Elemental composition of algal cells[Project home page] The instrument will be set up with the optical arrangement shown in part A of this figure, but without the coverslip in place. Mean elemental composition of at least 100 cells per sample will be obtained from discrete observations using the microscope-LIBS. These data will be compared with elemental composition determined by LIBS on bulk samples collected on filters and transferred to quartz slides. In addition, we shall compare the LIBS measurements of elemental composition with measurements made using two other methods:
The samples used to compare these methods will be obtained from phytoplankton cultures. We anticipate undertaking triplicate analyses on each of 50 samples (150 in total) from axenic algal cultures grown under a range of well-defined conditions. Culturing is routinely undertaken in Geider's and Underwood's laboratories. In particular, we will examine the coccolithophore Emiliania huxleyi and the diatom Thalassiosira weissfloggi; two of the model organisms that we use for research on photosynthetic physiology and nutrient limitation. These will be grown in nutrient replete, N-limiting, P-limiting and Fe-limiting conditions to obtain a range of elemental compositions. We will also examine the chrococcoid cyanobacterium Synechcococcus under nutrient-replete conditions as a test of the ability to determine the elemental composition of picophytoplankton. Cell abundance will be determined by direct counting in a haemocytometer slide. Filtration under gentle vacuum of samples for elemental analyses will be undertaken in a class 100 laminar flow hood to prevent contamination. All elements will be analysed in triplicate. For CHNS, material will be collected by filtration on ashed (550°C for 4 h) GFF filters (47 mm diameter). For O, material will be collected by filtration on ashed (550°C for 4 h) GFF filters (25 mm diameter) and transferred to silver crucible. For Si, Ca, P and trace metals (including Co, Cu, Fe, Mn, Ni and Zn), material will be collected by filtration on acid washed polycarbonate membrane filter (1 µm pore size, 25 mm) and washed using a trace-element clean reagent (Tovar-Sanchez et al. 2003) prior to analysis. All samples will be oven dried at 80°C and stored until analysis by the named project partner, Eric Achterberg, at the University of Plymouth. C, H and N will be analysed simultaneously using an elemental analyser (EA 1110) with combustion at 1800°C (flash combustion) in the presence of ultrapure oxygen. S will be analysed using an elemental analyser (EA 1110) at 1800°C (flash combustion) in the presence of ultrapure oxygen. O will be analysed using an elemental analyser (EA 1110), with combustion temperature of 1050°C undertaken in the presence of ultrapure helium in a pyrolytic column. For P and trace metals (incl. Co, Cu, Fe, Mn, Ni and Zn), material will be digested using HNO3–HF, with subsequent analysis using a Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (Axiom, VG).
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