Physiological adaptation to nutrient limitation in a marine oligotrophic ultramicrobacterium Sphingopyxis alaskensis

Sphingopyxis (formerly Sphingomonas) alaskensis, a numerically abundant species isolated from Alaskan watersand the North Sea represents one of the only pure cultures of a typical oligotrophic ultramicrobacterium isolatedfrom the marine environment. In this study, physiological and molecular characterization of an extinction dilutionisolate from the North Pacific indicate that it is a strain of Sphingopyxis alaskenis, extending the knowngeographical distribution of this strain and affirming its importance as a model marine oligotroph. Given theimportance of open ocean systems in climatic processes, it is clearly important to understand the physiology andunderlying molecular biology of abundant species, such as S. alaskensis, and to define their role in biogeochemicalprocesses.S. alaskensis is thought to proliferate by growing slowly on limited concentrations of substrates thereby avoidingoutright starvation. In order to mimic environmental conditions chemostat culture was used to study the physiologyof this model oligotroph in response to slow growth and nutrient limitation. It was found that the extent of nutrientlimitation and starvation has fundamentally different consequences for the physiology of oligotrophicultramicrobacteria compared with well-studied copiotrophic bacteria (Vibrio angustum S14 and Escherichia coli).For example, growth rate played a critical role in hydrogen peroxide resistance of S. alaskensis with slowlygrowing cells being 10, 000 times more resistant than fast growing cells. In contrast, the responses of V. angustumand E. coli to nutrient availability differed in that starved cells were more resistant than growing cells, regardless ofgrowth rate.In order to examine molecular basis of the response to general nutrient limitation, starvation and oxidative stress inS. alaskensis we used proteomics to define differences in protein profiles of chemostat-grown cultures at variouslevels of nutrient limitation. High-resolution two-dimensional electrophoresis (2DE) methods were developed and2DE protein maps were used to define proteins regulated by the level of nutrient limitation. A number of theseproteins were identified with the aid of mass spectrometry and cross-species database matching. The identifiedproteins are involved in fundamental cellular processes including protein synthesis, protein folding, energygeneration and electron transport, providing an important step in discovering the molecular basis of oligotrophy inthis model organism