Beach aquifers, located in the subsurface of sandy beaches, are unique ecosystems with steep chemical and physical gradients resulting from the mixing of terrestrial fresh groundwater and saline groundwater from the sea. While work has rapidly progressed to understand the physics and chemistry in this environment, much less is known about the microorganisms present despite the fact that they are responsible for vital biogeochemical processes. This paper presents a review of the current state of knowledge of microbes within beach aquifers and the mechanisms that control the beach aquifer microbiome. We review literature describing the distribution and diversity of microorganisms in the freshwater-saltwater mixing zone of beach aquifers, and identify just 12 papers. We highlight knowledge gaps, as well as future research directions: The understanding of beach aquifer microorganisms is informed primarily by 16S ribosomal RNA gene sequences. Metagenomics and metatranscriptomics have not yet been applied but are promising approaches for elucidating key metabolic and ecological roles of microbes in this environment. Additionally, variability in field sampling and analytical methods restrict comparison of data across studies and geographic locations. Further, documented evidence on the migration of microbes within the beach aquifer is limited. Taking into account the physical transport of microbes through sand by flowing groundwater may be critical for understanding the structure and dynamics of microbial communities. Quantitative measurements of rates of elemental cycling in the context of microbial diversity need further investigation, in order to understand the roles of microbes in mediating biogeochemical fluxes from the beach aquifer to the coastal ocean. Lastly, understanding the current state of beach aquifers in regulating carbon stocks is critical to foster a better understanding of the contribution of the beach aquifer microbiome to global climate models.

中文翻译：

---

沙滩含水层微生物组：研究差距和数据需求

位于沙滩地下的海滩含水层是独特的生态系统，其化学和物理梯度很陡，这是由于陆地新鲜淡水和海水中的地下水混合而成的。尽管了解这种环境中的物理和化学的工作已经迅速发展，但是尽管存在微生物对重要的生物地球化学过程起重要作用，但对它们的了解却很少。本文介绍了滩涂含水层中微生物的最新知识以及控制滩涂含水层微生物组的机制。我们回顾了描述海滩含水层淡水-盐水混合区中微生物分布和多样性的文献，仅发现了12篇论文。我们重点介绍知识差距以及未来的研究方向：对海滩含水层微生物的了解主要是通过16S核糖体RNA基因序列进行的。元基因组学和元转录组学尚未应用，但对于阐明这种环境中微生物的关键代谢和生态作用是很有前途的方法。此外，现场采样和分析方法的可变性限制了跨研究和地理位置的数据比较。此外，关于微生物在海滩含水层内迁移的文献证据有限。考虑到微生物通过流动的地下水通过沙子的物理传输对于理解微生物群落的结构和动力学可能至关重要。微生物多样性背景下元素循环速率的定量测量需要进一步研究，为了了解微生物在介导从海滩含水层到沿海海洋的生物地球化学通量中的作用。最后，了解海滩含水层在调节碳储量方面的现状对于培养对海滩含水层微生物组对全球气候模型的贡献的更好理解至关重要。