Photoalteration of dissolved organic matter (DOM) released from Microcystis aeruginosa in different growth phases: DOM-fraction distribution and biodegradability

Abstract

The photoalteration of dissolved organic matter (DOM) produced in different growth phases of the blue-green alga, Microcystis aeruginosa, was investigated by comparing the biodegradability and distribution of fractions of algal DOM after different ultraviolet (UV) treatments. The distribution of DOM-fractions (based on hydrophobic-hydrophilic and acidic-basic breaks) showed that two of the fractions, hydrophilic acids (HiA) and bases (HiB), were more abundant in all growth phases of M. aeruginosa than the other three fractions, hydrophobic acids (HoA), hydrophobic neutrals (HoN), and hydrophilic neutrals (HiN). The proportion of HiB increased, while the HiA fraction decreased with aging of the algae. After UV treatment, all algal DOM became recalcitrant to bacterial degradation without complete photo-degradation. This was more pronounced in DOM from older cultures (stationary phase) as compared to DOM from the exponential growth phase. The DOM distribution was also significantly different after UV exposure, implying photoalteration to the chemical composition of algal DOM. The proportions of the HiB fraction decreased as a percent of the total dissolved organic carbon pool by 1.5-8.1 % after UVA treatment and by 5.3-15.8 % after UVB treatment. In contrast, the HiA fraction increased by similar amounts. Analyses of fluorescent properties and some carboxylic acids confirmed the changes to the HiB and HiA fractions. However, the increased HiA fraction may not be linked to the recalcitrance of algal DOM after UV exposure, since there was no difference in the biodegradability of this fraction before and after UV treatments. The initially labile HiB fraction, however, became less available to bacteria after UV exposure depending on intensity and thus may be linked to the recalcitrance of algal DOM after UV exposure. Our results confirm earlier reports that algal DOM can be changed in its chemical composition as well as biodegradability by UV radiation, and suggest that the HiB fraction may be important in the formation of recalcitrant algal DOM.