Table 1 Chemical characterisation of different humic materials

Substances	mg C-Carb mg⁻¹ S	mg eHS mg⁻¹ S	nM Fe-L mg⁻¹ S	Log K_Fe’L
EPS bacteria	0.189 ± 0.034 (3)^9,39	0.011 ± 0.008 (2) ^9,39	4.724 ± 2.374 (4) ^9,39^,***	11.38 ± 0.25 (4) ^9,39,***
EPS phytoplankton	0.141 ± 0.040 (3) ^9,39	0.104 ± 0.025 (2)³⁹	62.63 ± 31.22 (2)³⁹	11.65 ± 0.27 (2)³⁹
EPS in situ	0.109 ± 0.011 (2) ^9,39	0.164 (1)³⁹	68.07 (1)³⁹	10.91 (1)³⁹
Carb - PS	0.101 (1)⁹	0.034 (1)⁹	0.376 (1)⁹	10.90 (1)⁹
Carb -MS	1.010 (1)⁹	0.075 (1)⁹	1.231 (1)⁹	9.855 ± 1.06 (2)^9,46
FA	0.077 (1)^,*	1.02 (1)⁹	16.70 (1)³²	10.60 (1)³²
HA	0.060 (1)^,*	--	31.90 (1)³²	11.10 (1)³²

Average contribution to hydrolysable carbohydrates (Carb, expressed as glucose equivalent), electroactive humic substances (eHS, expressed as Suwanee River fulvic acids equivalent), iron-binding ligands (Fe-L) and their conditional stability constant (log K_Fe’L) for different organic substances (S). Organic substances were chosen as compounds potentially controlling Fe biogeochemistry and include: Bacterial, phytoplankton and in situ exopolymeric substances (EPS), polysaccharides (carrageenan, PS), monosaccharides (glucuronic acid, MS) and Suwanee River humic substances (fulvic acids, FA, and humic acids, HA). Number of observations (n) is shown in brackets; average is presented with standard deviation for n ≥ 3 and with half-data gaps for n = 2, ND not detected.
^*estimated based on carboxylic carbon content^**, International humic substances data for SRFA and SRHA std 1^***, Hassler unpublished data.

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