In Situ Bacterioplankton Growth Partitioning by High-Resolution Metatranscriptomics

Area of Science:

• Marine microbial ecology
• Ocean biogeochemical cycles
• Phytoplankton bloom dynamics

Background:

• Microbial communities are vital for marine ecosystems, influencing trophic webs and elemental cycling.
• Accurate in situ measurements of microbial growth rates at high phylogenetic resolution are lacking.
• Understanding microbial activity is key to modeling marine food webs.

Purpose of the Study:

• To develop and apply a method for phylogenetically resolving microbial growth in situ.
• To track changes in microbial community composition and activity during a spring phytoplankton bloom.
• To differentiate between replication-associated and other cellular functions in marine microbes.

Main Methods:

• High temporal resolution transcriptomics to quantify gene expression.
• Measurement of ftsZ gene expression as an indicator of cell division and growth.
• Comparison of ftsZ expression with RNase P expression to assess resource allocation.
• Phylogenetic analysis of gene expression to determine active taxa.

Main Results:

• Two distinct microbial growth profiles were observed before and after the phytoplankton bloom.
• Prior to and after the bloom, Synechococcales, Pelagibacterales, and picoeukaryotes dominated ftsZ expression.
• As the bloom declined, Rhodobacterales, SAR92, and SAR86 showed increased ftsZ expression.
• High temporal resolution revealed disproportionate microbial contributions during bloom development and decline.

Conclusions:

• Phylogenetic distribution of ftsZ expression provides a scalable index of in situ microbial growth.
• Transcriptional activity reveals dynamic successional patterns in marine microbial communities.
• This approach enhances mechanistic models of planktonic food web dynamics and elemental cycling.