Algal polysaccharides constitute a diverse and abundant reservoir of organic matter for marine heterotrophic bacteria, central to the oceanic carbon cycle. We investigated the uptake of alginate, a major brown macroalgal polysaccharide, by microbial communities from kelp-dominated coastal habitats. Congruent with cell growth and rapid substrate utilization, alginate amendments induced a decrease in bacterial diversity and a marked compositional shift towards copiotrophic bacteria. We traced ¹³C derived from alginate into specific bacterial incorporators and quantified the uptake activity at the single-cell level, using halogen in situ hybridization coupled to nanoscale secondary ion mass spectrometry (HISH-SIMS) and DNA stable isotope probing (DNA-SIP). Cell-specific alginate uptake was observed for Gammaproteobacteria and Flavobacteriales, with carbon assimilation rates ranging from 0.14 to 27.50 fg C µm⁻³ h⁻¹. DNA-SIP revealed that only a few initially rare Flavobacteriaceae and Alteromonadales taxa incorporated ¹³C from alginate into their biomass, accounting for most of the carbon assimilation based on bulk isotopic measurements. Functional screening of metagenomic libraries gave insights into the genes of alginolytic Alteromonadales active in situ. These results highlight the high degree of niche specialization in heterotrophic communities and help constraining the quantitative role of polysaccharide-degrading bacteria in coastal ecosystems.

藻类多糖为海洋异养细菌构成了多样化且丰富的有机物库，是海洋碳循环的核心。我们研究了海藻酸盐（一种主要的棕色巨藻多糖）被来自海带为主的沿海生境的微生物群落的吸收。与细胞生长和快速底物利用相一致，藻酸盐改良剂引起细菌多样性的减少和向富营养型细菌的显着组成转变。我们使用卤素原位杂交结合纳米级二次离子质谱 (HISH-SIMS) 和 DNA 稳定同位素探测 (DNA-SIP)，将来自藻酸盐的 13 C 追踪到特定的细菌掺入器中，并在单细胞水平上量化吸收^活性。观察到Gammaproteobacteria和Flavobacteriales的细胞特异性藻酸盐吸收，碳同化率范围为 0.14 至 27.50 fg C µm ^-3 h ^-1。DNA-SIP 显示，只有少数最初罕见的黄杆菌科和互单胞菌类群将来自藻酸盐的¹³ C 纳入其生物量，占基于大量同位素测量的大部分碳同化。宏基因组文库的功能筛选深入了解了原位活性的藻素解互单胞菌基因。这些结果凸显了异养群落的高度专业化，并有助于限制多糖降解细菌在沿海生态系统中的定量作用。