Figure 4.

Relationships between log₁₀ (isiB/rnpB) of the Tten clade and log₁₀ ([Fe]/[PO₄]) (a), and dissolved [Fe] (b). Three dashed horizontal lines are shown on each plot. The bottom line is at the value log₁₀(isiB/rnpB)=−2.09, which is the value associated with basal expression in the Tten clade from previous work (Chappell and Webb, 2010). The middle line at the value log₁₀(isiB/rnpB) =−1.20, which is the value associated with a 50% reduction in N₂ fixation in the Tten clade from previous work that we use as a threshold value to indicate Fe limitation of N₂ fixation in Tten (Chappell and Webb, 2010). The top line is at the value log₁₀(isiB/rnpB)=−0.84, which is the highest expression value measured in Tten laboratory cultures (Chappell and Webb, 2010). Values from the Sargasso Sea are plotted as filled squares, values from the Equatorial Atlantic are plotted as open squares and values from the Pacific are plotted as open circles. The solid line in (a) represents the linear regression of log₁₀ ([Fe]/[PO₄]) verses log₁₀ (isiB/rnpB) with the dashed curves on either side representing the 95% confidence intervals. The equation for this line is log₁₀ (isiB/rnpB)=(−0.60±0.14) × log₁₀ ([Fe]/[PO₄]) + (−2.7±0.28), which has an R² value of 0.50. The solid line in (b) represents the linear regression of [Fe] versus log₁₀ (isiB/rnpB) excluding the data point that is circled, which was influenced by the Amazon River Plume (Station E21). The dashed curves on either side represent the 95% confidence intervals for this regression. The equation for this line is log₁₀ (isiB/rnpB)=(−1.51±0.38) × [Fe]+(−0.7±0.26), which has an R² value of 0.47. Error bars represent the s.d. of triplicate analyses.