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Detecting carbon uptake and cellular allocation by individual algae in multispecies assemblages

Justin N. Murdock
Limnology and Oceanography: Methods 2016 v.14 no.2 pp. 124-137
Bacillariophyceae, Chlorophyta, Spirogyra, algae, algae culture, aquatic ecosystems, carbon, carbon sequestration, cellulose, chemical composition, energy, isotopes, lipids, nutrient uptake, phytoplankton, proteins
Algal species vary in carbon (C) need and uptake rates. Understanding differences in C uptake and cellular allocation among species from natural communities will bring new insight into many ecosystem process questions including how species changes will alter energy availability and C sequestration in aquatic ecosystems. A major limitation of current methods that measure algal C incorporation is the inability to separate the response of individual species from mixed-species assemblages. I used Fourier-transform infrared micro-spectroscopy to qualitatively measure inorganic (13)C isotope incorporation into individual algal cells in single species, two species, and natural phytoplankton assemblages. Lateral shifts in spectral peaks from (13)C treatments were observed in all species. Comparison of peaks associated with carbohydrates, proteins, and lipids allowed for the detection of which individuals took in C, and which macromolecules the C was used to make. For example, shifts in Spirogyra spectral peaks showed substantial C incorporation in carbohydrates. Further, shifts in peaks at 1160 cm(-1), 1108 cm(-1), 1080 cm(-1), 1048 cm(-1), and 1030 cm(-1) suggested C was being allocated into cellulose. The natural phytoplankton assemblage demonstrated how C could be tracked into co-occurring species. A diatom had large shifts in protein and carbohydrate peaks, while a green alga and euglenoid had only a few shifts in protein related peaks. Fourier-transform infrared microspectroscopy is an established, label free method for measuring the chemical composition of algal cells. However, adding a label such as (13)C isotope can greatly expand the technique’s capabilities by qualitatively tracking C movement between inorganic and organic states within single cells.