Phytoplankton
Globally, almost 50% of all primary production is carried out by marine phytoplankton (Chisholm 2000). Photosynthetic marine microorganisms therefore play a critical role in the global carbon (C) cycle by influencing the carbon dioxide balance of the oceans and atmosphere. Particularly important in this context are those oceanic regions that exhibit high rates of ‘new production’. New production occurs where nutrient input leads to a net increase of biomass in the system. This biomass may be exported from the euphotic zone via the biological pump or be buried in marine sediments, thus leading to net C sequestration (i.e. a net removal of C dioxide from the system). My lab is interested in factors that influence the effectiveness of C sequestration as the result of new productivity, because they affect global climate prediction models.
(The figure shows a false color SeaWiFS image of the Gulf of Mexico. Lighter colors indicate elevated levels of chlorophyll (as well as certain artifacts such as bottom reflectence and Gelbstoff). Linking global patters of phytoplankton distribution to cellular physiology is an important interest of my lab.)
In oceanic systems, new production in the euphotic zone is often limited by nitrogen (N) input, which mostly occurs due to upwelling or coastal proximity (river plumes). With N amendment, phytoplankton bloom and biomass generally increases, but little is known about the population dynamics that underlie such processes. For example, it is unclear whether the same populations of phytoplankton that are responsible for high rates of N uptake are also those who exhibit equally high rates of C fixation and biomass increases.
We hypothesize that bulk N and C uptake measurements and traditional pigment analyses are inadequate to fully describe N and C cycling dynamics in natural systems, because these methods group microorganisms by size or class, rather than functional relationships. We are therefore developing methodology that will be able to explicitly test the linkage between N and C uptake in marine phytoplankton and bacterial communities at a species-level scale.
We are also interested in studying the diversity, distribution and activity of eukaryotic picoplankton in marine systems. A very large diversity of these small eukaryotic algae exists, but very little is known about their distribution and ecology.
Shown is a phylogenetic analsyis of chlorophyte-like rbcL sequence obtained from the Gulf of Mexico and the Atlantic Ocean. Many unknown clusters of sequences remain, indicating that yet to be cultured phytoplankton is abundant in the worlds oceans.