Photochemical transformations of surface and deep marine dissolved organic matter: Effects on bacterial growth

The effects of photochemical transformations on the bioavailability of marine dissolved organic matter (DOM) were investigated in surface and deep water from the Gulf of Mexico. Seawater samples were collected from eight depths (15–1,000 m), passed through 0.2‐µm pore‐size filters, and exposed to sunlight in quartz bottles in a flowing seawater deck incubator for 5‐9 h. Following sunlight exposure, samples were inoculated (1:10) with unfiltered seawater from 15‐m depth, and bacterial growth rates were estimated from rates of 3 H‐leucine incorporation in dark incubations. Exposure of surface‐water DOM to sunlight resulted in a 75% reduction in bacterial production, whereas exposure of deep‐water DOM resulted in a 40% enhancement in bacterial production. Photomineralization of bioreactive DOM likely contributed to the reduction of bacterial growth in surface water, but the photoproduction of biorefractory DOM also appeared to contribute to reduced bacterial growth. Enhanced bacterial growth in irradiated deep water was consistent with previous studies demonstrating the photoproduction of bioavailable substrates from deep‐water DOM. Phototransformations of DOM appeared to be multifaceted and to play a critical role in the cycling of DOM in the ocean.