Table_3_Diverse Bacteria Utilize Alginate Within the Microbiome of the Giant Kelp Macrocystis pyrifera.XLSX

Bacteria are integral to marine carbon cycling. They transfer organic carbon to higher trophic levels and remineralise it into inorganic forms. Kelp forests are among the most productive ecosystems within the global oceans, yet the diversity and metabolic capacity of bacteria that transform kelp carbon is poorly understood. Here, we use 16S amplicon and metagenomic shotgun sequencing to survey bacterial communities associated with the surfaces of the giant kelp Macrocystis pyrifera and assess the capacity of these bacteria for carbohydrate metabolism. We find that Macrocystis-associated communities are distinct from the water column, and that they become more diverse and shift in composition with blade depth, which is a proxy for tissue age. These patterns are also observed in metagenomic functional profiles, though the broader functional groups—carbohydrate active enzyme families—are largely consistent across samples and depths. Additionally, we assayed more than 250 isolates cultured from Macrocystis blades and the surrounding water column for the ability to utilize alginate, the primary polysaccharide in Macrocystis tissue. The majority of cultured bacteria (66%) demonstrated this capacity; we find that alginate utilization is patchily distributed across diverse genera in the Bacteroidetes and Proteobacteria, yet can also vary between isolates with identical 16S rRNA sequences. The genes encoding enzymes involved in alginate metabolism were detected in metagenomic data across taxonomically diverse bacterial communities, further indicating this capacity is likely widespread amongst bacteria in kelp forests. Overall, the M. pyrifera epibiota shifts across a depth gradient, demonstrating a connection between bacterial assemblage and host tissue state.

细菌在海洋碳循环中扮演着至关重要的角色，它们将有机碳转移至更高的营养层次，并将其重新矿化为无机形态。海藻森林是全球海洋中生产力最为丰富的生态系统之一，然而，关于转化海藻碳的细菌多样性和代谢能力，我们的理解却十分有限。在本研究中，我们运用16S扩增子测序和宏基因组鸟枪法测序技术，对巨型海带Macrocystis pyrifera表层的细菌群落进行了调查，并评估了这些细菌进行碳水化合物代谢的能力。我们发现，与Macrocystis关联的细菌群落与水体中的细菌群落存在显著差异，且随着叶片深度的增加（即组织年龄的替代指标），其多样性亦随之提高，成分亦发生转变。这些模式在宏基因组功能分析中亦得到体现，尽管更广泛的功能群体——碳水化合物活性酶家族——在样本和深度上总体保持一致。此外，我们还对从Macrocystis叶片及其周围水体中分离培养的超过250个菌株进行了藻酸盐利用能力的检测，藻酸盐是Macrocystis组织中的主要多糖。大多数培养细菌（66%）表现出了这种能力；我们发现，藻酸盐的利用在Bacteroidetes和Proteobacteria的多个属中分布不均，甚至在具有相同16S rRNA序列的分离株之间也存在差异。编码参与藻酸盐代谢的酶的基因在宏基因组数据中跨越了多种分类学上的细菌群落被发现，进一步表明这种能力在海带森林中的细菌中可能广泛存在。总体而言，M. pyrifera的共生生物群落随着深度梯度的变化而变化，这证明了细菌群落与宿主组织状态之间的联系。