The Measurement of Lipopolysaccharide (LPS) as an Indicator of Bacterial Biomass in the Oligotrophic Ocean

Abstract

Measurements of biomass, or the amount of carbon associated with living organisms, are fundamental to the study of microbial processes in the sea and their relationship to the marine ecosystem. A vast majority of all organic material in the ocean is in the dissolved state or in nonliving particulate material, making it difficult to determine the amount of carbon associated with microorganisms and bacteria in particular, with accuracy. The most common way of estimating bacterial biomass is by determining the number of bacterial cells, with epifluorescent microscopy or flow cytometry, in a water sample and then applying a carbon/cell conversion factor. It has been suggested that the use of a biochemical indicator such at LPS (lipopolysaccharide), which is the main constituent of the outer membranes of Gram-negative bacteria, may provide a more accurate estimate of biomass. The LPS method has not been firmly established in the field of oceanography yet, but it has the potential to complement cell enumeration techniques in the determination of bacterial biomass. The details of this method, most importantly the C/LPS conversion factor and the separation of dissolved from particulate LPS, have not been addressed beyond their initial descriptions. The goal of this project was to look at the use of the LPS method in the oligotrophic Pacific Ocean. I measured LPS and cell numbers in a seawater culture, compared two methods for the separation of dissolved from particulate LPS, and measured LPS in depth profiles. The CILPS value of 6.35 appears to be reasonable, there is no significant difference between the two methods of separation, and depth profiles show that there is a relatively constant pool of LPS in the oligotrophic Pacific Ocean .