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\DOI{10.5802/crgeos.269}
\datereceived{2024-03-21}
\daterevised{2024-06-04}
\dateaccepted{2024-06-24}
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\dateposted{2024-07-04}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Foraminiferal test dissolution reveals severe sediment
acidification in estuarine mudflats: new~perspectives for present and
historical assessment}

\alttitle{La dissolution de tests de foraminif\`{e}res r\'{e}v\`{e}le
une acidification s\'{e}v\`{e}re des s\'{e}diments dans les
vasi\`{e}res estuariennes : nouvelles perspectives pour
l'\'{e}valuation actuelle et historique}

\author{\firstname{Marie} \lastname{Fouet}\CDRorcid{0000-0002-6137-6349}\IsEqualContrib}
\address{UMR EPOC, Universit\'{e} de Bordeaux, CNRS, UMR 5805, Station
Marine d'Arcachon, 2 rue du Professeur Jolyet, 33120, Arcachon, France}
\email[M. Fouet]{marie.fouet@u-bordeaux.fr}

\author{\firstname{Maxime} \lastname{Daviray}\CDRorcid{0000-0003-1422-3398}\IsEqualContrib\IsCorresp}
\address{Univ Angers, Nantes Universit\'{e}, Le Mans Universit\'{e},
CNRS, LPG UMR 6112, F-49000~Angers, France}
\email[M. Daviray]{maxime.daviray@univ-angers.fr}

\author{\firstname{Emmanuelle} \lastname{Geslin}}
\addressSameAs{2}{Univ Angers, Nantes Universit\'{e}, Le Mans
Universit\'{e}, CNRS, LPG UMR 6112, F-49000~Angers, France}

\author{\firstname{Edouard} \lastname{Metzger}\CDRorcid{0000-0002-5838-4540}}
\addressSameAs{2}{Univ Angers, Nantes Universit\'{e}, Le Mans
Universit\'{e}, CNRS, LPG UMR 6112, F-49000~Angers, France}

\author{\firstname{Frans} \lastname{Jorissen}\CDRorcid{0000-0002-9325-6085}}
\addressSameAs{2}{Univ Angers, Nantes Universit\'{e}, Le Mans
Universit\'{e}, CNRS, LPG UMR 6112, F-49000~Angers, France}

\begin{abstract}
Foraminiferal mudflat communities in the Auray estuary, located along
the French Northwest Atlantic coast, were investigated in September
2020. Numerous living specimens of calcareous species showed marks of
dissolution. The number of affected individuals, together with their
dissolution degree, varied from one site to another. Moreover, pH
profiles showed porewater acidification of the subsurface sediment at
several stations. We propose two metrics quantifying the degree of
dissolution: (i) the ``Foraminiferal Test Dissolution'' (FTD), based on
binocular observations of the foraminiferal community, and (ii) the
``Calcareous Test Preservation Ratio'' (CTPR), surface ratio (0--0.5~cm)
of total living foraminiferal densities (0--1~cm). In stations
showing sediment acidification, the O\tsub{2}-pH profiles show the
typical biogeochemical signatures of cable bacteria activity (pH
increase in the oxic zone followed by strong acidification in the
suboxic zone). This suggests that bacterial activity could be
responsible for the strong pH decrease leading to corrosive porewaters
affecting calcareous tests, hence severe FTD in the first centimetre of
sediment leading to high CTPR. Furthermore, observations of FTD in
other estuaries on the French Atlantic coast indicate that the
phenomenon has become widespread. Re-investigated foraminiferal tests
sampled in 1995--1996 in the Auray estuary show no dissolution marks,
suggesting that the dissolution process observed in 2020 appeared over
the last thirty years. Therefore, the FTD and CTPR could be used as
rapid tools to establish the occurrence of dissolution processes,
pending geochemical confirmation. The FTD could also be used as
sediment acidification proxy in historical records, where geochemical
analyses are not possible. 
\end{abstract}

\begin{altabstract}
Les communaut\'{e}s de foraminif\`{e}res vivants des vasi\`{e}res de
l'estuaire d'Auray, situ\'{e} le long de la c\^{o}te atlantique
nord-ouest fran\c{c}aise, ont \'{e}t\'{e} \'{e}tudi\'{e}es en septembre
2020. Dans ces \'{e}chantillons, de nombreux sp\'{e}cimens
pr\'{e}sentaient des traces remarquables de dissolution. Le nombre
d'individus atteints et le stade de dissolution de leur test
\'{e}taient variables d'un site \`{a} l'autre. De plus, les profils de
pH ont montr\'{e} une acidification de l'eau interstitielle du
s\'{e}diment de subsurface \`{a} plusieurs stations. Pour quantifier le
degr\'{e} de dissolution, nous proposons deux m\'{e}triques : la
"Dissolution du Test des Foraminif\`{e}res" (FTD), \`{a} partir
d'observations \`{a} la loupe binoculaire de la communaut\'{e} de
foraminif\`{e}res, et le \og Ratio de Pr\'{e}servation des Tests
Calcaires \fg{} (CTPR), le rapport entre les abondances de
foraminif\`{e}res de surface (0--0.5~cm) et le nombre total (0--1~cm).
Dans les stations pr\'{e}sentant une acidification des s\'{e}diments,
les profils de pH et d'oxyg\`{e}ne \'{e}taient typiques de la signature
g\'{e}ochimique de l'activit\'{e} de bact\'{e}ries filamenteuses
(augmentation du pH dans la zone oxique suivi par une forte
acidification dans la zone suboxique). Cette activit\'{e}
bact\'{e}rienne pourrait \^{e}tre responsable de la forte diminution du
pH et pourrait conduire \`{a} une FTD s\'{e}v\`{e}re dans le premier
centim\`{e}tre de s\'{e}diment un CTPR \'{e}lev\'{e}. De plus,
l'\'{e}valuation de la FTD dans d'autres estuaires de la c\^{o}te
atlantique fran\c{c}aise indiquent que ce ph\'{e}nom\`{e}ne s'est
g\'{e}n\'{e}ralis\'{e}. Les r\'{e}-observations de tests de
foraminif\`{e}res \'{e}chantillonn\'{e}s en 1995--1996 dans l'estuaire
d'Auray ne montrent aucune marque de dissolution, ce qui sugg\`{e}re
que ce processus de dissolution est apparu au cours des trente
derni\`{e}res ann\'{e}es. Par cons\'{e}quent, la FTD et le CTPR
pourraient \^{e}tre utilis\'{e}es comme des outils rapides pour
\'{e}tablir l'occurrence des processus de dissolution, dans l'attente
d'une confirmation g\'{e}ochimique. La FTD pourrait \'{e}galement
servir d'indicateur de l'acidification des s\'{e}diments dans les
archives historiques, lorsque de telles analyses g\'{e}ochimiques ne
sont pas possibles.
\end{altabstract}

\keywords{\kwd{Benthic foraminifera}\kwd{Test preservation}\kwd{Cable
bacteria}\kwd{Sediment acidification proxy}}

\altkeywords{\kwd{Foraminif\`{e}res benthiques}\kwd{Bact\'{e}ries
filamenteuses}\kwd{Pr\'{e}servation des tests}\kwd{Proxy
d'acidification}}

\shortrunauthors

\thanks{CNRS-INSU (program LEFE-CYBER, project CB-FOR), Angers
University, OFB (French Office of Biodiversity, grant number
3976-CT-RD-AMI-18-SURV-FORESTAT)}

\maketitle

\pagebreak

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

Estuaries are complex, dynamic and very productive areas
\citep{Dayetal2012}. Despite their low diversity, estuarine habitats
host high abundances of macro- and meiofauna, because they provide
shelter \citep{ElliottMcLusky2002} and are storehouses of nutrients and
organic matter \citep{ElliottQuintino2007}. Organisms living in
estuarine intertidal mudflats must endure intense environmental stress,
as a result of daily (tidal cycles) to monthly (precipitation and river
runoff) changes in marine and freshwater inputs
\citep{ElliottMcLusky2002,Jorissenetal2022}.  Besides those stresses,
coastal environments are increasingly threatened by ocean acidification
\citep{Caietal2021,Cooleyetal2022,Woottonetal2008}. This phenomenon
jeopardises benthic organisms producing calcareous shells such as{\unskip\break}
foraminifera.

Benthic foraminifera are widely present in estuarine areas
\citep{Debenayetal2000b}. These unicellular protists have the
particularity to build a shell (called test), using different materials
for its construction. Some species produce an organic test, others
aggregate sediment particles, while a large group of species produce a
test of calcium carbonate \citep{deNooijeretal2009,Debenayetal2000a}.
Numerous authors have studied the distribution of foraminifera in
estuarine mudflats \citep[e.g.,][]{Morenoetal2005,
Debenayetal2006,Mojtahidetal2016,Francescangelietal2020,
ThibaultdeChanvalonetal2022}. The benthic foraminifera communities in
the Auray estuary (French Atlantic coast) were investigated 30 years
ago by \citet{Redois1996} and recently in 2019 by \citet{Fouetetal2022}
and 2020 by \citet{Davirayetal2024}. In 1996, Redois did not mention
any marks of dissolution of calcareous foraminifera, whereas
\citet{Fouetetal2022} and \citet{Davirayetal2024} noted at several
stations numerous specimens of calcareous foraminifera with partly or
completely dissolved{\unskip\break} test.

Numerous authors have described dissolution of calcareous tests of
living foraminifera in coastal environments and various hypotheses have
been put forward to explain the causes of the dissolution phenomenon.
For instance, abiotic factors such as trace metal pollution 
\citep{BuzasStephensetal2018}, re-oxidation of reduced dissolved or
particulate sulphidic compounds \citep{Cesbronetal2016} or freshwater
intrusion \citep{Charrieauetal2018,Haynertetal2012}
have been suggested to reduce the pH of pore water. Biotic factors
which can reduce the saturation state of calcite have also been
proposed such as intense microbial respiration of organic matter
\citep{SchonfeldMendes2022} or cable bacteria activity{\unskip\break}
\citep{Davirayetal2024}. 

Cable bacteria (CB) were discovered a decade ago in coastal marine
sediments \citep{Nielsenetal2010,Pfefferetal2012}. Their activity (CBA)
produces an electric gradient leading to ion migrations and a very
particular geochemical signature in the sediment: a pH maximum in the
oxic zone just below the sediment surface, followed by a strong pH
decrease in the few centimetres below
\citep{Meysmanetal2015,Nielsenetal2010,Pfefferetal2012,
RisgaardPetersenetal2015,RisgaardPetersenetal2012}. CB have been
described in various aquatic environments (e.g. open sea [e.g. Baltic
Sea, \citealp{Hermansetal2019}; Black Sea, \citealp{Hermansetal2020}], marine
lakes [e.g. Grevelingen, \citealp{Malkinetal2014}], intertidal salt
marshes [e.g. New England, \citealp{Larsenetal2014}], sandy intertidal
flats [e.g. Wadden Sea, \citealp{Malkinetal2017}] and also in freshwater
soils [e.g. Giber \AA{}, Denmark, \citealp{RisgaardPetersenetal2015}]), 
all around the world, see \citet{Burdorfetal2017} for a more extensive
review of CBA occurrence worldwide. In French marine environments, CB
were also observed especially in Mediterranean Lagoons such as Berre
\citep{Dametal2021},
Thau \citep{Burdorfetal2017} in the Gulf of Lion
and Urbino in Corsica  \citep{Burdorfetal2017}. Cable Bacteria were
also observed in French Atlantic intertidal mudflats such as Arcachon
bay and Auray estuary{\unskip\break} \citep{Davirayetal2024}.

Here, the new dataset of living foraminiferal communities sampled in
2020 in the Auray estuary is accompanied by oxygen and pH
microprofiling to better understand the biogeochemical processes behind
the dissolution of foraminiferal tests. To better constrain the
significance of these observations, the samples from the same area
collected in 1995--1996 \citep{Redois1996} and in 2019
\citep{Fouetetal2022} were re-examined. In addition, we re-investigated
samples from other estuaries located along the French Atlantic coast
\citep{Fouet2022}. This short article signals the emergence of CBA in
the sediments along the French Atlantic coast, and the repercussions in
terms of environmental chemistry, ecology, and taphonomic processes.

\section{Material and methods}\label{sec2}

The Auray estuary (Brittany, France; Figure~\ref{fig1}) is a typical
ria (drowned river valley), connected to the Morbihan Gulf (an enclosed
marine bay), located along the north French Atlantic coast. This zone
is subjected to a mesotidal to low macrotidal regime with a tidal range
of about 4~m. More details about the studied site can be found in
\citet{Fouetetal2022}. Living foraminiferal samples were collected in
September 2020. All 10 stations were sampled along the estuary at low
tide (Figure~\ref{fig1}B). Stations 6B, 4B, 2C of this study
corresponds to stations 1, 2, 3 respectively in
\citet{Davirayetal2024}, who also investigated the presence of CB by
qPCR at stations 1 (6B) and 2 (4B).

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}1A: Location of the studied estuaries
(in red: Auray estuary, in blue: eight other estuaries). 1B:~Location
of the stations in the Auray estuary sampled in 2019 and 2020 (oxygen
and pH profiles were measured in 2020 for stations indicated in red).
Modified from \citet{Fouet2022}.} 
\end{figure*}

At each station, the first two 0.5~cm of the sediment were collected by
hand using tubes with an internal diameter of 9.6~cm. Samples were
stained with 2~g/L Rose Bengal and preserved in 96\% ethanol. In the
laboratory, samples were sieved over a 125~$\upmu$m mesh. All stained
foraminifera were picked in water, using a Leica MZ16 stereomicroscope.
Based on stereomicroscope observations of calcareous tests contained in
each sample (examples of individuals showing dissolution marks are
shown in Figure~\ref{fig2}), three types of samples were distinguished,
characterised by different type of Foraminiferal Test Dissolution
(FTD):
\begin{itemize}
\item Type 1 (no visible test dissolution)---Most individuals have a
transparent, glassy test; less than 10\% of the test is whitish and
opaque; this is considered as the first sign of dissolution. 
\item Type 2 (moderate dissolution)---More than 10\% of the individuals
have a whitish, matt test, while some specimens show more advanced
stages of test dissolution. 
\item Type 3 (advanced to severe test dissolution)---Most specimens
show holes and cracks in the test with organic lining sometimes
becoming partially or completely visible. Some glassy specimens may
still be present in the assemblage.
\end{itemize}
The 0--0.5 and 0.5--1~cm levels were studied separately, and the
overall assessment of FTD type of each sample was based on the layer
which showed the most severe dissolution (nearly always the 0.5--1~cm
level). To investigate whether dissolution caused the loss of
foraminiferal tests in slightly deeper sediment layers, we calculated a
Calcareous Test Preservation Ratio (CTPR) that was defined as the ratio
between the density of tests of living calcareous foraminifera in the
0--0.5~cm level and their total density in the 0--1~cm layer. When CTPR
was below 0.6, high densities of calcareous specimens were recorded in
the subsurface layer (0.5--1~cm); when CTPR was between 0.6 and 0.90,
their densities were higher in the surface layer, and when CTPR was
above 0.90, the population living in the subsurface layer had largely
disappeared. 

\begin{figure*}[t!]
{\vspace*{-4pt}}
\includegraphics{fig02}
{\vspace*{-4pt}}
\caption{\label{fig2}Examples of gradual foraminiferal test
dissolution. (1) Glassy, transparent tests, (2) Whitish, matt tests, (3)
Tests with signs of advanced dissolution, (4) Tests affected by severe
dissolution, for \textit{Haynesina germanica} (1-a, 1-b, 2-a,
2-b, 3-a, 3-b, 4-a), \textit{Ammonia} spp. (1-c, 1-d, 2-c,
2-d, 3-c, 3-d, 4-b, 4-c, 4-d) and \textit{Quinqueloculina oblonga}
(1-e, 3-e, 4-e). For each dissolution stage, figures a, c
and e have been produced with an optical microscope, figures b and d
with a Scanning Electronic Microscope. White arrows indicate the
organic lining; scale bars: 100~$\upmu$m.}
{\vspace*{-4pt}}
\end{figure*}

To investigate both the spatial and historical extent of dissolution
processes, additional samples were re-examined using a stereomicroscope
to define the FTD. Concerning the spatial extent, samples from 
\citet{Fouet2022} were used to describe the FTD of samples coming from
eight other estuaries (133 samples) (Figure~\ref{fig1}A). Concerning
the historical aspect, Auray estuary samples collected 25 years ago
\citep{Redois1996} and in 2019 \citep{Fouetetal2022} were also
inspected  for FTD and CTPR values. Samples collected by Fouet and
co-workers in 2019 used the same methodology as presented here.
However, their study performed triplicates. The methodology used by
\citet{Redois1996} was slightly different. He studied the whole 0--1~cm
level, collected with a spoon on a surface of 100~cm\tsup{3}, stained
with Rose Bengal and preserved in ethanol, washed on a 50~$\upmu$m mesh
size sieve and dried the samples for binocular observation. Therefore,
CTPR could not be calculated for 1995--1996 samples, and these samples
were only investigated for FTD. 

Oxygen and pH microprofiling were performed at each station. After
retrieval in the field, cores were immerged in an aquarium with
estuarine water at \textit{in~situ} temperature, where oxygen
saturation was maintained by air-bubbling. Two Unisense\textcopyright{}
profiling systems were used simultaneously. One consisted of two
Clark-type oxygen microsensors with a  50~$\upmu$m tip
\citep{Revsbech1989,RevsbechJrgensen1986}, and the other of a pH sensor
with a 500~$\upmu$m tip diameter (PH500, Unisense\textcopyright{}).
Both microsensors were mounted on a motorised micromanipulator linked
to a computer, and connected to a MultiMeter S/N. We used a 50-$\upmu$m
increment for oxygen, whilst for pH we chose a 100-$\upmu$m increment
around the sediment-water interface, which was further adapted in
real-time according to the evolution of the observed pH profile, down
to 4--5 cm depth. To calibrate the oxygen microsensor, two points were
used, the water column for 100\% saturation and pore waters in the
anoxic sediment for 0\%. For pH calibration, three NBS buffers were
used (values 4.0, 7.0 and 9.2).

\section{Results and discussion}\label{sec3}
\subsection{Sediment geochemistry}\label{ssec31}

Figure~\ref{fig3} shows oxygen and pH profiles at and below the
Sediment-Water Interface (SWI) for 10 stations in the Auray estuary
taken in September 2020. All stations showed typical pore-water oxygen
profiles with a strong decrease in the topmost sediment. The oxygen
penetration depth (OPD) ranged from 0.7 to 1.5~mm depth. Based on the
pH profiles, which showed greater variability, three groups of stations
could be distinguished:
\begin{itemize}
\item Group I (stations 2A, 6A, 8A, 8B): the pH strongly decreased from
${\sim}$7.8 at the SWI to ${\sim}$7.0 at 2--4 mm depth, to remain
relatively stable in deeper sediment layers. 
\item Group II (stations 1C and 6B): the stations showed pH profiles
with slightly acidic pore waters in the suboxic zone. Station 1C showed
a minor pH peak (7.9) just below the SWI, but deeper down, the pH
remained stable around 6.6 until 4~cm depth. Station 6B showed a minor
acidification in the suboxic zone (pH\ ${=}$\ 6.8). According to
\citet{Davirayetal2024}, DNA sequencing analysis at station 6B 
(station~1 in their article) showed low cable bacteria densities.
\item Group III (stations 1A, 2B, 2C, 4B): the pH decreased to a
minimum well below 7.0 between 0.5 and 1.0 cm depth, in the suboxic
zone (i.e., without detectable oxygen). It increased again towards
greater depth but remained below 7.0. In addition, the profiles of
stations 2C and 4B showed a small but distinct pH peak just below the
SWI. According to \citet{Davirayetal2024}, DNA sequencing analysis at
station 4B (station 2 in their article) showed high CB densities.
\end{itemize}
O\tsub{2}-pH profile groups can be discriminated according to the
knowledge of early diagenesis processes and geochemical fingerprint of
cable bacteria activity \citep[CBA;][]{Meysmanetal2015,Nielsenetal2010,
Pfefferetal2012,RisgaardPetersenetal2012,RisgaardPetersenetal2015}. 
Profiles of group I are considered as typical for marine environments
without CBA. Conversely, the profiles of group III suggest
acidification stems from CBA as shown in \citet{Nielsenetal2010}; 
\citet{Pfefferetal2012} and \citet{Meysmanetal2015}.
\citet{Davirayetal2024} confirmed the presence of CB
[\textit{Candidatus} {Electrothrix sp.;}  \citealp{Trojanetal2016}] at
station 4B by DNA analysis and estimated a CB filament density of 
74.4 ${\pm}$ 5.0~m${\cdot}$cm\tsup{-3} per bulk sediment in the first sediment
centimetre. Finally, in group II, O\tsub{2}-pH profiles show only a
slight pH decrease and are not typical of intense CBA. Even if the pH
profile do not show a clear pH decrease, \citet{Davirayetal2024}
indicated the presence of genetical material of cable bacteria in
station 6B in 2020 with a low CB filament density of 7.4 ${\pm}$ 
0.4~m${\cdot}$cm\tsup{-3} per bulk sediment. Therefore, these two profiles
are interpreted as sustaining moderate~CBA. 

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig03}
{\vspace*{-4pt}}
\caption{\label{fig3}pH (orange) and oxygen (blue) profiles measured in
September 2020 in the Auray estuary. Three groups were established
according to the shape of the O\tsub{2}-pH profiles (group I: low
acidification; group~II: moderate acidification; group~III: strong
acidification). Profiles from stations 2C, 4B and 6B are also published
in \citet{Davirayetal2024}.} 
{\vspace*{-4pt}}
\end{figure*}

\subsection{Foraminiferal assemblages}

The density of living foraminifera (excluding organic linings) in the
first centimetre varied from 76 to 5120 individuals per 50~cm\tsup{3}
with 52--2164 individuals per 50~cm\tsup{3} in the surface half
centimetre and 22--3575 individuals per  50~cm\tsup{3} in the
subsurface (0.5--1 cm) layer (Figure~\ref{fig4}). These values are
comparable to those observed in 2019 \citep{Fouetetal2022} and in other
estuaries 
\citep[e.g.,][]{Camachoetal2015,Debenayetal2006,Francescangelietal2021,Mojtahidetal2016}. 
The foraminiferal density was generally higher in the superficial
sediment layer (0--0.5~cm) than in the 0.5--1~cm layer, except for
station 8B. For many stations (i.e., 1A, 4B), the densities were an
order of magnitude lower in the second layer.

\begin{figure*}
\includegraphics{fig04}
{\vspace*{-2pt}}
\caption{\label{fig4}Foraminiferal density in the ${>}$125~$\upmu$m
fraction for the 0--0.5 and 0.5--1~cm levels, expressed in individuals
per 50~cm\tsup{3} (for samples collected in July 2019 by
\citet{Fouetetal2022} and September 2020 (this study)). The three
replicates of 2019 were averaged, and the standard deviation is
presented overhead. The stations are ordered from the outer (left) to
the inner part of the estuary (right).}
{\vspace*{-2pt}}
\end{figure*}

The Calcareous Test Preservation Ratio (CTPR), which describes the
density differences within the first centimetre, varied between 0.27
and 0.96 (Table~\ref{tab1}). Five stations showed values of CTPR higher
than 0.6 (1A, 1C, 2B, 2C and 4B) suggesting a low occurrence of
calcareous living foraminifera in the subsurface layer (0.5--1~cm). At
stations of acidification groups II and III, dissolution, as shown by
FTD was much more intense in the 0.5--1 than in the 0--0.5 cm layer,
and consequently, foraminiferal densities were much higher in the first
layer (Figure~\ref{fig4}), leading to a high CTPR (Table~\ref{tab1}).
This is not surprising in view of the pH profiles of group III
(Figure~\ref{fig3}), which show that the strong pH minimum (5.3 ${<}$ pH
${<}$ 6.3) was always positioned between 0.5 and 1.0 cm depth. 

%tab1
\begin{table*}
\caption{\label{tab1}The first row indicates the O\tsub{2}-pH profile
groups from green (group I: low acidification) through yellow (group
II: moderate acidification) to red (group III: strong acidification).
The second row describes the CTPR, with ${<}$0.6: green,
0.6--0.90: yellow, ${>}$0.90: red. The last row summarises observations
of foraminiferal test dissolution (FTD), ranging from ``no visible
dissolution'' (type 1, green), via moderate (type 2, yellow) to
``advanced to severe dissolution'' (type 3, red)\vspace*{2pt}}
\begin{tabular}{@{}c@{}}
\tbody
\inlinefig{fx01}
\end{tabular}
\vspace*{2pt}
\end{table*}

Table~\ref{tab1} shows the comparison of the results of the pH
profiles, Foraminiferal Test Dissolution (FTD) and the Calcareous Test
Preservation Ratio (CTPR). In many cases, scores yielded by these three
methods converge. All three indicators suggest that there was intense
sediment acidification at stations 1A and 4B, and to a lesser degree at
stations 2B and 2C. At these stations, FTD was maximal, especially in
the 0.5--1 cm layer, leading to high CTPR values (${>}$0.6). Conversely,
at stations where the pH profile did not indicate sediment
acidification (i.e., 2A, 6A, 8A and 8B), the observed FTD was minimal
and the CTPR was always below 0.6, indicating that calcareous
individuals were well preserved in both sediment levels. Stations of pH
group II (with atypical pH profiles) showed intermediate FTD and CTPR
values, confirming our initial interpretation that these stations were
slightly affected by acidification. However, there are some exceptions
to this pattern. At stations 2A and 8B, where the pH profile suggested
no acidification, moderate FTD was observed. Nevertheless, it appears
that in the case of pH profiles indicative of strong acidification, the
living foraminiferal assemblages show advanced to severe FTD, which is
confirmed by an important loss of calcareous tests in the 0.5--1.0 cm
layer, leading to a high CTPR. 

These results show a good correspondence between observations of
foraminiferal test dissolution and measurements of sediment
acidification. In the literature, different causes of foraminiferal
test dissolution have been proposed. First, global ocean acidification
is one of the threats that could alter organisms that produce
calcareous shells such as foraminifera \citep{GuamanGuevaraetal2019,
Kuroyanagietal2021}. However, these changes are expected to impact all
sites of the same ecosystem equally, so this explanation was not
retained to explain the differences in sediment acidification observed
between the different stations of this study. Therefore, the
possibility of freshwater intrusion to explain the FTD
\citep{Charrieauetal2018,Haynertetal2012} is not
retained either, because stations located in the inner part, and
receiving more freshwater input (stations 8A and 8B), do not show
stronger acidification than stations located in the outer part of the
estuary (1A and 1C). \citet{BuzasStephensetal2018} proposed that trace
metal pollution can explain FTD, but in our stations, measurement of
trace metal did not show any sign of major pollution
\citep{Fouetetal2022}. \looseness=1

Based on the similarities between our pH profiles showing sediment
acidification associated with oxygen profiles (group III) and the
representative one of Cable Bacteria Activity, as presented in
\citet{Meysmanetal2015}, \citet{Nielsenetal2010} and 
\citet{Pfefferetal2012}, we hypothesised that CBA could be responsible
of Foraminiferal Test Dissolution at these Auray estuary stations (1A,
2B, 2C, 4B). However, some \mbox{discrepancies} between the pH profiles and
the observations of FTD and CTPR could be noted, for which two
explanations can be advanced. First, as suggested by differences in FTD
between stations collected in the same mudflat (i.e., station 1A versus
1C, and station 2A versus 2C and 2B or station 6A versus 6B), CBA could
have a patchy distribution on the mudflat. The pH measurements are
based on a few 1D punctual profiles (500~$\upmu$m scale) across the
depth of the sediment, whereas foraminiferal observations included a
large sediment volume (72~cm\tsup{3}). The foraminiferal sampling zone
may have been subject to scattered~bacterial activity. Next, while pH
measurements suggest acidification at the moment of sampling, the
living foraminiferal community could reflect dissolution that has
occurred over the last few weeks \citep{Charrieauetal2018,
Charrieauetal2002,LeCadreetal2003}. Indeed, several
authors have shown that CBA may vary considerably throughout the year
\citep{Seitajetal2015,SuluGambarietal2016}. 

In order to explore whether FTD also occurs on individuals collected in
other estuaries, samples from eight other estuaries 
[Figure~\ref{fig1}; \citealp{Fouet2022}] were visually inspected for
marks of dissolution. The \mbox{results} of these observations are presented
in Table~\ref{tab2}, using the same three types of sample
classification, characterised by different stages of FTD, as in the
Auray estuary. A few stations from Odet, La\"{i}ta and Crac'h estuaries
showed moderate marks of test dissolution. Both Aulne and Belon
estuaries had one station showing advanced to severe marks of test
dissolution (Table~\ref{tab2}). By analogy with Auray, we hypothesise
that CBA could be present in these estuaries as well. This still
remains to be confirmed by more distinctive methods such as pH
profiling or molecular identification. Conversely, no FTD was observed
in the Vie (two different field campaigns, in October 2018 and June
2020), Elorn and Vilaine estuaries. Also in the Loire estuary,
re-inspection of foraminiferal assemblages described by 
\citet{Mojtahidetal2016} did not show any signs of FTD suggesting the
absence of foraminiferal test dissolution at these locations (com.
pers. Mojtahid). Moreover, concerning the geographical extent of this
strong acidification phenomenon, advanced to severe test dissolution of
living foraminifera have earlier been described in Arcachon Bay
\citep{Cesbronetal2016}. The authors explained this as the result of
decreased pH due to bacterial activity around decomposing eelgrass
roots. However, closer inspection of their pH profiles suggests a CBA
signature. Together with the strong decrease of calcareous tests
densities in the first centimetre of the sediment could also plead to
intense seasonal cable bacteria activity. Summarising, in several
intertidal mudflats of French transitional environments (estuaries and
semi-enclosed bays), FTD has been observed that could be the result of
CBA. It therefore appears that this acidification phenomenon could be
widespread along the French coast.

%tab2
\begin{table*}
\caption{\label{tab2}Values of FTD (Foraminiferal Test Dissolution)}
\begin{tabular}{@{}c@{}}
\tbody
\inlinefig{fx02}
\end{tabular}
\tabnote{Stereomicroscope observations of foraminiferal test
dissolution, ranging from ``no visible test dissolution'' (group 1,
green) to ``moderate'' (group 2, yellow), ``advanced to severe
dissolution'' (group 3, red) for samples collected in nine different
estuaries studied in \citet{Fouet2022}. The letters a, b, and c are the
three replicates sampled in 2018 (Vie estuary) and 2019 (Auray
estuary).}
\end{table*}

In addition, to look into the historical aspect of the presence of this
phenomenon, we re-examined two foraminiferal datasets from Auray
estuary: \mbox{samples} with three replicates per station collected in July
2019 \citep{Fouetetal2022} and samples collected during a 10-month
survey in 1995--1996 \citep{Redois1996}. Samples were collected on the
same mudflat as the present study (see method section). Concerning the
samples collected in 2019 (Table~\ref{tab3}), advanced to severe test
dissolutions (FTD type 3) was observed at several stations and in
several replicates that corresponds to similar FTD type found in the
present study. It can be noted that CTPR values from calcareous
foraminiferal communities collected in 2019 (Table~\ref{tab3})  were in
good agreement with those observed in 2020 for stations 1A, 1C, 2B, 4A,
5B, 7. CTPR values (Table~\ref{tab3}) for the stations 2A, 2C, 6B and
8A show higher values in 2019 than in 2020 (${>}$0.9 on average) while
station 5A shows a higher CTPR value higher in 2020 (${>}$0.95).
Differences between samples could be attributed either to interannual
variability (between 2019 and 2020) or to spatial heterogeneity
(between 2019 replicates) of CBA. Concerning the samples collected in
1995--1996, although the sampling methodology and treatment were slightly
different (0--1~cm layer sampled and samples dried so that organic
linings were not preserved), visual inspection of the calcareous
foraminiferal tests was still possible. The re-examination of the
foraminiferal tests of these samples did not yield any marks of
foraminiferal test dissolution, and all examined samples were
classified as Type 1. This suggests that in the Auray estuary, sediment
acidification is a recent phenomenon (present at least since 2019) that
appeared in this area in the last 25 years.

%tab3
\begin{table*}
\caption{\label{tab3}Values of CTPR: ratio between the number of
calcareous individuals in the 0--0.5~cm level and the total number of
calcareous foraminifera in the 0--1~cm sample, for the three replicates
(a, b, c) sampled in 2019 \citep[after][]{Fouetetal2022} and 2020 at
all the fifteen stations along the Auray estuary}
\begin{tabular}{@{}c@{}}
\tbody
\inlinefig{fx03}
\end{tabular}
\tabnote{``-'' indicates the sample where the density of calcareous
species is ${<}$10 specimens.}
\end{table*}

To better assess the spatial and temporal dynamics of this phenomenon,
the re-examination of foraminiferal tests collected in earlier studies
is possible. Observation of archived samples may show FTD indicative of
former acidic conditions at places where their presence had not been
suspected in the past. This method represents a first, rapid and
inexpensive approach for directing further analyses in order to
characterise present and past dissolution processes using other
approaches (elemental and isotopic analyses, sedDNA, etc.). We
therefore recommend for future investigations studying foraminiferal
living assemblages to use a resolution of 0.5 cm in the uppermost
sediment to allow CTPR calculation. In addition, the ideal condition
for observing FTD is a sediment that has been kept humid, in which
foraminifera have been coloured (i.e., Rose Bengal or CTG) allowing
inner organic linings observation. Thereby, the recognition of FTD in
fossil assemblages preserved in sediment archives could yield
\mbox{information} about the historical evolution of CBA, as shown by 
\citet{Richirtetal2022}. These authors observed in a sediment core from
Grevelingen Lake (The Netherlands) very low densities of calcareous
foraminifera at 20 cm depth, corresponding to a period of major change,
the reoxygenation of bottom water due to the increase of water
exchanges after the opening of the lake to the sea. They hypothesise
that such changes could have promoted the enhancement of CBA since
2003, and therefore carbonate test dissolution. Such investigations
could help to better understand the spread of these recently discovered
bacteria, remaining poorly unknown, and their consequences for the
ecology of the surrounding organisms. Species with a calcareous test
could be more fragile, and others (such as agglutinated foraminifera or
soft-walled organisms) more adapted to resist to dissolution, so this
additional stress could lead to changes in the community structure
\citep{Davirayetal2024}. In addition, sediment acidification can result
in a loss of sediment archives \citep{Davirayetal2024}, which could
have implications for the interpretation of palaeontological data. 

\section{Conclusion}\label{sec4}

The foraminifera from sediment samples collected in September 2020 in
the Auray estuary showed severe marks of dissolution. This process was
not homogenous along this estuary. In addition, pH profiles showed
acidification of the porewater at a few millimetres depth after the
oxic zone in several stations. These profiles are typical of cable
bacteria activity signature. We suggest that an important loss of
calcareous foraminifera in the first half centimetre (high CTPR)
coupled with foraminiferal test dissolution (FTD) can be considered as
an indicator of a potential occurrence of porewater acidification, such
as cable bacteria activity. It appears therefore that these metrics
could be used as a rapid tool of sediment acidification, pending
geochemical confirmation. FTD could also be used as a proxy of acidic
conditions in historical records, where pH can no longer be measured.
The study of FTD in samples collected at the Auray estuary in the 90s
suggests that these acidic conditions have emerged over the last thirty
years. Re-examination of calcareous foraminifera coming from other
estuaries on the French Atlantic coast allows to propose that the
corrosive phenomenon is or has become widespread over the last few
decades.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}

The authors received funding from the CNRS-INSU (program LEFE-CYBER,
project CB-FOR), from the Angers University and the OFB (French Office
of Biodiversity, grant number 3976-CT-RD-AMI-18-SURV-FORESTAT).

\section*{Acknowledgements}

The authors are grateful to Nils Risgaard-Petersen for his advice
during the early stages of the writing process, and Robin Fentimen for
his proofreading and correction language of the text. The authors thank
the participants in the field trips (2019 and 2020).

\CDRGrant[OFB]{3976-CT-RD-AMI-18-SURV-FORESTAT}

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