A single Prochlorococcus ecotype dominates the tropical Bay of Bengal with ultradian growth

• Published in: Environmental microbiology 2024-03, Vol.26 (3), p.e16605-n/a
• Main Authors: Grone, Jonathan, Poirier, Camille, Abbott, Kathleen, Wittmers, Fabian, Jaeger, Gualtiero Spiro, Mahadevan, Amala, Worden, Alexandra Z.
• Format: Article
• Language: English
• Subjects: Abundance; Cell cycle; Cell division; Ecotypes; Growth rate; Mixed layer; Phytoplankton; Picoplankton; Plankton; Population density; Primary production; Prochlorococcus; rRNA; Surface mixed layer; Surface water; Water masses
• ISSN: 1462-2912; 1462-2920
• DOI: 10.1111/1462-2920.16605

The Bay of Bengal (BoB) spans >2.2 million km2 in the northeastern Indian Ocean and is bordered by dense populations that depend upon its resources. Over recent decades, a shift from larger phytoplankton to picoplankton has been reported, yet the abundance, activity, and composition of primary producer communities are not well‐characterized. We analysed the BoB regions during the summer monsoon. Prochlorococcus ranged up to 3.14 × 105 cells mL−1 in the surface mixed layer, averaging 1.74 ± 0.46 × 105 in the upper 10 m and consistently higher than Synechococcus and eukaryotic phytoplankton. V1‐V2 rRNA gene amplicon analyses showed the High Light II (HLII) ecotype formed 98 ± 1% of Prochlorococcus amplicons in surface waters, comprising six oligotypes, with the dominant oligotype accounting for 65 ± 4% of HLII. Diel sampling of a coherent water mass demonstrated evening onset of cell division and rapid Prochlorococcus growth between 1.5 and 3.1 div day−1, based on cell cycle analysis, as confirmed by abundance‐based estimates of 2.1 div day−1. Accumulation of Prochlorococcus produced by ultradian growth was restricted by high loss rates. Alongside prior Arabian Sea and tropical Atlantic rates, our results indicate Prochlorococcus growth rates should be reevaluated with greater attention to latitudinal zones and influences on contributions to global primary production. The extremely rapid growth of a single Prochlorococcus ecotype at the surface of the Indian Ocean leads to its role as a key primary producer in this tropical marine ecosystem. The discoveries have major implications for how we model biomass and production in the tropics.