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Abstract: Benthic diatoms are important primary producers in intertidal marine sediments and form the basis of the food web in these ecosystems. In order to investigate the carbon flow within diatom mats, we performed in situ 13C pulse‐chase labeling experiments and followed in detail the biochemical fate of carbon fixed by the diatoms for five consecutive days. These labeling experiments were done at approximately 2‐monthly intervals during 1 yr in order to cover seasonal variations. The fixed carbon was recovered in individual carbohydrates including extracellular polymeric substances (EPS), amino acids, fatty acids, and nucleic acid bases. In addition, we assessed a variety of environmental parameters and photosynthetic characteristics. The fixed carbon was initially mainly stored as carbohydrate (glucose) while nitrogen‐rich compounds (e.g., amino acids and RNA/DNA) were produced more slowly. During the year, the diatoms distributed the photosynthetically fixed carbon differently among the various carbon pools that were measured. In summer, the diatoms decreased carbon fixation and accumulated relatively more lipid as a storage compound (27% ± 2% vs. 12% ± 5% in other seasons). The percentage of fixed carbon that was excreted as EPS was lower in summer compared to other seasons, amounting 9% ± 4% and 21% ± 6%, respectively. Hence, it seemed that the physiology of the microphytobenthos was different during summer and caused by higher light intensity and a shift in nitrogen source. [ABSTRACT FROM AUTHOR]

Anthropogenic activities resulted in major shifts in nutrient inputs to coastal seas, which may have altered the biomolecule composition of phytoplankton because of different C : N : P requirements for biosynthesis. In order to understand the effects of N- and P-limitation on the allocation of photosynthetically fixed C, we directly measured seasonal and spatial dynamics of amino acid, fatty acid and carbohydrate concentrations and biosynthesis rates in the phytoplankton of the North Sea using a novel 13C-tracer approach. Nutrient limitation and season had large effects on composition and biosynthesis rates of all biochemical classes, for instance the contribution of protein derived amino acids varied up to fourfold in concentration and up to eightfold in biosynthesis rates. Nutrient specific effects and the prevailing nutrient limitations were identified by short-term (24-72 h) nutrient addition experiments. Addition of the growth-limiting nutrient increased amino acid synthesis while storage compound synthesis decreased concurrently. The strongest effect was detected in N-limited flagellates, where amino acid synthesis increased up to fivefold within 24 h upon relief of nutrient limitation. P-limitation was only detected in diatom-dominated near-coast stations, and amino acid synthesis responded much slower, likely because of the necessary preceding synthesis of ribosomes. This metabolic elasticity of phytoplankton in response to nutrient availability will have major consequences for their nutritional value within the food web and subsequently the carrying capacity of coastal ecosystems. [ABSTRACT FROM AUTHOR]

Measurement of total primary production using 13C incorporation is a widely established tool. However, these bulk measurements lack information about the fate of fixed carbon: the production of major cellular compounds (carbohydrates, amino acids, fatty acids, and DNA/RNA) is affected by for instance nutrient availability as their C:N:P requirements differ. Here, we describe an approach to combine established methods in gas chromatography/isotope ratio mass spectrometry (GC/C-IRMS) and recently developed methods in liquid chromatography/IRMS (LC/IRMS) to trace stable isotope incorporation into neutral carbohydrates, amino acids, and fatty acids, and compare their production to total carbon fixation rates. We conducted a trial study at stations in the North Sea where different nutrients were limiting. There was variation in the fate of fixed carbon at these sites. The majority of fixed carbon (64-71%) was incorporated into neutral carbohydrates, followed by amino acids (19-32%) and fatty acids (4-9%). The sum of carbon fixation into these three fractions accounted for 81-116% of total carbon fixation. P- and N-limitation increased the biosynthesis of storage lipids and storage carbohydrates while N-limitation decreased synthesis of amino acid proline with a concurrent increase in glutamic acid + glutamine. This new approach provides the capability to determine direct effects of resource limitation and the consequences for the physiological state of a phytoplankton community. It may thereby enable us to evaluate the overall quality of phytoplankton as a food source for higher trophic levels or trace consumption of phytoplankton through the food web. [ABSTRACT FROM AUTHOR]