Ultraviolet radiation blocks the organic carbon exchange between the dissolved phase and the gel phase in the ocean

Dissolved organic carbon (DOC) is one of the major reservoirs of active organic carbon on Earth. Although the bulk of the marine DOC pool is largely composed of small refractory polymeric material, new evidence suggests that similar to10% of the DOC pool (10(16) g C) can enter the microbial loop by forming microscopic gels that can eventually be colonized and degraded by bacteria. Marine microgels result from a spontaneous and reversible assembly/dispersion equilibrium of DOC polymers forming hydrated Ca-bonded tangled polymer networks. Here we test the hypothesis that ultraviolet (UV) photocleavage should strongly inhibit the formation of microgels, because the stability of tangled networks decreases exponentially with polymer length. Because of the loss of ozone shielding, the UV-B spectral component of solar radiation (lambda = 280-320 nm) has undergone a dramatic increase in the past few decades, particularly in the polar regions. We used dynamic laser-scattering spectroscopy and flow cytometry to investigate UV-induced DOC polymer cracking and the effect of UV on DOC assembly/dispersion equilibrium in 0.2 mum filtered seawater. Results indicate that exposure of seawater to UV-B fluxes equivalent to those found in Antarctica during summer solstice can cleave DOC polymers, inhibit their spontaneous assembly, and/or disperse assembled microgels. Our results agree with previous observations that indicated that fragmentation produced by UV photolysis increases exponentially with exposure time and suggested that UV could limit the supply of microbial substrate by hindering microgel formation. UV cleavage yields short-chain polymers that do not assemble and could eventually account for the old refractory DOC pool found in seawater.