Submitted to: Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2010
Publication Date: 10/1/2010
Citation: Murdock, J.N., Dodds, W.K., Reffner, J.A., Wetzel, D.L. 2010. Measuring cellular-scale nutrient distribution in algal biofilms with synchrotron confocal infrared microspectroscopy. Spectroscopy, 10(25):32-41. Interpretive Summary: In addition to increasing the amount of algae in a stream, nutrient pollution can change algal species composition, which can lead to communities dominated by species that are less edible, less efficient at removing nutrients, or produce toxins. Measuring how individual species in natural algal communities respond is difficult because it is not easy to separate species in large enough quantities for standard nutritional measurements. We used infrared microspectroscopy to measure the changes in the nutrient content of single algal cells taken from natural algal communities during exposure to reduced nutrient availability. We found that the two most abundant species differed in their physiological response, but both increased lipid content, which is a sign of cell stress. We also found that algal colonies with more individuals responded faster to changing nutrients. Understanding how individual species within an algal biofilm respond to changing nutrient availability can help to better predict how increases, or decreases, in nutrient pollution will impact algal communities in regards to species composition and nutrient uptake.
Technical Abstract: Infrared microspectroscopy (IMS) and chemical imaging is ideal for measuring nutrient distribution in single algal cells on a cellular and subcellular level. The study of small algal cells, or cells within a colony requires enhanced spatial resolution IMS. Synchrotron infrared microspectroscopy with confocal image plane masking provides spatial resolution at the diffraction limit, which allows the study of micro-ecological interactions with a minimum target width of 5 µm. Many species that contribute substantially to ecosystem productivity and nutrient cycling require this optical advantage. Also, this spatial resolution allows the measurement of individuals within a colony, providing insight into physiological advantages of colony formation. We used synchrotron confocal IMS to measure the response of two dominant algal species from a stream biofilm to a reduction in nutrient availability: Achnanthes affinis, a single cell (5 ' 10 µm) diatom, and Fragilaria virescens, a colonial diatom (5 ' 15 µm cells connected lengthwise). Both species initially had similar variation in macromolecular content but had differing responses to nutrient limitation in carbohydrate, phosphorous compound, and lipid distribution over time. The response of F. virescens colonies was at the cellular level, but the cellular response of individuals varied with colony length, as long colonies had a stronger response than short colonies. Better IMS spatial resolution allows the biochemical analysis of smaller, more closely associated algae and increases our ability to predict how algal communities will be affected by changing environmental conditions.