An ultra-pure water supply for preparation of artificial seawater medium to culture phytoplankton

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Phytoplankton contribute to the global carbon cycle through photosynthetic uptake and transfer of carbon dioxide to the deep sea. Their growth is limited in large regions by low concentrations of trace metals that reduce the efficiency of CO2 uptake. Determining how phytoplankton take up metals and what are the types and amounts they require for growth are important questions in Biological Oceanography. Study of the trace metal nutrition of phytoplankton has been greatly facilitated by the use of artificial seawater medium in which the composition and chemistry of the trace metals is known and understood. Artificial seawater replicates the composition of the major salt constituents in natural waters and can be prepared in large quantities at labs located far from sea. Its composition of nutrients and metals is precisely known and replicable, making it better suited than natural seawater for culture studies. An essential requirement for the preparation of artificial seawater is a large supply of ultra-pure water free of metal contaminants. In Aquil, the most widely used artificial medium, the concentrations of the various species of elements are tightly controlled and regulated to study their effect on phytoplankton growth. Such knowledge is essential to understand how phytoplankton acclimate and adapt to low concentrations of iron and other trace metals in the surface ocean. Large quantities of ultra-pure water are also needed for rinsing and cleaning flask and containers to remove contaminating metals and in the analysis of the cellular concentrations of these elements.