Biophysical contexture of coastal biofilm-sediments varies heterogeneously and seasonally at the centimeter scale across the bed-water interface

Coastal sediments filter and accumulate organic and inorganic materials from the terrestrial and marine environment, and thus provide a high diversity of microbial niches. However, sediment-based analyses typically examine bulk samples and seldom consider variation at a scale relevant to changes in environmental conditions, due to the lack of mid-long term field data which can cover both the seasonal and sediment depth variations. In this study, microbial production and bacterial community structure were determined together with grain parameters over 10 months of intertidal silty sands on Jiangsu Coast, China. We demonstrated that the microbiological effects did not merely present on the surface, but greatly varied and stratified in both physical and biological contexture within the top 4 cm layer. Bacterial community structure showed a clear vertical variation with higher operational taxonomic unit (OTU) numbers at 1~2 cm depth than in the top 2 mm, probably because of the decreasing disturbance by hydrodynamic forces. However, the microbial production rates and metabolic activities, represented by the production of extracellular polymeric substances (EPS), were always higher in the top. Seasonal changes were strongly reflected in the vertical patterns of EPS but could not explain the variation across sites. The overall EPS secretion in spring and summer was generally at high level than that in autumn and winter, with the maximum value of 5~6 times higher. Interestingly, the stratification of biological and physical properties followed a fixed relationship, where with the decrease of the grain size D50, the EPS content increased exponentially, and this relationship was independent of temporal or spatial variation. Despite the significant seasonal variation of microbial activity and sedimentary grain size individually, the basic function between EPS content and D50 however did not alter. Filling these knowledge gaps will not only help to decipher the fate of grain-biofilm aggregates and organic matter burial under global changes, but also provide field evidence for the development of sediment transport models as well as blue carbon models incorporating microbial processes.

海岸沉积物可过滤并累积陆地与海洋环境中的有机及无机物质，由此孕育出高度多样的微生物生态位。然而，基于沉积物的分析通常仅针对整体样品开展，且极少考虑与环境条件变化相关尺度下的异质性——这是由于缺乏能够覆盖季节与沉积物深度变化的中长期野外监测数据。本研究针对中国江苏海岸的潮间带粉砂质砂开展了为期10个月的监测，同步测定了微生物生产、细菌群落结构以及沉积物粒度参数。研究发现，微生物相关效应并非仅存在于沉积物表层，而是在表层4 cm范围内的物理与生物组成中均呈现显著变化与分层特征。细菌群落结构表现出清晰的垂直分异：1~2 cm深度处的操作分类单元（OTU）数量显著高于表层2 mm，这可能与水动力扰动的减弱有关。不过，以胞外聚合物（EPS）产量表征的微生物生产速率与代谢活性，始终在表层处于更高水平。季节变化对EPS的垂直分布模式具有显著影响，但无法解释不同采样位点间的差异。春季与夏季的整体EPS分泌量普遍高于秋冬两季，峰值可达秋冬的5~6倍。值得注意的是，沉积物物理与生物特性的分层关系遵循固定规律：随着粒度中值D50的降低，EPS含量呈指数增长，且该关系不受时间与空间变异的干扰。尽管微生物活性与沉积物粒度各自存在显著的季节变异，但EPS含量与D50之间的基本函数关系并未发生改变。填补这一研究空白，不仅有助于解析全球变化背景下颗粒-生物膜聚集体的归宿与有机质埋藏过程，还可为沉积物输运模型以及纳入微生物过程的蓝碳模型的开发提供野外实证依据。