Monothalamous foraminifera are useful bioindicators for DNA-based benthic monitoring of North Sea offshore platforms

Benthic foraminifera are widely recognized as excellent bioindicators for assessing the Ecological Quality Status (EcoQS) of marine environments. However, most of the current foraminiferal biomonitoring is based on hard-shelled taxa. Here, we analyze soft-walled monothalamous foraminifera in sedimentary environmental DNA metabarcoding data to assess the EcoQS and the response of the benthic community to impacts associated with the activity of an offshore platform in the North Sea, based on two-year sampling campaigns. Our results confirm the dominance of monothalamids in the metabarcoding data and indicate that their diversity tends to increase with distance from the platform. We also calibrated and tested an ecological index based on foraminiferal ecological indices, foram-gAMBI, to assess the EcoQS. The foram-gAMBI is significantly and strongly correlated to most metals, hydrocarbons (r = 0.74-0.92, p < 0.05), and moderately correlated with organic matter (r = 0.48-0.55, p < 0.05). The correlation values are higher than those found for the macrofauna-based AMBI and nematode-gAMBI, supporting the higher sensitivity of foram-gAMBI to environmental stress. The EcoQS assignment based on four ecological indices shows a very high agreement and consistency among them. The classification of stations into three impact classes according to selected environmental variables (Ba, Zn, Cu, THC) allows the identification of some monothalamids as indicators of high or low impact. Our study provides new evidence of the usefulness of foraminiferal metabarcoding for monitoring anthropogenic impacts and highlights the need to include monothalamids among foraminiferal bioindicators.