Validation of a modified Landry-Hassett dilution technique to quantify the growth and mortality rates of subpopulations of planktonic microbes

Due to their importance in global ocean ecology and biogeochemistry, much is known about the abundance and diversity of marine microbes and this data informs our studies of microbial genetic variability and niche partitioning. However, much less is known about marine microbes’ vital rates, including phylogenetically-resolved intrinsic growth and mortality. Here, we use cyanobacteria to validate a recently-developed, sequence-based modification of the Landry-Hassett Dilution (LHD) technique, which simultaneously assesses the growth and grazer-induced mortality of microbial subpopulations. Using laboratory cultures, we show that most approaches for quantifying cyanobacterial intrinsic growth yield similar results, providing a foundation for methods comparisons in complex communities. We then leverage these findings to validate the modified LHD approach by sampling from a highly-dynamic, coastal marine ecosystem. We find that the LHD 16S rRNA gene-based method returns the same intrinsic growth as other biomass estimates. Further, field sampling yields distinct intrinsic growth among closely-related lineages of both cyano- and heterotrophic bacteria. While grazer-mediated mortality was less variable, we did observe distinct mortality rates within the broader microbiome, supporting the importance of examining vital rates in subpopulations. This validated methodology (and initial field findings) opens the door to answer fundamental ecological and biogeochemical questions about the microbial oceanography of spatially and temporally-variable coastal and open ocean ecosystems.