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

The carbon isotope ratio (δ^(13)C 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. CO_2 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 δ^(13)C values for specific phytoplankton populations in the wild. For three samples collected off Scripps Pier in 2010–2011, Synechococcus averages δ^(13)C values of −25.7 ± 2.0‰, Prochlorococcus averages −23.0 ± 1.3, and diatoms average −20.8 ± 1.7‰. Diatoms were ∼3‰ enriched in ^(13)C 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 δ^(13)C among different populations of particles can be exploited to follow carbon flow through successive trophic levels, and throughout organic matter remineralization, sinking, and preservation.