Experimental assessment of benthic foraminifera as salinity bioindicators: Integrating morphological and eDNA approaches

Benthic foraminifera are extensively used as bioindicators for paleoenvironmental reconstruction, and environmental DNA (eDNA) analysis provides a powerful lens to uncover their community diversity and environmental responses. Foraminifera are commonly used to assess salinity changes in estuary settings, but quantitative experimental studies of their responses to salinity gradients are scarce. Here, sediments from the intertidal zone of Qingdao Bay were subjected to a 10-week controlled culture across 13 salinity levels (0–60 Practical Salinity Units, PSU), and community dynamics were analysed using both morphological and eDNA approaches. Foraminifera exhibited broad salinity tolerance (0–60 PSU). Analyses of both morphological and molecular datasets revealed a significant increase in community diversity with salinity (p<0.05), accompanied by a marked decrease in the relative abundance of calcified Rotaliida (p<0.01). The inverse relationship for Rotaliida was quantitatively robust and well-described by a linear regression model. The eDNA analysis revealed that soft-bodied Monothalamiids, often overlooked morphologically, reached up to 76.2 % relative abundance (average 56.1 %). In contrast, the salinity-driven increase in high-Mg calcite-shelled Milioliida was observed only through morphological analysis. These results demonstrate the distinct but complementary nature of morphological and molecular methods. This study addresses the scarcity of experimental constraints on salinity responses, offering a calibrated reference for applying foraminifera in both ecological assessment and paleo-reconstruction in marginal marine environments.