The effect of ultraviolet irradiation on diatom-derived dissolved organic matter used as a bacterial growth substrate was investigated. The dissolved organic matter fraction of 100–1000 daltons (low molecular weight) was prepared from a culture of a marine diatom, Chaetoceros sp. This dissolved matter was then exposed to ultraviolet-A and -B irradiation at naturally occurring intensities of 0–44 kJ m-2. The concentration of low-molecular-weight dissolved organic matter decreased as ultraviolet irradiation increased, which was a reflection of photolytic mineralization of the material.
The irradiated organic matter was added to the culture of a Vibrio strain to assess its bioavailability. Bacterial growth was enhanced with the irradiated material, which was probably due to the photolytic production of more bioavailable monomeric substrates. The loss of the low-molecular-weight dissolved organic material concentration was counterbalanced by enhanced bioavailability. Thus, ultraviolet irradiation of the natural diatom-derived low-molecular-weight dissolved organic matter pool may be net-positive for the growth of marine bacteria.