Table_1_Acclimation of marine invertebrate osmolyte systems to low salinity: A systematic review & meta-analysis.xlsx

Salinity is a major environmental factor shaping the distribution and abundance of marine organisms. Climate change is predicted to alter salinity in many coastal regions due to sea level rise, evaporation, and changes in freshwater input. This exerts significant physiological stress on coastal invertebrates whose body fluid osmolality follows that of seawater (‘osmoconformers’). In this study, we conducted a systematic review and meta-analysis of osmolytes (both organic and inorganic) utilized by osmoconforming marine invertebrates during a >14-day acclimation to reduced salinity. Of the 2,389 studies screened, a total of 56 fulfilled the search criteria. Thirty-eight studies reported tissue osmolyte. Following acclimation to reduced salinity, tissue concentrations of six organic compounds and sodium were consistently reduced across phyla. This suggests that intracellular inorganic ions are not only utilized as a rapid response system during acute exposure to low salinity stress but also, in concert with reductions in organic osmolyte concentrations, during longer-term acclimation. Our systematic review demonstrates that only a few studies (n = 13) have quantified salinity-induced long-term changes in intracellular ion concentrations. In addition, no study has compiled a complete intracellular osmolyte budget. Alanine, betaine, glycine, and taurine are the major organic osmolytes that are universally employed across five phyla. The characterization of organic osmolytes was heavily weighted towards free amino acids (FAAs) and derivatives—neglecting methylamines and methylsulfonium compounds, which can be as important as FAAs in modulating intracellular osmolality. As a consequence, we suggest best-practice guidelines to streamline experimental designs and protocols in osmoregulation research in order to better understand the conserved mechanisms that define the limits of salinity acclimation in marine invertebrates. To our best knowledge, this is the first systematic review and meta-analysis on osmolyte concentrations in osmoconformers acclimated to low salinity. It creates a valuable baseline for future research and reveals large research gaps. Our meta-analysis suggests that there are common osmolyte actors employed across phyla but no uniform concept since osmolyte pool composition and proportions were taxon-specific. In light of future salinity changes and their potential consequences, it becomes more important to understand salinity tolerance capacities and limits.

盐度（Salinity）是塑造海洋生物分布与种群丰度的核心环境因子。据预测，气候变化将通过海平面上升、蒸发作用以及淡水输入量的改变，对众多沿海区域的盐度产生扰动。这会对体液渗透压随海水变化的沿海无脊椎动物——即“渗透压顺应者（osmoconformer）”——造成显著生理胁迫。
本研究针对渗透压顺应型海洋无脊椎动物在14天以上的低盐驯化过程中所利用的渗透溶质（osmolyte，包括有机与无机两类）开展了系统综述（systematic review）与元分析（meta-analysis）。在筛选的2389项研究中，仅有56项符合检索标准。其中38项报道了组织渗透溶质的含量变化。经低盐驯化后，跨多个动物门的受试生物体内六种有机化合物与钠离子的组织浓度均持续下降。这表明，细胞内无机离子不仅是急性低盐胁迫暴露时的快速响应系统，还可与有机渗透溶质浓度降低协同作用，参与长期驯化过程。
本系统综述显示，仅有13项研究量化了盐度诱导的细胞内离子浓度长期变化。此外，尚无研究完整统计细胞内渗透溶质收支情况。丙氨酸、甜菜碱、甘氨酸与牛磺酸是五大动物门普遍使用的核心有机渗透溶质。现有研究对有机渗透溶质的表征高度聚焦于游离氨基酸（free amino acids, FAAs）及其衍生物，却忽略了甲基胺类与甲硫鎓化合物——这类物质在调节细胞内渗透压方面的重要性可与游离氨基酸比肩。
为此，我们提出了优化渗透压调节研究的实验设计与实验方案的最佳实践指南，以期更好地解析决定海洋无脊椎动物盐度驯化极限的保守机制。据我们所知，本研究是首个针对低盐驯化环境下渗透压顺应者体内渗透溶质浓度的系统综述与元分析，可为未来研究提供宝贵的基准数据，并揭示了大量尚未填补的研究空白。
我们的元分析结果表明，不同动物门共享部分核心渗透溶质，但不存在统一的渗透溶质组合模式，因为渗透溶质池的组成与比例具有类群特异性。鉴于未来盐度变化及其潜在生态影响，解析海洋生物的盐度耐受能力与极限变得愈发重要。