The large temporal and spatial variability in carbon isotope fractionation of marine phytoplankton (E,,) is thought to reflect differences in environmental conditions. Meaningful interpretation of this variab&ty requires an understanding of the processes responsible for phytoplankton isotope fractionation. While numerous factors have been suggested to potent~ally influence %, recent theoretical and experimental evidence has emphasized the primary role of phytoplankton growth rate ( p ) and CO2 concentration ([CO2aqj) in controlling E,,. Experimental examination of the relationship of E, with p and [C02aq] in studies using different experimental approaches, however, has ylelded inconsistent results. Here we directly compare new and previously published data on c, as a function of CO2 concentration and growth rate for the marine diatom Phaeodactylum tncornutum. When grown under nitrogen-deficient conditions (nitrate-limited chemostat), E,, of P. tn'cornutum decreases with increasing growth rate. In contrast, under N-replete conditions E, values are considerably lower at comparable growth rates and CO2 concentrations and are largely insensitive to a 3-fold increase in growth rate due to increasing photon flux density. In both experimental approaches, E, shows a relatively small CO2 sensitivity in the range of CO2 concentrations naturally occurring in the ocean (8 to 25 p 0 1 kg-'). Below ca 5 pm01 CO2 kg-', a strong decline in E,, with decreasing [C02aq] is observed. The apparent difference in E, responses between nitrate-limited and light-controlled cultures of P. tricornutum suggests a principal difference in carbon acquisition for different growth-rate-limiting resources. A mechanistic explanation is proposed and potential implications for the interpretation of phytoplankton carbon isotope fractionation are discussed.

SUMMARYPhotoacclimation of Scenedesmus protuberans F'ritsch and Microcystis aeruginosa Kutzing emend. Elenkin to high and fluctuating PPFD was studied in continuous cultures with computer-controlled variable light regimes. The aim of the work was to provide a better understanding of species-specific acclimation to high PPFD (as encountered by cyanobacteria in surface waterblooms), and of suppression of the growth of colony-forming cyanobacteria during periods of prolonged mixing in lakes. The dynamics of a set of variables was followed during the light period, including pigment composition, maximum rate, efficiency and minimum quantum requirement of photosynthesis, PS 11 cross-sections, and fluorescence variables. Both the green alga and the cyanobacterium displayed strong photo-inhibition of photosynthesis in the sinusoidal light regime, which simulated a natural light regime in the absence of mixing. P,,,,,^, a, QR and the ratio of variable to maximum fluorescence declined, and the number of inactive PS II centres and PS II,, centres increased towards midday. Introduction of oscillations in the diurnal light regime, simulating different intensities of wind-induced mixing in lakes, mitigated photo-inhibition. Microcystis showed a prompt non-photochemical quenching of fluorescence in all light regimes, even at low to moderate PPFD. The sustained presence of zeaxanthin in Microcystis possibly induced instant, thermal dissipation of excitation energy from the antenna. Microcyslis also exhibited a more reluctant acclimation to fluctuating PPFD. Growth rate of Scenedesmus was higher in all light regimes. This implied that if (known) differences in loss processes were ignored, Scenedesmus would outcompete Microcystis in lakes. Tbe results underlined the importance of buoyancy regulation in increasing the daily light dose of cyanobacteria (but at the same time preventing over-excitation), and ultimately in tbe success in Microcystis in stable lakes.

The availability of dissolved nutrients such as nitrate under extreme low temperatures is a strong determinant in the development and growth of ice diatoms. Consequently we investigated regulation of photosynthesis in a mixed culture of three diatom species, which grew in chemostats at À1 C, 15 mmol photons m À2 s À1 under N-limitation. When nitrogen is limiting, pigment-protein complexes are one of the most affected structures under low-light conditions. The loss of integral polar thylakoid components destabilized the bilayer structure of the membrane with consequences for lipid composition and the degree of fatty acid desaturation. N-Limitation caused a decrease in monogalactosydiacylglycerol (MGDG) and a simultaneous increase in bilayer forming digalactosyldiacylglycerol (DGDG). Their ratio MGDG:DGDG decreased from 3.4AE 0.1 to 1.1 AE 0.4, while 20:5 n-3 fatty acids of chloroplast related phospholipid classes such as phosphatidylglycerol (PG) increased under N-limitation. These data reveal that lipids are important components, required to sustain membrane structure under a deficiency of integral membrane bound proteins and pigments. Nonetheless, energy conversion at photosystem II is still affected by N-limitation despite this structural regulation. Photosynthetic quantum yield (F v /F m ) and electron transport rates decreased under N-limitation caused by an increasing amount of electron acceptors (second stable electron acceptor=Q B ) which had slower reoxidation kinetics. The energy surplus under these conditions is stored in triacylglycerols, the main energy sink in Antarctic sea ice diatoms under N-limitation. #

Low photosynthetic active radiation is a strong determinant in the development and growth of sea ice algae. The algae appear to have universal mechanisms to overcome light limitation. One important process, which is induced under light limitation, is the desaturation of chloroplast membrane lipids. In order to discover whether this process is universally valid in sea ice diatoms, we investigated three species coexisting in chemostats illuminated with 15 and 2 mmol photons m À2 s À1 at À1 C. Growth under 2 mmol photons m À2 s À1 caused a 50% increase in monogalactosyldiacylglycerols (MGDG) thylakoid membrane related 20:5 n-3 fatty acids. This fatty acid supports the fluidity of the thylakoid membrane and therefore the velocity of electron flow, which is indicated by increasing rate constants for the electron transport between Q A (first stable electron acceptor) and bound Q B (second stable electron acceptor) (11.16 AE 1.34 to 23.24 AE 1.35 relative units). Two mmol photons m À2 s À1 furthermore resulted in higher amounts of non-lipid bilayer forming MGDG in relation to other bilayer forming lipids, especially digalactosydiacylglycerol (DGDG). The ratio of MGDG:DGDG increased from 3.4 AE0.3 to 5.7 AE0.3. The existence of bilayer thylakoid membranes with high proportions of non. bilayer forming lipids is only possible when sufficient thylakoid pigment-protein complexes are present. If more thylakoid pigment-protein complexes are present in membranes, as found under extreme light limitation, less bilayer forming lipids such as DGDG are required to stabilize the bilayer structure. Differences in protein contents between both light intensities were not found. Consequently pigment contents which nearly doubled under 2 mmol photons m À2 s À1 must be responsible in balancing the potential stability loss resulting from an increase in MGDG:DGDG ratio. #

Light‐emitting diodes (LEDs) were used as the sole light source in continuous culture of the green alga Chlorella pyrenoidosa. The LEDs applied show a peak emission at 659 nm with a half‐power bandwidth of 30 nm. Selection of this wavelength range, which is optimal for excitation of chlorophylls a and b in their “red” absorption bands makes all photons emitted potentially suitable for photosynthesis. No need for additional supply of blue light was found. A standardized panel with 2 LEDs cm−2 fully covered one side of the culture vessel. At standard voltage in continuous operation the light output of the diode panel appeared more than sufficient to reach maximal growth. Flash operation (5‐μs pulse duration) enables potential use of higher operating voltages which may render up to three times more light output. Flat airlift fermentor‐type continuous culture devices were used to estimate steady state growth rates of Chlorella pyrenoidosa as a function of the light flux (μmol photons · m−2 · s−1) and the flashing frequency of the light‐emitting diodes (which determines the duration of the dark “off” time between the 5‐μs “on” pulses). At the fixed voltage and turbidostat setting applied a 20‐kHz frequency, which equals dark periods of 45 μs, still permitted the maximum growth rate to become nearly reached. Lower frequencies fell short of sustaining the maximal growth rate. However, the light flux decrease resulting from lowering of the flash frequency appeared to reduce the observed growth rates less than in the case of a similar flux decrease with light originating from LEDs in continuous operation. Flash application also showed reduction of the quantum requirement for oxygen evolution at defined frequencies. The frequency domain of interest was between 2 and 14 kHz. LEDs may open interesting new perspectives for studies on optimization of mixing in mass algal culture via the possibility of separation of interests in the role of modulation on light energy conversion and saturation of nutrient supply. Use of flashing LEDs in indoor algal culture yielded a major gain in energy economy in comparison to luminescent light sources. © 1996 John Wiley & Sons, Inc.