As Arctic multiyear ice is being replaced with first-year sea ice, a key question pertains to the cycling and fate of the increasingly dominant first-year ice carbon upon its release into the water column during melt. In this study, we experimentally tested the hypothesis that changes in sea ice dissolved organic matter (DOM) fractions affect the response of under-ice microbial communities and, as a corollary, the cycling of sea ice carbon in surface waters.Size-fractionated DOM fractions were isolated from first-year ice, producing three pools of dissolved organic carbon comprising the full DOM spectrum, large exopolymeric substances (EPS > 100 KDa molecular weight), and small EPS (< 100 KDa and > 10 KDa molecular weight). Enrichment experiments with these fractions revealed, over a 216 h period, significant and contrasting changes in prokaryotic abundance and production, substrate utilization rates, and microbial diversity between enrichments. The different DOM fractions differed in terms of molecular composition, elemental ratios and carbohydrate contribution, with the full DOM spectrum showing the highest proportion of carbohydrates and highest number of low-molecular weight unique compounds. Despite an abundance of low-molecular weight compounds in the DOM enrichment, the EPS fractions induced the strongest and fastest response of underice microbial communities, with highest growth rates and substrate utilization for the small EPS fraction. Shifts in microbial diversity were also observed over the course of the experiment, resulting in different community composition between enrichments. Notably, the EPS and DOM enrichments led to a dominance of Colwellia and Psychromonas, respectively.The results support our hypothesis, showing the fundamental role of the composition of the sea ice dissolved organic matter (DOM) pool and its dual impact, through changes in microbial diversity and substrate utilization, on organic matter cycling at the sea-ice interface. These findings have far-reaching consequences in terms of the cycling and export of sea ice associated carbon in the climatically-impacted Arctic and its role in global biogeochemical cycles.