Macroalgae, often referred to as seaweeds, are rightfully recognized as ecosystem engineers, given that they are the most productive marine macrophytes, thriving in intertidal coastal zones. They are increasingly acknowledged for their role in global carbon cycling, as they exhibit shorter growth cycles, higher photosynthetic rates, and high primary productivity. This enables them to sequester atmospheric CO2 effectively, an essential function in the pursuit of carbon neutrality. Seaweed ecosystems contribute to oceanic carbon sequestration through the production of both dissolved and particulate organic carbon. Notably, seaweeds are known to produce highly recalcitrant compounds, some of which have been reported to get buried in sediments or exported to the deep sea, allowing for carbon storage on millennial timescales. In addition, macroalgae can absorb sulfur and nitrogen oxides, further contributing to the reduction of atmospheric pollutants and greenhouse gas equivalents. The full potential of macroalgae in achieving carbon neutrality is best realized when considering their intricate associations with diverse microbial assemblages. These microbiomes, comprising bacteria, archaea, and fungi, form a functionally integrated consortium with their macroalgal hosts that support essential physiological processes, including nutrient acquisition, organic matter transformation, stress tolerance, and even morphogenesis. Macroalgae are known to enrich microbial communities involved in carbon and nitrogen cycling, further amplifying their role in regulating ecosystem-level carbon dynamics. This chapter examines the synergistic roles of macroalgae and their associated microorganisms in facilitating carbon capture and long-term storage. It highlights microbial contributions to algal growth optimization, carbon flux modulation through decomposition and mineralization, and long-term carbon burial through particulate organic matter. With the aid of emerging genomic and metagenomic tools, the complexity and metabolic versatility of these macroalgae-associated microbiomes are being progressively understood. Beyond their role in carbon neutrality, these biomes support ecosystem resilience and marine biodiversity, placing them at the forefront of nature-based climate mitigation strategies.