Defining the Realized Niche of the Two Major Clades of Trichodesmium: A Study on the West Florida Shelf
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abstract
The cyanobacteriumTrichodesmiumplays an essential role supporting ocean productivity by relieving nitrogen limitation via dinitrogen (N2) fixation. The two common Trichodesmium clades,T. erythraeumandT. thiebautii, are both observed in waters along the West Florida Shelf (WFS). We hypothesized that these taxa occupy distinct realized niches, whereT. thiebautiiis 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 genernpBfrom both clades were enumerated via quantitative PCR. We conducted a suite of statistical analyses to assess Trichodesmiumclade abundances in the context of the physicochemical data. We observed a consistent coastal vs. open ocean separation of the two clades:T. erythraeumwas found in shallow waters where the concentrations of dissolved iron (dFe) and the groundwater tracer Ba were significantly higher, whileT. thiebautiiabundance 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 50m isobath, suggesting that both clades are subject to Fe limitation on the outer shelf. Whereas, previous work has observed thatT. thiebautiiis more abundant thanT. erythraeumin open ocean surface waters, this is the first study to examineTrichodesmiumniche 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.
