AUTHOR=Confesor Kristina A. , Selden Corday R. , Powell Kimberly E. , Donahue Laura A. , Mellett Travis , Caprara Salvatore , Knapp Angela N. , Buck Kristen N. , Chappell P. Dreux 

TITLE=Defining the Realized Niche of the Two Major Clades of Trichodesmium: A Study on the West Florida Shelf

JOURNAL=Frontiers in Marine Science

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.821655

DOI=10.3389/fmars.2022.821655

ISSN=2296-7745

ABSTRACT=<p>The cyanobacterium <italic>Trichodesmium</italic> plays an essential role supporting ocean productivity by relieving nitrogen limitation <italic>via</italic> dinitrogen (N<sub>2</sub>) fixation. The two common <italic>Trichodesmium</italic> clades, <italic>T. erythraeum</italic> and <italic>T. thiebautii</italic>, are both observed in waters along the West Florida Shelf (WFS). We hypothesized that these taxa occupy distinct realized niches, where <italic>T. thiebautii</italic> is the more oceanic clade. Samples for DNA and water chemistry analyses were collected on three separate WFS expeditions (2015, 2018, and 2019) spanning multiple seasons; abundances of the single copy housekeeping gene <italic>rnpB </italic>from both clades were enumerated <italic>via</italic> quantitative PCR. We conducted a suite of statistical analyses to assess <italic>Trichodesmium</italic> clade abundances in the context of the physicochemical data. We observed a consistent coastal vs. open ocean separation of the two clades: <italic>T. erythraeum</italic> was found in shallow waters where the concentrations of dissolved iron (dFe) and the groundwater tracer Ba were significantly higher, while <italic>T. thiebautii </italic>abundance was positively correlated with water column depth. The Loop Current intrusion in 2015 with entrained Missisippi River water brought higher dFe and elevated abundance of both clades offshore of the 50 m isobath, suggesting that both clades are subject to Fe limitation on the outer shelf. Whereas, previous work has observed that <italic>T. thiebautii</italic> is more abundant than <italic>T. erythraeum</italic> in open ocean surface waters, this is the first study to examine <italic>Trichodesmium</italic> niche differentiation in a coastal environment. Understanding the environmental niches of these two key taxa bears important implications for their contributions to global nitrogen and carbon cycling and their response to global climate change.</p>