Angel White: Biological Productivity in the Ocean 'Deserts'
The global ocean covers 70% of the planet and the mid-ocean gyres, with their circular surface currents, comprise about 40-50% of this ocean. Researchers working on global climate change try to understand what factors control the biomass and productivity in these gyres, previously considered "desert" regions, and how these changes in productivity feedback into the climate. Phytoplankton—microscopic floating plantlife at the bottom of the marine food chain and producers of half the oxygen produced on Earth by plants—can soften the impacts of climate change by absorbing carbon dioxide.
Angel White, recent COAS alumni and now a research associate post doc working with Ricardo Letelier, studies the controls of productivity in the mid-ocean gyres. On a recent cruise from Noumea, New Caledonia to Honolulu, Hawaii, White sailed with other researchers trying to understand the diversity of the microbial community, including Trichodesmium, a large colony-forming cyanobacterium known to form large surface blooms.
Colonies of Trichodesmium can be seen from a distance as a slick on the surface. Upon seeing this for the first time, Captain Cook called it "sea sawdust." A colony is an aggregation of several hundred filaments that are each, in turn, composed of hundreds of cells.
Trichodesmium grow in the tropical and subtropical gyres, far away from nutrient sources. Their growth is favored in environments that are warm, well-stratified, and well lit. In the open ocean where most life forms are limited in their growth by the availability of reduced nitrogen, Trichodesmium are able to utilize the nearly unlimited sources of nitrogen gas dissolved in seawater from the atmosphere, thereby sidestepping the problem of nitrogen limitation. This ability permits it to grow in the N-poor open ocean and, in the process, support the growth of a robust photosynthetic community.
Interesting as well, these colonies act as a kind of communal kibbutz, a nitrogen-rich raft for a slew of other organisms including copepods, protists, and other bacteria. Since Trichodesmium are drivers of production, nitrogen fixation, and carbon export into the deep ocean in these vast biomes, the photosynthetic production rates of these cyanobacteria can be very important.
White's research aims to understand how elements are coupled in these charismatic macrofauna, particularly how phosphorus controls carbon and nitrogen fixation and how that can be modeled on a basin or global scale. White notes, "Ricardo Letelier, Yvette Spitz and I are trying to put the phosphorus kinetics of Trichodesmium into the broader scheme of things. On this last cruise, there were research groups looking at iron limitation, nitrogen fixation and other microbial community dynamics. Through these collaborative efforts we hope to be able to put our results into a much broader context.”
White became interested in the ecology and physiology of Trichodesmium while doing her PhD with Letelier and Spitz; she was looking for a project that would let her work across discipline boundaries. "I wanted to work on a project that allowed me to use my existing skills (modeling) as well as work outside my box, learning new tools and new approaches. In practice, this meant that I ended up combining numerical modeling, remote sensing, and physiological studies to try and understand elemental dynamics in open ocean cyanobacteria.”
White sees her work as adding to the inquiry of many others. "Captain Cook described Trichodesmium in 1768; Darwin wrote about it in The Voyage of the Beagle in 1845. Since the 1940s, there has been an exponential increase in publication; in 2006, approximately 140-150 articles focusing on Trichodesmium made it into scientific journals. However, we are still asking essentially the same question: what limits the growth and controls the distribution of this organism." White hopes that cross-disciplinary approaches to old problems will provide new answers.
White's graduate work on Trichodesmium was awarded the 2007 OSU Distinguished Dissertation Award, which recognizes a doctoral recipient whose dissertation makes an unusually significant contribution to a discipline in biological and life sciences. Her dissertation, "Phosphorus Physiology and Environmental Forcing of Oceanic Cyanobacteria, Primarily Trichodesmium spp," will be advanced as OSU's nominee in the national competition.
|
