Mark Brzezinski

Research

Dr. Brzezinski's research focuses on a dominant group of marine phytoplankton, the diatoms. Diatoms are unique among the phytoplankton in that they require silicon to grow which they deposit in their ornately patterned cell walls. That requirement for Si is obligatory and without a source of dissolved silicon diatoms cease to grow. A major focus of Brzezinski's research is to assess the role of silicon as a limiting resource for diatom growth in the sea. His studies of silicon limitation of diatoms in Gulf Stream warm-core rings, in the Sargasso Sea and in the coastal waters off Southern California and in the Southern Ocean have established silicic acid availability as a strong determinant of the level of diatom activity in these systems.

Another focus of Brzezinski's research is the role of diatoms in the biogeochemistry of silicon. Although diatoms are microscopic, diatoms control the marine silica cycle. Each atom of Si that enters the sea from rivers and other sources is incorporated into a diatom frustule and then dissolves back into the seawater an average of about 40 times before being buried in the sea bed. The amount of silicon utilized by diatoms is prodigious amounting to some 250 Teramoles of particulate silica each year. Brzezinski's research seeks to quantify the level of silica production and silica dissolution in different ocean habitats and to understand the mechanisms that control diatom growth in the sea. One of the main unresolved questions is the source of silicon supporting diatom growth. The source of silicic acid sets an upper limit on the level of net silica production in the surface waters and the contribution of diatoms to the transport of carbon to the deep sea. The latter factor is particularly relevant today given the growing concern over rising level of greenhouse gases in the atmosphere and the need to quantify the ability of the oceans to serve as a sink for atmospheric CO2 to mediate future greenhouse effects. Silicic acid is regenerated in the sea through the dissolution of biogenic silica. That process can occur locally in the surface waters through the dissolution of diatom frustules in the euphotic zone or it can occur in the deep sea with requiring vertical mixing to resupply the surface waters with silicic acid. A high level of Si recycling in the surface waters means that little diatom silica can be exported without dramatically curtailing silica production rates. In contrast, a given input of silicic acid from deep waters can support equivalent level of silica export. Exported diatoms carry both particulate silica and organic material, and in particular carbon, to the deep sea. Thus, the relative importance of new versus recycled silicic acid can control the amount of CO2 fixed by diatoms that is exported to the deep sea and ultimately the contribution of diatoms to the oceanic carbon dioxide sink.

In recent years members of Brzezinski's laboratory discovered that diatoms fractionate isotopes of silicon when building their siliceous frustules and that this tool can be used to examine silicic acid use by diatoms onetime scales from days to millennia. Research is underway to use this tool to reconstruct relative diatom silicic acid use from diatom frustules recovered from dated sediment cores. Work on cores from the Southern Ocean has shown that diatom productivity has changed dramatically on glacial-interglacial cycles which has profound implications for ocean productivity, carbon export and climate