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.
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.