AUTHOR=Bock Nicholas , Subramaniam Ajit , Juhl Andrew R. , Montoya Joseph , Duhamel Solange 

TITLE=Quantifying Per-Cell Chlorophyll a in Natural Picophytoplankton Populations Using Fluorescence-Activated Cell Sorting

JOURNAL=Frontiers in Marine Science

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.850646

DOI=10.3389/fmars.2022.850646

ISSN=2296-7745

ABSTRACT=<p>Marine phytoplankton play a central role in global biogeochemical cycling, carbon export, and the overall functioning of marine ecosystems. While chlorophyll <italic>a</italic> (Chl <italic>a</italic>) is widely used as a proxy for phytoplankton biomass, identifying the proportion of Chl <italic>a</italic> attributable to different phytoplankton groups remains a major challenge in oceanography, especially for the picophytoplankton groups that often represent the majority of phytoplankton biomass in the open ocean. We describe a method for measuring picophytoplankton per-cell Chl <italic>a</italic> in field samples using fluorescence-activated cell sorting followed by solvent-based Chl <italic>a</italic> extraction and fluorescence quantification. Applying this method to surface samples from the Gulf of Mexico, we determined per-cell Chl <italic>a</italic> to be 0.24 ± 0.07, 0.6 ± 0.33, and 26.36 ± 20.9 fg Chl <italic>a</italic> cell<sup>-1</sup> for <italic>Prochlorococcus</italic>, <italic>Synechococcus</italic>, and PPE, respectively (mean ± SD). Measurements of per-cell Chl <italic>a</italic> using this method are precise to within 1.7, 2.1, and 3.1% for <italic>Prochlorococcus</italic>, <italic>Synechococcus</italic>, and PPE, respectively. We demonstrate that this approach can be used to obtain estimates of group-specific Chl <italic>a</italic> for <italic>Prochlorococcus</italic>, <italic>Synechococcus</italic>, and picophytoeukaryotes, the latter two of which cannot be captured by existing methods. We also demonstrate that measurements of per-cell Chl <italic>a</italic> made using this method in field samples are sufficiently precise to capture relationships between per-cell Chl <italic>a</italic> and cytometer red fluorescence, providing a bridge between biomass estimates from cell counts and bulk measurements of total Chl <italic>a</italic>.</p>