Microbial size spectra from natural and nutrient enriched ecosystems

Microbial size spectra, including bacteria through nanophytoplankton, were measured by use of how cytometry across the western north Atlantic Ocean an d during two nutrient enrichment studies: bottle enrichments in the Sargass o Sea and an in situ iron enrichment in the equatorial Pacific (IronEx II). Spectral shapes, or the relative conformity to a function described by a p ower law, ranged from smooth and log linear during the spring bloom in the Sargasso Sea to being distinctly non-log linear in coastal waters. Overall, the individual spectra within large regions characterized by similar ecolo gical conditions showed remarkable consistency, inviting speculation that p owerful organizing mechanisms are at work in these communities. Moreover, t he ensemble average of all of the spectra along the transect displays clear power-law behavior. Slopes ranged from -1.0, in which biomass was equally distributed between all size classes, to -1.4, in which proportionally more biomass was contained in smaller size classes; there was no clear relation ship between nutrient concentrations and spectral slopes over the entire da ta set. Species succession in nutrient-enriched bottles caused spectra to e volve from relatively smooth power laws to distributions showing preferred sizes (i.e., nonlinear on a log-log plot). The IronEx II spectra, however, remained similar over the course of the experiment. It could be that the el imination of bottle effects in this experiment buffered the system in ways that maintained the size structure of the microbial community over the size range we measured. Our results suggest conditions that lead to log-linear size distributions; these should be verified over a broader range of scales and environments.