Metabolic differences between attached and free‐living marine bacteria: inadequacy of liquid cultures for describing in situ bacterial activity

Marinobacter sp. strain CAB was cultivated with or without porous glass beads as solid support. Two substrates were used: the hydrophilic sodium lactate and a hydrophobic C 18 ‐isoprenoid ketone (6,10,14‐trimethylpentadecan‐2‐one (TMP)). The substrate adsorption onto the beads was measured. Bacterial adhesion was determined by a direct count technique and amounted to 70% of total cells. In the immobilised cell cultures (ICC), generation times were 1.5 and 1.8 times shorter than in the planktonic cultures (FCC) with sodium lactate and with TMP, respectively. In ICC, the growth yields were lower (15.3 FCC ×10 9 and 0.8 ICC ×10 9 bacteria mg −1 of sodium lactate; 50 FCC ×10 9 and 35 ICC ×10 9 bacteria mg −1 of TMP). The mineralisation of substrates was estimated after mass spectrometric determination of the CO 2 production rates of both free and immobilised cell cultures. The results indicated a higher specific CO 2 production rate in the ICC with sodium lactate (3.1 FCC ±0.2 and 3.5 ICC ±0.3 nmol CO 2 mg −1 protein min −1 ) but not in the ICC with TMP (1.9 FCC ±0.7 and 0.5 ICC ±0.3 nmol CO 2 mg −1 protein min −1 ). The affinities for the two substrates were lower in the presence of the solid support ( K m,ICC =18.2±0.2 μM and 37.1±2.0 μM, for sodium lactate and TMP, respectively) than without support ( K m,FCC =8.5±1.5 μM and 8.4±1.2 μM, for sodium lactate and TMP, respectively). Moreover, the presence of a solid support showed a lower inhibition by the TMP ( K i,FCC =3.8±1.0 μM and K i,ICC =12.2±2.5 μM) which may explain why the immobilised cell cultures degraded hydrophobic TMP more efficiently than the planktonic cultures.