Potential impacts of cable bacteria activity on hard-shelled benthic foraminifera: a prelude to implications for their interpretation as bioindicators or paleoproxies

Cable bacteria (CB) are filamentous bacteria coupling sulphide oxidation to oxygen reduction over centimetre distances. This bacterial activity generates a strong pH gradient within the first few centimetres of the sediment that affects the microhabitats occupied by benthic foraminifera. Hard-shelled foraminifera are protists able to build a calcareous or agglutinated shell (called “test”). Here we study the impact of sediment acidification induced by CB activity (CBA) on calcareous test preservation. For this study, sediment cores were sampled in the macrotidal Auray estuary located on the French Atlantic coast. Living and dead foraminifera were quantified (until 5-cm depth) and discriminated using the Cell-TrackerTM Green vital marker. CBA was assessed with pH and oxygen profiles combined with quantitative Polymerase Chain Reaction (q-PCR). On two different intertidal mudflats, volumetric filament densities have been measured. They were comparable to those observed in the literature for coastal environments, with 7.4 ± 0.4 and 74.4 ± 5.0 m.cm−3 per bulk sediment respectively. Highly contrasting CBA (from low to very intense) were described with lowest pH at 5.8. This seems to lead to various dissolution stages of the foraminiferal calcareous test from intact to fully dissolved tests revealing the organic lining. The dissolution scale is based on observations of living Ammonia spp. and Haynesina germanica specimens under a Scanning Electronic Microscope. Furthermore, dead foraminiferal assemblages showed a strong calcareous test loss and an organic lining accumulation throughout depth under low pH, hampering the test preservation in deep sediment. These changes in both living and dead foraminiferal assemblages imply that CB must be strongly considered in ecological monitoring and historical studies using foraminifera as bioindicator and paleoenvironmental proxy.