Endolithic cyanobacteria in larger benthic foraminifera: a morphotype-based classification framework for interpreting bioerosion and micritization

Larger benthic foraminifera (LBF) are key carbonate producers and nutrient recyclers in shallow tropical–subtropical seas, with much of their ecological success attributed to photosymbiosis with diverse microalgae. Beyond these well-studied living associations, foraminiferal tests also host post-mortem microbial colonizers that can drive early diagenetic alteration. Endolithic cyanobacteria are among the most effective carbonate microborers, yet their diversity and trace-forming styles in LBF tests remain poorly documented, limiting interpretation of micritization pathways and preservation bias in both modern and fossil foraminifera tests.

Here we present a preliminary, morphotype-based classification of endolithic cyanobacteria associated with five common LBF taxa collected from Aziziyah Corniche (near the King Fahd Causeway), Eastern Saudi Arabia (Arabian Gulf): Coscinospira hemprichii, Peneroplis planatus, P. pertusus, P. arietinus, and Sorites orbiculus. Specimens were hand-picked from a scoop of marine beach sediment, selecting tests that showed visible evidence of cyanobacterial infestation/bioerosion; therefore, the dataset is intended to characterize endolithic forms rather than quantify infestation frequency. We examined >40 tests using an embedding–casting approach (Logitech type 301 two part epoxy resin infiltration of microborings in vacuum,  carbonate dissolution with dilute HCl to recover casts) combined with incident-light stereo microscopy and SEM. Endolithic forms were categorized by diagnostic boring architecture (e.g., filament diameter, branching frequency, chamber-wall penetration style, and distribution across whorls/chambers), with taxonomic assignment based on cast morphology where possible.

Across hosts, endolithic assemblages were dominated by Hyella (including an H. imanis-like morphotype; ~90% of observations), with Hyella forms consistently abundant in all five host taxa. At least three additional endolithic cyanobacterial morphotypes were observed but could only be assigned to genus-level. Boring patterns indicate active colonization of test walls that plausibly facilitates structural weakening and subsequent micritization during early taphonomy.

This morphotype inventory offers a practical reference for recognizing cyanobacterial microborings in LBF tests and for comparing bioerosion and micritization signatures among host taxa. The observed boring patterns further suggest that endolithic cyanobacteria can contribute to post-mortem test alteration, including micritization pathways that influence preservation in the fossil record.