Oxygen supply for biostimulation of enzymatic activity in organic-rich marine ecosystems

Abstract

The depletion of oxygen and the use of high energy cost electron acceptors for mineralisation processes are considered as one of the main reasons limiting degradation rates in aquatic organic-rich ecosystems. In this research the effect of two different biostimulation oxygen-releasing protocols were investigated using extracellular β-glucosidase activity rates. Organic-rich sediment and water was collected from the harbour of Genoa (Italy, North-Western Mediterranean) and oxygenation was generated either by positioning air tubes into the overlying water, or by placing oxygen release compounds (ORC) directly into the sediment, in laboratory microcosms. The increase in enzymatic activity was significant in the water compartment of the oxygenated microcosms (ANOVA, p<0.05) and greater for the ORC treatment. In particular, in the water, air tubes caused an increase in the β-glucosidase activity during the first 24 h (from 18.1 nmol l −1 h −1 ( T=0, before the treatment) to 50.9 nmol l −1 h −1 ( T=24 h)), while ORC induced a β-glucosidase activity increase over the whole period (from 12.4 nmol l −1 h −1 ( T=0) to 63.7 nmol l −1 h −1 ( T=60 d)) together with a reduction in carbohydrate (from 1.91 mg l −1 ( T=0) to 0.76 mg l −1 ( T=60 d)) and a sedimentary pH increase (from 8.04 ( T=0) to 8.22 ( T=60 d)). The response to ORC treatment in the overlying water was also evident in the potential carbohydrate turnover rate, decreasing from 0.86 h ( T=0) to 0.06 h ( T=60 d) and cell specific enzymatic activity, increasing from 3.8 ( T=0) to 22.8 nmol l −1 h −1 cell −1 ( T=60 d). A less marked change was observed in the sediment. This research shows that the supply of oxygen directly into the sediment rather than water aeration is a better way of naturally enhancing enzymatic degradation rates in organic-rich marine ecosystems. The marked effect observed in the water following the sediment treatment suggests the occurring of a close association of decomposition processes within water and sediment compartments.