Summary• The balance of energy flow from light absorption into biomass was investigated under simulated natural light conditions in the diatom Phaeodactylum tricornutum and the green alga Chlorella vulgaris .• The energy balance was quantified by comparative analysis of carbon accumulation in the new biomass with photosynthetic electron transport rates per absorbed quantum, measured both by fluorescence quenching and oxygen production. The difference between fluorescence-and oxygen-based electron flow is defined as 'alternative electron cycling'.• The photosynthetic efficiency of biomass production was found to be identical for both algae under nonfluctuating light conditions. In a fluctuating light regime, a much higher conversion efficiency of photosynthetic energy into biomass was observed in the diatom compared with the green alga.• The data clearly show that the diatom utilizes a different strategy in the dissipation of excessively absorbed energy compared with the green alga. Consequently, in a fluctuating light climate, the differences between green algae and diatoms in the efficiency of biomass production per photon absorbed are caused by the different amount of alternative electron cycling.

A mid-infrared spectroscopic method was developed for the simultaneous and quantitative determination of total protein, carbohydrate and lipid contents of microalgal cells. Based on a chemometric approach, measured FTIR (Fourier transform infrared) spectra from algal cells were reconstructed by a partial least square algorithm, using the spectra of the reference substances to determine their relative contribution to the overall cell spectrum. From this specific absorption, absolute macromolecular cell composition [pg cell(-1)] can be calculated using calibration curves, which have been validated by independent biochemical methods. The future potential of this method for photosynthesis research is shown by its application to follow time-resolved changes in the cellular composition of microalgae during an illumination period of several hours. We show how the macromolecular composition can be investigated by FTIR spectroscopy methods. This can substantially increase the efficiency of screening processes like bioreactor monitoring and may be beneficial in metabolic engineering of algal cells.

The energy balance of Phaeodactylum tricornutum cells from photon to biomass have been analysed under nutrient-replete and N-limiting conditions in combination with fluctuating (FL) and non-fluctuating (SL) dynamic light. For this purpose, the amount of photons absorbed has been related to electrons transported by photosystem II, to gas exchange rates, and to the newly formed biomass differentially resolved into carbohydrates, proteins, and lipids measured by means of Fourier transform infrared (FTIR) spectroscopy. Under high nutrient conditions, the quantum efficiency of carbon-related biomass production (Phi(C)) and the metabolic costs of carbon (C) production were found to be strongly controlled by the light climate. Under N-limited conditions, the light climate was less important for the efficieny of primary production. Thus, the largest range of Phi(C) dependent on the nutrient status of the cells was observed under non-fluctuating light conditions which are comparable with stratified conditions in the natural environment. It is evident that N limitation induced pronounced changes in the composition of macromolecular compounds and, thus, influenced the degree of reduction of the biomass as well as the metabolic costs of C production. However, Phi(C) and the metabolic costs are not predictable from the photosynthesis rates. In consequence, the results clearly show that bio-optical methods as well as gas exchange measurements during the light phase can severely mismatch the true energy storage in the biomass especially under high nutrient in combination with non-fluctuating light conditions.

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which could consequently lead to a depletion of this energy reserves before the end of the polar night. On the other hand, the membrane building phospho-and glycolipids remained unchanged during the 8 weeks darkness, indicating still intact thylakoid membranes. These results explain the shorter survival times of polar diatoms with increasing water temperatures during prolonged dark periods. Abstract The Arctic represents an extreme habitat for phototrophic algae due to long periods of darkness caused by the polar night (~4 months darkness). Benthic diatoms, which dominate microphytobenthic communities in shallow water regions, can survive this dark period, but the underlying physiological and biochemical mechanisms are not well understood. One of the potential mechanisms for long-term dark survival is the utilisation of stored energy products in combination with a reduced basic metabolism. In recent years, water temperatures in the Arctic increased due to an ongoing global warming. Higher temperatures could enhance the cellular energy requirements for the maintenance metabolism during darkness and, therefore, accelerate the consumption of lipid reserves. In this study, we investigated the macromolecular ratios and the lipid content and composition of Navicula cf. perminuta Grunow, an Arctic benthic diatom isolated from the microphytobenthos of Adventfjorden (Svalbard, Norway), over a dark period of 8 weeks at two different temperatures (0 and 7 °C). The results demonstrate that N. perminuta uses the stored lipid compound triacylglycerol (TAG) during prolonged dark periods, but also the pool of free fatty acids (FFA). Under the enhanced temperature of 7 °C, the lipid resources were used significantly faster than at 0 °C, Keywords

SummaryDiatoms are important players in the global carbon cycle. Their apparent photosynthetic affinity for ambient CO 2 is much higher than that of ribulose 1,5-bisphosphate carboxylase/ oxygenase (Rubisco), indicating that a CO 2 -concentrating mechanism (CCM) is functioning. However, the nature of the CCM, a biophysical or a biochemical C 4 , remains elusive. Although 14 C labeling experiments and presence of complete sets of genes for C 4 metabolism in two diatoms supported the presence of C 4 , other data and predicted localization of the decarboxylating enzymes, away from Rubisco, makes this unlikely.We used RNA-interference to silence the single gene encoding pyruvate-orthophosphate dikinase (PPDK) in Phaeodactylum tricornutum, essential for C 4 metabolism, and examined the photosynthetic characteristics.The mutants possess much lower ppdk transcript and PPDK activity but the photosynthetic K 1/2 (CO 2 ) was hardly affected, thus clearly indicating that the C 4 route does not serve the purpose of raising the CO 2 concentration in close proximity of Rubisco in P. tricornutum. The photosynthetic V max was slightly reduced in the mutant, possibly reflecting a metabolic constraint that also resulted in a larger lipid accumulation.We propose that the C 4 metabolism does not function in net CO 2 fixation but helps the cells to dissipate excess light energy and in pH homeostasis.

Fourier transform infrared (FTIR) spectra were measured from cells of Microcystis aeruginosa and Protoceratium reticulatum, whose growth rates were manipulated by the availability of nutrients or light. As expected, the macromolecular composition changed in response to the treatments. These changes were species-specific and depended on the type of perturbation applied to the growth regime. Microcystis aeruginosa showed an increase in the carbohydrate-to-protein ratio with decreased growth rates, under nutrient limitation, whereas light limitation induced a decrease of the carbohydrate-to-protein ratio with decreasing proliferation rates. The macromolecular pools of P. reticulatum showed a higher degree of compositional homeostasis. Only when the lowest light irradiance and nutrient availability were supplied, an increase of the carbohydrate-to-protein FTIR absorbance ratio was observed. A species-specific partial least squares (PLS) model was developed using the whole FTIR spectra. This model afforded a very high correlation between the predicted and the measured growth rates, regardless of the growth conditions. On the contrary, the prediction based on absorption band ratios generally used in FTIR studies would strongly depend on growth conditions. This new computational method could constitute a substantial improvement in the early warning systems of algal blooms and, in general, for the study of algal growth, e.g. in biotechnology. Furthermore, these results confirm the suitability of FTIR spectroscopy as a tool to map complex biological processes like growth under different environmental conditions.

Alternative electron sinks are an important regulatory mechanism to dissipate excessively absorbed light energy particularly under fast changing dynamic light conditions. In diatoms, the cyclic electron transport (CET) around Photosystem II (PS II) is an alternative electron transport pathway (AET) that contributes to avoidance of overexcitation under high light illumination. The combination of nitrogen limitation and high-intensity irradiance regularly occurs under natural conditions and is expected to force the imbalance between light absorption and the metabolic use of light energy. The present study demonstrates that under N limitation, the amount of AET and the activity of CETPSII in the diatom Phaeodactylum tricornutum were increased. Thereby, the activity of CETPSII was linearly correlated with the amount of AET rates. It is concluded that CETPSII significantly contributes to AET in P. tricornutum. Surprisingly, CETPSII was found to be activated already at the end of the dark period under N-limited conditions. This coincided with a significantly increased degree of reduction of the plastoquinone (PQ) pool. The analysis of the macromolecular composition of cells of P. tricornutum under N-limited conditions revealed a carbon allocation in favor of carbohydrates during the light period and their degradation during the dark phase. A possible linkage between the activity of CETPSII and degree of reduction of the PQ pool on the one side and the macromolecular changes on the other is discussed.