and their photosynthetic activity.     

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.     

A STUDY OF IN VITRO ELECTRON TRANSPORT ACTIVITY IN MARINE PHYTOPLANKTON AS A FUNCTION OF TEMPERATURE1

The in vitro temperature inactivation of the electron transport system (ETS) activity was measured for 11 phytoplankton species. The average inactivation temperature is between 18–23 C, but exceptions with inactivation temperatures as high as 43 C were noted for a tide pool phytoplankton, Dunaliella tertiolecta Butcher. Despite differences in temperatures at which the ETS activity begins to decrease, experimental values for the energies of activation (E) of electron transport are very similar and average ca. 12 kcal/mol. The results show the necessity of applying temperature corrections to in vitro ETS activity measurements when oceanic in situ oxygen consumption is computed from ETS activity measurements. The progress of thermal inactivation of the ETS activity in phytoplankton species studied yields biphasic curves. The biphasic nature of the curves is expressed when using both relatively high and low temperature of inactivation and in more or less heat sensitive species. These curves are described in terms of microsomal and mitochondrial substrate dependence. Based on the obtained temperature response of the ETS activity, the adaption of phytoplankton species to growth temperature is discussed.     

A CHLOROPHYLL FLUORESCENCE INDEX TO ESTIMATE SHORT‐TERM RATES OF PHOTOSYNTHESIS BY INTERTIDAL MICROPHYTOBENTHOS1

An index based on chl a fluorescence quenching analysis was tested as a predictor of photosynthetic rates of undisturbed intertidal microphytobenthic assemblages. The fluorescence index, P_fluo, was derived from the combination of different chl a fluorescence parameters chosen to represent the two main sources of short‐term variability in the community‐level microphytobenthic photosynthesis: 1) the quantum yield of photosynthesis of the microalgae present in the photic zone of the sediment, φP, and 2) the community‐level efficiency of photosynthetic light absorption, ?, determined by the microalgal concentration in the photic zone. Variations in φP were traced by the fluorescence index ΔF/F_m′ (the effective quantum yield of charge separation at PSII), whereas changes in ? were followed by the fluorescence parameter F_o (dark or minimum fluorescence level). Gross photosynthetic rate, P, and fluorescence parameters were measured nondestructively and simultaneously under in situ conditions, on the same samples, using oxygen microelectrodes and pulse amplitude modulation fluorometry, respectively. Despite the large and uncorrelated hourly variability in irradiance, photosynthetic rate, and fluorescence parameters included in P_fluo, highly significant correlations between P_fluo and P were found for all the sampling periods, encompassing hourly, biweekly, and seasonal time scales. The variability in P explained by P_fluo ranged from 84.3% to 91.4% when sampling periods were considered separately and reached 81.1% when all data were pooled. The results of the study show that despite its simplicity, the index P_fluo can be used to trace short‐term variations in the photosynthetic rate of undisturbed microphytobenthic assemblages undergoing rhythmic vertical migration.     

INORGANIC CARBON REPLETION CONSTRAINS STEADY‐STATE LIGHT ACCLIMATION IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS1

Cyanobacteria show high metabolic plasticity by re‐allocating macromolecular resources in response to variations in both environmental inorganic carbon (Ci) and light. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Nägeli across a 50‐fold range of growth irradiance at either a dissolved [Ci] <0.1 mM, sufficient to induce strongly the carbon‐concentrating mechanism (CCM) or a dissolved [Ci] of ～4 mM, sufficient to strongly induce the CCM to basal constitutive activity. There was no detectable growth cost of acclimation to low Ci across the entire range of irradiance and growth was nearly light saturated at 50 l mol photons·m^?2·s^?1. Cells acclimated to low Ci significantly re‐allocated macromolecular resources to support their CCM, while maintaining near homeostatis of metabolic flux per unit photosynthetic complex. Changing growth irradiance also drove re‐organization of the photosynthetic machinery to balance excitation flux and metabolic demands, but flux per complex varied widely across the range of tolerable growth irradiances. Across the range of growth irradiance, low Ci cells had significantly less phycocyanin than high Ci cells, which corresponded to a lower PSII absorbance capacity. Furthermore, low Ci cells maintained more PSI per cell^?1 than high Ci cells under high growth irradiance. Low Ci cells could therefore maintain more of their PSII reaction centers open at high growth irradiance than could high Ci cells, which experienced a significant PSII closure. Thus, acclimation to growth under high available Ci actually constrained acclimation to high light by restricting electron transport downstream from PSII in S. elongatus.     

PHOTOINHIBITION OF TEMPERATE LAKE PHYTOPLANKTON BY NEAR-SURFACE IRRADIANCE: EVIDENCE FROM VERTICAL PROFILES AND FIELD EXPERIMENTS1

To evaluate the in situ occurrence of phytoplankton photoinhibition, the light-mediated depression of chlorophyll in vivo fluorescence (IVF) and of the cellular fluorescence capacity (CFC) of phytoplankton was determined in three southeastern United States reservoirs. Vertical profiles of a fluorescence depression index (FDI) and of the CFC for reservoir phytoplankton showed that near-surface photoinhibition of fluorescence properties occurred in association with high surface irradiance and weak vertical mixing of the water column. To characterize the time scales of photochemical and photosynthetic responses to and recovery from exposure to supraoptimal light intensity, phytoplankton IVF responses and ¹⁴C-fixation rates were measured infield experiments. Phytoplankton chlorophyll IVF, CFC, and photosynthetic ¹⁴C fixation were rapidly (20–40 min) depressed when reservoir phytoplankton were exposed to surface irradiances (1700–2000 μE·m^?2·s^?1). Light-mediated increases in the FDI declined rapidly (20–40 min) to pre-exposure levels during a subsequent low-light (75–200 μE·m^?2·s^?1) period, but CFC and ¹⁴C fixation recovered more slowly (>40 min). Exposure of reservoir phytoplankton to a light-intensity gradient revealed both intensity and time thresholds for IVF and CFC depression. Phytoplankton photochemical responses to bright light operate on time scales that, in conjunction with vertical mixing, should limit the occurrence of photoinhibition to extreme irradiance environments. Our results support the hypothesis that the photoinhibition of phytoplankton productivity occurs less commonly than is indicated by fixed-depth incubation measurements.     

PHYTOPLANKTON PLASMA MEMBRANE REDOX ACTIVITY: EFFECT OF IRON LIMITATION AND INTERACTION WITH PHOTOSYNTHESIS1

Phytoplankton plasma membrane electron transport activity was determined by monitoring the reduction of the impermeant artificial electron acceptor ferricyanide in a range of diatoms. The results revealed that constitutive plasma membrane electron transport activity of marine diatoms is high compared with chlorophytes and higher plant cells. Diatom plasma membrane electron transport activity was not significantly increased by iron limitation. This lack of induction on iron limitation indicates that diatoms have an iron acquisition strategy that is distinct from chlorophytes and the dicotyledon higher plants that exhibit marked increases in plasma membrane ferricyanide reductase activity on iron limitation. The interaction of the constitutive plasma membrane electron transport with photosynthesis was also investigated. We found that 1) ferricyanide reduction at the plasma membrane was progressively inhibited in response to increasing irradiances; 2) the presence of extracellular ferricyanide, but not the reduced couple ferrocyanide, caused a marked inhibition of carbon fixation at high irradiance; and 3) extracellular electron acceptors ferricyanide and hexachloroiridate (but not ferrocyanide) induced an immediate and reversible decrease in fluorescence yields (F_o and F_m). The extent to which extracellular electron acceptors affected CO₂ fixation, F_o, and F_m was related to the level of constitutive ferricyanide reductase activity, the species with highest ferricyanide reduction rates being most sensitive. The data suggest that consumption of electrons and/or reductant at the plasma membrane by external acceptors may compete directly with CO₂ fixation for electrons, alter cytosolic‐chloroplast redox poise, and/or induce a redox‐signaling cascade that alters photosynthetic metabolism.     

USING A NONANALYTICAL APPROACH TO MODEL NONLINEAR DYNAMICS IN PHOTOSYNTHESIS AT THE PHOTOSYSTEM LEVEL1

Nonlinear dynamics in photon capture and uptake at the photosystem level may have a strong effect on photosynthetic yield. However, the magnitude of such effects is difficult to estimate theoretically because nonlinear systems often cannot be represented accurately using equations. A nonanalytical simulation was developed that used a simple decision tree and Monte Carlo methods, instead of equations, to model how a population of photosystems absorbs and utilizes photons from an ambient light field. This simulation replicated realistic kinetics in the closure and variable fluorescence yield of PSII on the single‐turnover timescale, as well as the saturating behavior in light‐driven electron flow that is observed in nature with increasing irradiance. This simulation indicated that the transfer of absorbed photon energy among PSII units can introduce strong nonlinear enhancement in light‐driven electron flow. However, this effect was seen only in populations with particular photosynthetic states as determined by physiological properties of PSII. Other populations with the same degree of energy transfer but with different photosynthetic states exhibited little enhancement in electron flow and, in some cases, a reduction. This nonanalytical approach provides a simple means to quantify theoretically how nonlinear dynamics in photosynthesis arise at the photosystem level and how these dynamics may act to enhance or constrain photosynthetic rates and yields. Such simulations can provide quantitative insight into different physiological bases of nonlinear light‐harvesting dynamics and identify those that would have the strongest theoretical influence and thus warrant closer experimental examination.     

PHOSPHORUS LIMITATION AND CARBON METABOLISM IN A UNICELLULAR ALGA: INTERACTION BETWEEN GROWTH RATE AND THE MEASUREMENT OF NET AND GROSS PHOTOSYNTHESIS1

Chlamydomonas reinhardii Dangeard was grown in continuous culture under P limitation at a range of dilution rates. Carbon uptake measurements were performed using double isotope (¹²C/¹⁴C) techniques and the fluxes of carbon in the light and dark were analysed over the range of growth rates. ¹⁴C uptake was shown to be equal to gross photosynthesis only at maximum relative growth rates; at low relative growth rates ¹⁴C uptake approximated net photosynthesis. The altered pattern of C uptake was found to be due to the suppression of dark respiration in the light and the release of ¹⁴C0₂ from respiratory pathways at low relative growth rates. Metabolic channelling of ¹⁴C from photosynthetic pathways to respiratory pathways occurred at low growth rates as the specific activity of the respired CO₂ reached 45% of the input gas mixture. These data are discussed in the light of the controversy concerning the measurement of gross and net photosynthesis in natural populations and in the light of models of ¹⁴C uptake in single celled algae. Existing models are shown to be adequate for high relative growth rates but not for low relative growth rates under P limitation.     

IN SEARCH OF A PHYSIOLOGICAL BASIS FOR COVARIATIONS IN LIGHT‐LIMITED AND LIGHT‐SATURATED PHOTOSYNTHESIS1

The photosynthesis‐irradiance (PE) relationship links indices of phytoplankton biomass (e.g. chl) to rates of primary production. The PE curve can be characterized by two variables: the light‐limited slope (α^b) and the light‐saturated rate (P^b_max) of photosynthesis. Variability in PE curves can be separated into two categories: that associated with changes in the light saturation index, E_k (=P^b_max/α^b) and that associated with parallel changes in α^band P^b_max (i.e. no change in E_k). The former group we refer to as “E_k‐dependent” variability, and it results predominantly from photoacclimation (i.e. physiological adjustments in response to changing light). The latter group we refer to as “E_k‐independent” variability, and its physiological basis is unknown. Here, we provide the first review of the sporadic field and laboratory reports of E_k‐independent variability, and then from a stepwise analysis of potential mechanisms we propose that this important yet largely neglected phenomenon results from growth rate–dependent variability in the metabolic processing of photosynthetically generated reductants (and generally not from changes in the oxygen‐evolving PSII complexes). Specifically, we suggest that as growth rates decrease (e.g. due to nutrient stress), reductants are increasingly used for simple ATP generation through a fast (<1s) respiratory pathway that skips the carbon reduction cycle altogether and is undetected by standard PE methodologies. The proposed mechanism is consistent with the field and laboratory data and involves a simple new “twist” on established metabolic pathways. Our conclusions emphasize that simple reductants, not reduced carbon compounds, are the central currency of photoautotrophs.     

MORPHO‐FUNCTIONAL PATTERNS OF PHOTOSYNTHESIS IN THE SOUTH PACIFIC KELP LESSONIA NIGRESCENS: EFFECTS OF UV RADIATION ON 14C FIXATION AND PRIMARY PHOTOCHEMICAL REACTIONS1

The morpho‐functional patterns of photosynthesis, measured as ¹⁴C‐fixation and chl fluorescence of PSII, also as affected by different doses of UV radiation in the laboratory were examined in the South Pacific kelp Lessonia nigrescens Bory of the coast of Valdivia, Chile (40°S). The results indicated the existence of longitudinal thallus profiles in physiological performance. In general, blades exhibited higher rates of carbon fixation and pigmentation as compared with stipes and holdfasts. Light‐independent ¹⁴C fixation (LICF) was high in meristematic zones of the blades (3.5 μmol ¹⁴C·g^?1 fresh weight [FW]·h^?1), representing 2%–16% (percentage ratio) of the photosynthetic ¹⁴C fixation (20 μmol ¹⁴C·g^?1 FW·h^?1). Exposures to UV radiation indicated that biologically effective UV‐B doses (BED_{photoinhibition300}) of 200–400 kJ·m^?2 (corresponding to current daily doses measured in Valdivia on cloudless summer days) inhibit photosynthetic ¹⁴C fixation of blades by 90%, while LICF was reduced by 70%. The percentage ratio of LICF to photosynthetic ¹⁴C fixation increased under UV exposure to 45%. Primary light reactions measured as maximum quantum yield (F_v/F_m) and electron transport rate (ETR) indicated a higher UV susceptibility of blades as compared with stipes and holdfasts: after a 48 h exposure to UV‐B, the decrease in the blades was close to 30%, while in the stipes and holdfasts it was <20%. The existence of translocation of labeled carbon along the blades suggests that growth at the meristem may be powered by nonphotosynthetic processes. A possible functional role of LIFC, such as during reduction of photosynthetic carbon fixation due to enhanced UV radiation, is discussed. These results in general support the idea that the UV‐related responses in Lessonia are integrated in the suite of morpho‐functional adaptations of the alga.     

PHOTOSYNTHESIS AND COLD ACCLIMATION: MOLECULAR EVIDENCE FROM A POLAR DIATOM1

The psychrophilic diatom Fragilariopsis cylindrus (Grunow) Krieger in Helmcke & Krieger was used to investigate photosynthesis and growth under freezing temperatures. Gene expression during a temperature shift from +5° C to ?1.8° C was studied under 3 and 35 μmol photons·m^?2·s^?1 by using a macroarray. These measurements were paralleled by determination of fluorescence induction at PSII and pigment analysis. The shift to ?1.8° C at 35 μmol photons·m^?2·s^?1 caused a marginal decrease of photosynthetic quantum yield (F_v/F_m) from 0.61 to 0.52 with fast recovery after 1 day. The ratio of chl c to chl a increased from 3.1 to 5.5, and the ratio of diatoxanthin to diadinoxanthin increased from 0.7 to 5.0. Genes encoding proteins of PSII (psbA, psbC) and for carbon fixation (rbcL) were down‐regulated, whereas genes encoding chaperons (hsp70) and genes for plastid protein synthesis and turnover (elongation factor EfTs, ribosomal protein rpS4, ftsH protease) were up‐regulated. In contrast, cold exposure at 3 μmol photons·m^?2·s^?1 induced a marginal increase in F_v/F_m from 0.61 to 0.63 and a strong increase in fucoxanthin concentrations from 0.04 up to 0.12 pg·cell^?1. This was paralleled by up‐regulation of fcp genes. The ratio of chl c to chl a also increased from 3.1 to 4.2, as did the ratio of diatoxanthin to diadinoxanthin from 0.7 to 2.2. Down‐regulation of psbA, psbC, and rbcL could also be measured but not up‐regulation of hsp70, EfTs, rpS4, and the ftsH protease. The latter genes are probably necessary to avoid cold shock photoinhibition only at higher light intensities.     

RELATIONSHIP OF GROWTH AND PHOTOSYNTHESIS WITH COLONY SIZE IN AN EDIBLE CYANOBACTERIUM,GE‐XIAN‐MI NOSTOC (CYANOPHYCEAE)1

Growth and photosynthesis of an edible cyanobacterium, Ge‐Xian‐Mi (Nostoc), were investigated with differently sized colonies. Both photosynthesis and growth were dependent on the colony size. Compared with larger ones, smaller colonies grew faster regardless of the levels of light and temperature for culture and showed higher values of maximal net photosynthetic rate, apparent quantum yield, light‐saturating and compensating points, and dark respiration. The ratios of chl a content and mass to surface area of a colony increased and that of chl a to mass or mass to volume of a colony decreased with increased colonial sizes. A Ge‐Xian‐Mi colony appeared to increase its chl a content per surface area, enhancing the light‐shading effect; however, at the same time it decreased its mass density on a volume basis, minimizing the enhanced effects of shading and diffusion barrier caused by the thickening outer layer with increasing colony size during growth.     

杉木人工林单叶至冠层光合作用的扩展与模拟研究

根据野外条件下杉木（Ｃｕｎｎｉｎｇｈａｍｉａ ｌａｎｃｅｏｌａｔａ（Ｌａｍｂ．）Ｈｏｏｋ．）针叶光合作用的测定结果。考虑到光合作用对光的非线性响应特性,及其与叶所处的实际冠层环境变量和冠层的空气动力学特性的相关,以筒化的林冠辐射传输模型为基础,结合不同部位和年龄针叶的光响应曲线,实现了叶室测量结果向冠层环境的调整,并进行了冠层光合作用模拟的初步研究。经过调整后的冠层光合作用平均比未经过调整的值高１     

RAPID AMMONIUM‐ AND NITRATE‐INDUCED PERTURBATIONS TO CHL a FLUORESCENCE IN NITROGEN‐STRESSED DUNALIELLA TERTIOLECTA (CHLOROPHYTA)1

When NH₄ ⁺ or NO₃ ^? was supplied to NO₃ ^? ‐stressed cells of the microalga Dunaliella tertiolecta Butcher, immediate transient changes in chl a fluorescence were observed over several minutes that were not seen in N‐replete cells. These changes were predominantly due to nonphotochemical fluorescence quenching. Fluorescence changes were accompanied by changes in photosynthetic oxygen evolution, indicating interactions between photosynthesis and N assimilation. The magnitude of the fluorescence change showed a Michaelis‐Menten relationship with half‐saturation concentration of 0.5 μM for NO₃ ^? and 10 μM for NH₄ ⁺ . Changes in fluorescence responses were characterized in D. tertiolecta both over 5 days of N starvation and in cells cultured at a range of NO₃ ^? ‐limited growth rates. Variation in responses was more marked in starved than in limited cells. During N starvation, the timing and onset of the fluorescence responses were different for NO₃ ^? versus NH₄ ⁺ and were correlated with changes in maximum N uptake rate during N starvation. In severely N‐starved cells, the major fluorescence response to NO₃ ^? disappeared, whereas the response to NH₄ ⁺ persisted. N‐starved cells previously grown with NH₄ ⁺ alone showed fluorescence responses with NH₄ ⁺ but not NO₃ ^? additions. The distinct responses to NO₃ ^? and NH₄ ⁺ may be due to the differences between regulation of the uptake mechanisms for the two N sources during N starvation. This method offers potential for assessing the importance of NO₃ ^? or NH₄ ⁺ as an N source to phytoplankton populations and as a diagnostic tool for N limitation.     

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.     

THE ACCLIMATED RESPONSE OF GROWTH,PHOTOSYNTHESIS, COMPOSITION,AND CARBON BALANCE TO TEMPERATURE IN THE PSYCHROPHILIC ICE DIATOM NITZSCHIA SERIATA1

Nitzschia seriata Cleve, a common member of marine bottom ice communities in the Arctic, was grown in unialgal batch cultures to test for compensatory mechanisms for the low temperatures (?1.8° C) typical of its natural habitat. The upper lethal limit for growth was between 12° and 15°C, and the optimum was between 6° and 12° C. The Arrhenius function adequately (R²= 73%) fitted the relationship between growth rate and temperature from – 1.6° up to 10° C, with an average Q₁₀ of 1.9 over the entire range. Light-saturated and light-limited rates of photosynthesis (normalized to chlorophyll a or cell carbon) showed complete compensation from 12° to 4° C. Photosynthetic rates, especially at light saturation, declined rapidly at temperatures below 4° C. Susceptibility to photoinhibition was greatest at the lowest growth temperatures. Cellular composition (chlorophyll a, protein, polysaccharide, and lipid contents) was not systematically related to temperature in any simple way, although cell size (carbon per cell) was maximal at the lowest growth temperature. Dark respiration was unmeasurably low (<0.015 day^?1) at all growth temperatures. The strategy of adaptation in N. seriata may be characterized as optimizing efficiency and compensation, rather than maximization, of growth rate.     

INTERACTIONS BETWEEN UV‐B EXPOSURE AND PHOSPHORUS NUTRITION. I. EFFECTS ON GROWTH,PHOSPHATE UPTAKE,AND CHLOROPHYLL FLUORESCENCE1

The interactive effects of P starvation and exposure to UV radiation on growth rates, quantum efficiency of PSII electron transport, and P‐uptake capacity of the chlorophyte microalga Dunaliella tertiolecta Butcher are presented. Ultraviolet radiation did not in itself cause marked changes in growth rate, though it did induce changes in the effective quantum yield of PSII. Depriving cells of phosphate resulted in significant changes in all parameters examined. The decline of growth rate and fluorescence parameters after P starvation was significantly faster in the presence of UV radiation. Ultraviolet radiation also stimulated the magnitude of the transient changes in chl fluorescence (nutrient‐induced fluorescence transient) exhibited by P‐starved cells after resupply of that nutrient.     

CELL DIVISION RATES OF EUCARYOTIC ALGAE MEASURED BY TRITIATED THYMIDINE INCORPORATION INTO DNA: COINCIDENT MEASUREMENTS OF PHOTOSYNTHESIS AND CELL DIVISION OF INDIVIDUAL SPECIES OF PHYTOPLANKTON ISOLATED FROM NATURAL POPULATIONS1

The cell cycle marker event of DNA replication in eucaryotic algae was identified using ³H-Thymidine (³H-TdR) incorporation. The frequency of cells (F) within a population undergoing DNA replication was estimated and the cell division rate (μF) calculated. In laboratory cultures the rates of cell division calculated from changes in cell numbers (μN) and μF were similar. Dual labelling with ³H-TdR and NaH¹⁴CO₃ enabled rates of cell division and photosynthesis to be coincidently measured for individual species of algae. Using these single species radioisotope techniques, several distinct photosynthesis irradiance and cell division irradiance relationships were found for: (1) different species of phytoplankton isolated from the same sample, and (2) the same species isolated from different environments. These techniques allow the coupling between photosynthesis and cell division to be examined with high resolution for algae in situ.     

DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2‐FIXATION RATE,BIOMASS, AND C:N:P STOICHIOMETRY1

Trichodesmium N₂ fixation has been studied for decades in situ and, recently, in controlled laboratory conditions; yet N₂‐fixation rate estimates still vary widely. This variance has made it difficult to accurately estimate the input of new nitrogen (N) by Trichodesmium to the oligotrophic gyres of the world ocean. Field and culture studies demonstrate that trace metal limitation, phosphate availability, the preferential uptake of combined N, light intensity, and temperature may all affect N₂ fixation, but the interactions between growth rate and N₂ fixation have not been well characterized in this marine diazotroph. To determine the effects of growth rate on N₂ fixation, we established phosphorus (P)–limited continuous cultures of Trichodesmium, which we maintained at nine steady‐state growth rates ranging from 0.27 to 0.67 d^?1. As growth rate increased, biomass (measured as particulate N) decreased, and N₂‐fixation rate increased linearly. The carbon to nitrogen ratio (C:N) varied from 5.5 to 6.2, with a mean of 5.8 ± 0.2 (mean ± SD, N = 9), and decreased significantly with growth rate. The N:P ratio varied from 23.4 to 45.9, with a mean of 30.5 ± 6.6 (mean ± SD, N = 9), and remained relatively constant over the range of growth rates studied. Relative constancy of C:N:P ratios suggests a tight coupling between the uptake of these three macronutrients and steady‐state growth across the range of growth rates. Our work demonstrates that growth rate must be considered when planning studies of the effects of environmental factors on N₂ fixation and when modeling the impact of Trichodesmium as a source of new N to oligotrophic regions of the ocean.