Separation of algae from detritus for stable isotope or ecological stoichiometry studies using density fractionation in colloidal silica

Abstract

Stable isotope and ecological stoichiometry investigations of aquatic food webs require separate measurements of microalgae and detritus, but fine particulate matter collected as seston or scraped from biofilms typically is an unknown mixture of these two components plus other material. This paper describes an economical method to partition fine particulate matter into predominantly algal and detrital components by centrifugation in colloidal silica. Centrifugation using this density also worked well to separate algae from inorganic matter. The protocol was tested by sampling a broad range of water bodies in Queensland, Australia. Seston was concentrated using an inexpensive continuous centrifuge and organic matter was collected from surfaces of rocks, mud, or plants. Separation in colloidal silica was achieved using a standard benchtop centrifuge. When the colloidal silica was adjusted to a density of 1.27 g/cm 3 , the light fraction (supernatant) tended to be dominated by algal organic matter while the heavy fraction tended to be more detrital. Ratios of organic carbon to chlorophyll a indicated the efficacy of the separations. Stable C and N isotope ratios often differed considerably between the algal and detrital fractions, demonstrating the need to perform such separations in many aquatic environments.

A multiple biomarker approach to tracking the fate of an ice algal bloom to the sea floor

“…Ludox has been previously used to separate algal material and microphytobenthos from detritus and sediment (Blanchard et al 1988;Hamilton et al 2005). Here, the centrifugation of sediment, Ludox, and distilled water creates a density gradient, allowing diatoms to aggregate in a layer at the interface between the Ludox-sediment and water layers.…”

Strong evidence for terrestrial support of zooplankton in small lakes based on stable isotopes of carbon, nitrogen, and hydrogen

“…Although the majority of these studies suggest significant terrestrial support of zooplankton (Table S1), this interpretation is debatable for three reasons: (i) The pathways outlined above are hard to quantify and gut contents are difficult to determine in zooplankton (35); (ii) approaches using stable isotopes can be problematic because it is difficult to directly measure the isotopic signature of phytoplankton. Suspended particulate organic matter [seston, or particulate organic matter (POM)] is only partially comprised of phytoplankton, and physically isolating the phytoplankton is only possible under certain conditions (36); and (iii) even where measurement or estimation is possible, phytoplankton can be isotopically similar to terrestrial organic matter, especially for carbon (37).…”

Comparison of methods to determine algal δ13C in freshwater

“…Although direct measurements of pelagic algal δ 13 C are possible for large-sized taxa (e.g., Volvox, see Rautio and Vincent 2007), there is little evidence to suggest that a single algal genus could be used to represent the carbon signature of a complex community. Despite promising efforts to develop new techniques that refine isotopic signatures of particular groups of organisms (Boschker and Middelburg 2002, Pel et al 2004, Hamilton et al 2005, Pond et al 2006, the majority of studies still rely on indirect methods to infer algal signatures. The most common methods are based on particulate organic carbon (POC) or dissolved inorganic carbon (DIC).…”