The samples were withdrawn from each tube periodically and measured for optical density using a GeneSys 50 spectrophotometer (ThermoFisher Scientific; Waltham, MA, USA). An aliquot of this was removed for direct cell counts using a Zeiss Axioscope 40 fluorescence microscope (ZEISS; Dublin, CA, USA) after staining with the LIVE/DEAD™ BacLight™ nucleic acid stain (Invitrogen/Life Technologies; Carlsbad, CA, USA). The remainder of the sample was passed through an Amicon 100 kD centrifugal filter to remove biomass. A portion of the filtrate was diluted in 2% nitric acid containing an indium internal standard and analyzed for Nd concentration using an Agilent 7900 inductively coupled plasma mass spectrometer (Agilent Technologies; Santa Clara, CA, USA). Replicate filtrates were pooled for pH measurements.
The percentage of biologically influenced Nd was determined from the difference between the abiotic and biotic sample concentrations divided by the abiotic sample concentration. A higher percentage of influence indicates that more Nd was removed from the solution.
The percentage of biologically influenced Nd was determined from the difference between the abiotic and biotic sample concentrations divided by the abiotic sample concentration. A higher percentage of influence indicates that more Nd was removed from the solution.