Dissolved proteins were assessed in surface and deep seawater by two-dimensional isoelectric focusing (IEF) OFFGEL-lab-on-chip (LOC) electrophoresis after tangential flow ultrafiltration followed by centrifugal ultrafiltration (preconcentration factor of 3000). Dissolved protein isolation was performed by treating the ultrafiltrated retentate with cold acetone and also with chloroform as precipitating reagents. The best electrophoretic behavior of the isolated proteins was obtained after protein precipitation with chloroform before different rinsing stages for removing methanol and water interferences. Metals bound to proteins in the different OFFGEL fractions were assessed by inductively coupled plasma-optical emission spectrometry and electrothermal atomic absorption spectrometry, under optimized operating conditions. Experiments regarding stability of the metal-binding proteins [superoxide dismutase (SOD) and alcohol dehydrogenase (ADH) as protein models] showed the integrity of the Zn-binding SOD/ADH under the OFFGEL electrophoretic conditions. However, stability of Cu bound to SOD is not guaranteed. The first electrophoretic dimension (IEF OFFGEL) showed that dissolved proteins in surface seawater exhibit alkaline isoelectric points (pIs of 8.10 and 8.37) and also acid Ips (4.82, 5.13, 5.43, and 5.73), while LOC showed that the isolated proteins exhibit a spread molecular weight range (within 15 - 63 kDa); although, high molecular weights were the most commonly found. Regarding deep seawater, isolated proteins were of acid Ips (from 3.30 to 4.22) and low molecular weight (within the 21-24 kDa range). Elements such as Cd, Cu, Mn, and Ni were mainly associated with dissolved proteins of alkaline pIs in surface seawater, while Zn was mainly associated to proteins of acid pIs. However, only Cu and Mn were found to be bound to dissolved proteins of higher Ips in deep seawater, and the amount of Mn (from 68 to 84 μg L(-1)) was higher than that found in dissolved proteins in surface seawater (22.4 μg L(-1)).