Agglutinated foraminifera as early indicators of microplastic pollution in two Mediterranean marine caves

Marine caves are considered extreme environments for aquatic life because, due to their enclosed setting, they are affected by extreme conditions for scarcity or absence of light, limited nutrient availability, the difficulty of larval dispersal, and possible hypoxic conditions. Moreover, caves connected with karst systems receive terrestrial contributions (water, sediments, and organic matter) that, interacting with the marine ones, determine wide spatial and temporal environmental variability. Combining all these variables determines a strong environmental gradient from the outer to the inner parts of caves that is reflected in the biota’s zonation; despite these unfavorable conditions, marine caves are biodiversity reservoirs. However, they are extremely fragile environments because they are affected by natural environmental stress and potentially exposed to contamination from both the land and the sea; for this reason, they are included in the European Habitats Directive for protection and conservation. Recently, microplastic pollution was recorded for the first time in the water and sediments of a Mediterranean marine cave. Benthic foraminifera were recognized as reliable environmental indicators in these environments, where they develop more and more different assemblages from those of the surrounding marine area with the increasing distance from the cave entrance. In particular, the agglutinated taxa, not abundant in the shallow water Mediterranean environment, are relatively numerous in cave assemblages. This work considered two Italian marine caves with different characteristics: the huge Bue Marino cave (Sardinia), which receives terrestrial contributions through the vast karst system of the Gulf of Orosei during rainy periods, and the small Argentarola cave (Tuscany archipelago), which is permanently characterized by fully marine conditions. Sediment samples from the two caves were analyzed for grain size, organic carbon (Ctot), microplastic and benthic foraminifera. In the Bue Marino cave, sediment ranged from sandy silt to sand, it was rich in Ctot (up to 8.07%), and microplastics were 10-27 items kg-1. The agglutinated, infaunal, and opportunist Eggerelloides advena was the dominant species, with Ammonia tepida and Ammonia inflata. In the Argentarola cave, sediment was silty clay or clayey silt, poor in Ctot (at most 0.13%), and microplastics were 70-300 items kg-1. The most abundant foraminiferal taxa were the epifaunal calcareous Spirillina vivipara and Patellina corrugata, and the infaunal agglutinated Lagenammina difflugiformis, while Glomospira charoides was common. Microplastics were searched in the tests of the three agglutinated species through Micro Fourier Infrared Spectroscopy (FTIR) because it was supposed that these items could be incorporated by specimens collecting grains during the building of new chambers; moreover, it is easier to recognize the signal associated with the presence of MPs in the agglutinated than in the calcareous tests. FTIR spectra revealed the presence of plastic components in E. advena (Bue Marino) and L. difflugiformis but not in G. charoides (Argentarola). The different responses among species may be attributed to their different ability to select the grains from the sediment for building new chambers. This result is particularly alarming from an environmental point of view because it testifies that microplastics enter a biological matrix, being included in the trophic chain, without the need to be ingested and metabolized. On the other hand, this study demonstrated that some species of agglutinated foraminifera are powerful proxies of microplastic pollution in sediments. Moreover, because the peculiar conditions of marine caves favour the presence of agglutinated species, agglutinated foraminifera may be successfully applied as early indicators of this pollution in these environments.