Single-celled bioturbators: benthic foraminifera mediate oxygen penetration and prokaryotic diversity in intertidal sediment
Abstract. Bioturbation processes influence particulate (sediment reworking) and dissolved (bioirrigation) fluxes at the sediment-water interface. Recent works showed that benthic foraminifera largely contribute to sediment reworking in intertidal mudflats; yet their role in bioirrigation processes remains unknown. In a laboratory experiment, we showed that foraminifera motion-behavior increased the oxygen penetration depth and decreased the total organic content. Their activity in the top 5 mm of the sediment also affected prokaryotic community structure. Indeed, in bioturbated sediment, bacterial richness was reduced and sulfate reducing taxa abundance in deeper layers was also reduced, probably inhibited by the larger oxygen penetration depth. Since foraminifera can modify both particulate and dissolved fluxes, their role as bioturbators can no longer be neglected. They are further able to mediate the prokaryotic community, suggesting that they play a major role in the benthic ecosystem functioning and may be the first described single-celled eukaryotic ecosystem engineers.
Dewi Langlet et al.
Status: open (until 15 Jun 2023)
- RC1: 'Comment on egusphere-2023-705', Anonymous Referee #1, 29 May 2023 reply
- RC2: 'Comment on egusphere-2023-705', Anonymous Referee #2, 01 Jun 2023 reply
Dewi Langlet et al.
Dewi Langlet et al.
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In general, the manuscript is well written, and the topic is highly interesting. However, prior to publication I have some questions and remarks that should be clarified prior to publication. For more details, please see below.
Experimental set up -section needs some clarification. Currently it is not clear how the sediment was homogenised? How was it ensured? The process is also likely to have had an impact on the porosity and water content of the sediment and hence the sediment structure may be different to what it is at the field situation. Could this have implications for the results and how the situation is in nature?
What size of foraminifera were picked and introduced into the experiment, also >125 um as sieved at the end of the experiment?
It is also not clear how often and how many times in total the oxygen profiling was conducted. Based on the Figure 3 profiles were conducted during 13 days? Was every control core (n=6) and foram core (n=6) measured on these days as a triplicate? Or were some cores measured more than others? In the section 2.6.2 it is stated that each core was divided into 5 areas to avoid multiple measurements in a same spot, implying that each core was measured max 5 times. The frequency and number or measurements per core should be clarified. I would also be interested to know if the authors have considered the impact of O2 profiling on the oxygen penetration depth in the sediment. In the end a considerable number of profiles were made, each producing a small vertical burrow down to several mm into sediment.
Please clarify in section 2.7 how the sampling was conducted, and from which depth intervals samples were taken. I assume that somehow the core was sliced, or was sediment scooped straight from the core? If latter, how was mixing of the sediment avoided, and contamination? Please clarify the text.
Section 3.2 did you measure the TOC and N-content at the start of the experiment? What was it at the start prior to incubations and how did it differ from the control cores at the end of the experiment? The freezing of the sediment used for the experiments would have introduced some fresh OM to the sediment, consisting of meiofauna etc. Foraminiferal are known the feed on various prey (e.g. Chronopoulou et al. 2019 DOI: 10.3389/fmicb.2019.01169), so wondering how much of the OM was processed by foraminifera, and if the difference in the Corg content at the end is due to foraminiferal grazing or enhanced OM-processing by bacteria due to bioirrigation?
Section 3.3. It seems that the O2 penetration depth was relatively shallow at 22 days. Could some of the differences in the OPD-measurements be due to different numbers of O2 profiles made in the cores? Or do authors have an idea why ODP was shallow at 22 days?
Section 3.4. It is a shame that the authors did not take a molecular sample at the start of the incubation, to observed how to prokaryote community developed from T=0 to T=85. If such a sample was taken, it would be very valuable to analyse it and include the results here.
Section 4.1 line 275. Without the Corg and N-measurements for the T=0, the authors cannot for certain confirm that that the Corg declined during the experiment.
Section 4.1 and elsewhere: I suggest that authors do not call control cores undisturbed but always as control cores, as the control cores were disturbed during the experiment with O2 microprofiling. It would be interesting, for authors to discuss the influence of profiling on the results. I.e. if some control cores were measured more than others, is there a difference in Corg, N, or OPD or O2 uptake etc?
Line 293, and 303: how were the burrows measured? Is it possible there were longer burrows inside the sediment cores that are not visible from observing the core from outside? And also that burrows were made inside the cores after 40 days.
Line 332-334 (and elsewhere related to bacterial richness estimate from OTUs) Bacterial richness estimation is based on number of OTUs. This, however, is somewhat problematic as OTUs do not directly translate to number of taxa (as some OTUs can be from same species). The more recent concept of an amplicon sequence variant (or ASV) could be a more appropriate means of estimating number of taxa present/bacterial richness. Authors could consider looking into ASVs, or at least they should be careful when interpreting bacterial richness based on OTUs and explain the limitations. Especially here, as the Shannon index is not showing the same trends.
Line 358-360. Regarding the influence of foraminifera on sediment N-content. It is more likely that the sedimentary N-content here is a reflection of OM degradation than related to foraminiferal denitrification. During degradation of OM, typically N-containing molecules (e.g. amino acids) are broken down preferentially, hence causing a shift in C/N ratio of OM left in the sediment (e.g. Schneider et al 2003, https://doi.org/10.1029/2002GB001871)
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