OBSERVATIONS ON SCENEDESMUS OBLIQUUS: DOES MAGNESIUM SUPPLY ALTER THE EFFICIENCY OF PHOSPHORUS USE?

A distinguishing characteristic of life is its ability to grow. Because biomass is represented by approximately 25 elements, growth is influenced by their availability. Carbon (C) ranks as the most abundant element in the cell and accounts for ~50% of the cellular dry mass. Other abundant elements in a cell are nitrogen (N) and phosphorus (P) that are part of proteins and nucleic acids. Approximately 50% of cellular P is found in the ribosomal RNA (rRNA), a key molecule in the machinery for protein synthesis. ATP hosts another substantial amount of cellular P. The negative charge of P in both RNA and in ATP is stabilized by another essential element, magnesium (Mg). Although historical data indicate the role of P input to freshwater algal growth, we still cannot predict algal blooms accurately; likely because there is a crucial knowledge gap in our understanding of the role of other elements, such as Mg, in algal growth. Across lakes in the United States, there is considerable variation in Mg, ranging from 100–4000 µg/L, although the role of Mg in algal growth, particularly in an interactive context with P is unknown. We hypothesized that Mg affects the efficiency with which P is used in the cell, with the greatest efficiency occurring at a 2:1 (P:Mg) reflecting biochemical demands. In separate laboratory experiments we grew Scenedesmus obliquus in a gradient of Mg concentration, and a gradient of P concentrations and measured growth rate. A significant effect of Mg:P ratio on algal growth rate was observed in both experiments. Put another way, Mg availability impacted the efficiency of P use (i.e., new biomass produced per unit P assimilated), and the Mg:P ratio at which highest growth rate was observed ranged from 1.5 – 4.5. Because Mg supply in reservoirs varies considerably along temporal and spatial dimensions, due to anthropogenic and local geochemical factors, we plan to explore Mg x P interactions in shaping algal growth and communities in nature.