Measuring the in situ carbon isotopic composition of distinct marine plankton populations sorted by flow cytometry

Abstract

The carbon isotope ratio (δ13C value) of marine particulates is a potentially useful tracer for elucidating pathways of carbon flow in the marine environment. Different species of phytoplankton vary in fractionation vs. CO2 by up to 24‰ in laboratory cultures under varying nutrient and growth conditions, a signal that should propagate through the microbial food web. However, such contrasts have been difficult to confirm in field measurements due to analytical limitations. Here, we combine fluorescence-activated cell sorting (FACS) with a specialized micro-combustion interface and isotope-ratio mass spectrometry (SWiM-IRMS) to provide some of the first direct measurements of whole-cell δ13C values for specific phytoplankton populations in the wild. For three samples collected off Scripps Pier in 2010-2011, Synechococcus averages δ13C values of -25.7±2.0‰, Prochlorococcus averages -23.0±1.3, and diatoms average -20.8±1.7‰. Diatoms were ∼3‰ enriched in 13C when measured during a bloom (March 2011) as compared with mid-summer (July 2010). Sorted particles thought to represent living heterotrophic bacteria averaged -25.4±2.5‰, whereas total filterable particles averaged -19.6±1.0‰, indicating a strong similarity to diatom biomass. These variations demonstrate that in situ differences in δ13C among different populations of particles can be exploited to follow carbon flow through successive trophic levels, and throughout organic matter remineralization, sinking, and preservation.