Contrasting effects of solar radiation on dissolved organic matter and its bioavailability to marine bacterioplankton

The effect of ultraviolet radiation on the bioavailability of dissolved organic matter (DOM) to marine bacterioplankton was investigated in the Mediterranean Sea (Strait of Gibraltar, South and North Aegean Sea) and in the Caribbean Sea off Curacao. Surface-water samples (collected between 1 and 85 m in depth) exposed to solar radiation did not show a distinct pattern in subsequent bacterial growth. However, samples collected from a pronounced chlorophyll maximum (two stations in the Strait of Gibraltar and four stations in the Aegean Sea) displayed in a 50% lower bacterial activity in the radiation-exposed treatments compared with the dark controls. In contrast, mesopelagic water samples (200-350 m in depth) exposed to surface solar radiation exhibited a two- to fourfold increase in bacterial activity compared with the corresponding dark controls. Addition of the model protein bovine serum albumin (BSA) to mesopelagic-water samples and subsequent exposure to solar radiation resulted in a 50% lower rate of bacterial activity compared with the dark treatments, thus indicating the occurrence of photochemically induced changes to this labile compound. This decrease in bacterial activity in BSA-amended, irradiated water was also detectable in surface waters sampled off Curacao, whereas BSA amendment to surface water from the Mediterranean Sea did not result in a distinct pattern of bacterial activity. Our data indicate that exposure of DOM to solar radiation causes a reduced bioavailability of the exposed DOM to bacterioplankton, if the bacterial activity:dissolved organic carbon (DOC) concentration ratio is high (indicative of labile DOM). If the bacterial activity:DOC ratio is low (indicative of more refractory DOM), the bioavailability of the DOM is increased upon exposure to solar radiation.