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

The effect of acidification of coastal marine sediments by cable bacteria on foraminifera has been documented in the intertidal mudflats of the Auray river (Morbihan, 56). This sharp pH decrease (1 < ∆pH < 2.4) leads to the dissolution of the carbonate tests of living and dead foraminifera, threatening their preservation in the sediment and their robustness as bioindicators or historical proxies.

Vertical distributions of living foraminifera densities per 10 cm3 of sediment at the three stations (> 125 µm fraction). 0 is the position of the sediment-water interface (SWI). Sediment pH microprofiles (orange diamonds) and neutral pH (vertical red line)

Foraminifera, microorganisms capable of secreting a carbonated shell, called a test, or of agglutinating particles from their environment, are commonly used as both bioindicators and palaeoproxies of marine environments because they are present in abundance and their tests are preserved in sediments. However, a recently observed phenomenon could threaten the robustness of this tool. Discovered in 2010, cable bacteria induce a strong acidification of subsurface sediments, down to a pH of 6 in the marine environment. These bacteria have a worldwide distribution, and the strong dissolution process associated with their activity could threaten the development of foraminifera and the preservation of their tests in sediments. This study seeks to characterise the impact of this bacterial activity on the state of carbonate tests of living and dead foraminifera.

To this end, high-resolution pH measurements revealed strong acidification of the sediment of intertidal mudflats in the Auray river estuary (1 < ∆pH < 2.4). Coupled with DNA analysis (qPCR), we determined that this process resulted from the activity of cable bacteria. This contrasting variability in sediment acidification appeared to induce various stages of dissolution of carbonate tests of foraminifera. We studied the living communities in the first five centimetres of sediment and characterised these dissolution stages using scanning electron microscopy. We observed that species with carbonated tests were dominant in the assemblages and that while the tests were in good state at the sediment surface, they were in advanced stages of dissolution in the more acidic subsurface layer, even to the point of complete dissolution of the test remaining only the organic lining. Species with an agglutinated test then dominated the community. At depth in the sediment, where the pH is slightly less acidic (5 cm), the few living specimens showed more contrasting stages of dissolution, resulting from a mixture of the upper layers. As for the dead communities, they were more strongly affected by this corrosive process: the deeper we went into the sediment, the greater the loss of carbonate tests, resulting in an accumulation of agglutinated tests and organic linings that were very different from the living communities.

Gradual dissolution of the foraminiferal test of a genus Ammonia specimen, from an intact stage (1.a) to a severe stage (2.a) until the complete loss of the test (3). Zoom on the last chambre of the test at the intact (1.b) and severe (2.b) stages

These changes in the assemblages of living and dead foraminifera suggest that cable bacteria are an important component of the taphonomic processes in the marine sediments they colonise and that they must be taken into account in the interpretation of ecological monitoring data and historical studies using carbonate microorganisms as bioindicators and palaeoenvironmental proxies. Understanding the seasonal dynamics of this bacterial activity and its effect on the carbonate system would enable us to grasp the integration of this impact over several months by carbonate benthic communities and their potential resilience.

> Reference : Daviray, M., Geslin, E., Risgaard-Petersen, N., Scholz, V. V., Fouet, M., and Metzger, E.: Potential impacts of cable bacteria activity on hard-shelled benthic foraminifera: implications for their interpretation as bioindicators or paleoproxies, Biogeosciences, 21, 911–928, https://doi.org/10.5194/bg-21-911-2024, 2024

This reasearch was supported by the OFB (project FORESTAT), the CNRS (LEFE-CYBER, project CB-For) and the LPG.

Published on March 15th, 2024