Benthic biofilm potential for organic carbon accumulation in salt marsh sediments 1

16 Coastal salt marshes are productive environments with high potential for carbon accumulation 17 and storage. Even though organic carbon in salt marsh sediment is typically attributed to plant 18 biomass, it can also be produced by benthic photosynthetic biofilms. These biofilms, generally 19 composed of diatoms and their secretions, are known for their high primary productivity and 20 contribution to the basal food web. In this study, we conducted laboratory experiments to test (1) 21 if biofilms can potentially accumulate carbon in marsh soil and (2) how different sedimentation 22 rates affect the amount of carbon accumulation. Containers filled with a settled mud bed were 23 inoculated with natural biofilms collected from a marsh surface and allowed to grow with 24 favorable light exposure, nutrient supply, and absence of grazing. Mud was added weekly in 25 different amounts, resulting in an equivalent sedimentation rate from 12 to 189 mm/yr. After 11 26 weeks, the sediment columns were sampled and analyzed for chlorophyll (chl a), loss on ignition 27 (LOI), and total organic carbon (TOC). Chl a accumulation rates ranged from 123-534 28 mg/cm/yr, organic matter accumulation ranged from 86-456 g/m/yr, and TOC accumulation 29 rates ranged from 31-211 g/m/yr. All three metrics (chl a, organic matter, and TOC) increased 30 with increased sedimentation rate. These results show that biofilms can potentially contribute to 31 carbon accumulation in salt marsh soils. Furthermore, areas with high sedimentation rates have 32 the potential for higher amounts of organic matter from biofilms in the sediment. 33 34


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Organic carbon (OC), ubiquitous in wetland soils, is important for food web dynamics 40 (rapid carbon dynamics) and carbon sequestration (long-term carbon dynamics). Labile OC 41 serves as the base of the food web, providing nutrients and energy to higher tropic levels (Kwak This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.

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were covered in dark paper to ensure light came only from the provided source. Control 107 containers did not receive the inoculum, were treated with 150 uL of bleach, and kept in the dark 108 to prevent biofilm growth. The cylinders were kept on the orbital shaker, which provided a 109 gentle agitation and promoted vertical mixing of the water column.

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The sedimentation experiment began after the observed colonization of the sediment 111 surface by biofilms (two weeks of growth). A slurry of bentonite clay mixed with the medium 112 was added according to five sedimentation rates ( and carbon in each layer was then summed together and divided by the duration of the 134 experiment and the surface area, thus obtaining accumulation rates per unit of area.

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The LOI and TOC data were fit according to the form: where is the accumulation rate of LOI or TOC, is the maximum rate of accumulation

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of OM or C mediated by sediment deposition, is the deposition rate, and is a fitting 139 parameter. to grow in all experiments in a replicable way.

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The height of the sediment-water interface in each container demonstrated that the 147 addition of bentonite increased the height of the sediment column and the rate of height increase 148 depended on the amount of sediment added ( Figure 2B). The height of the containers increased 149 by 4 mm to 45 mm, for the lowest and highest mineral sedimentation rate respectively over the 150 11-week experiment. Following each sediment addition, there was an initial increase in bed This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.

Chlorophyll-a 153
Sediment chl a accumulation rate increased with increasing vertical accretion ( Figure   154 3A). The containers with the lowest vertical accretion contained on average 123 mg/cm 2 /yr C 155 and the containers with the highest vertical accretion rate contained on average 534 mg/cm 2 /yr C.

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As sedimentation rate increased, more organic matter was stored in the sediments ( Figure   158 3B). The average amount of organic matter for the highest vertical accretion rate was 456 159 g/m 2 /yr, which is over five times the average amount of organic matter measured in the 160 containers with the lowest vertical accretion rates (86 g/m 2 /yr). The sedimentation rate was 16 161 times higher in the treatment with the highest vertical accretion rate compared to the lowest.

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Similarly, the amount of carbon increased with increasing rates of vertical accretion ( Figure 3C).

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The containers with the lowest accretion rates contained 31 g/m 2 /yr C, while those with the 164 highest accretion rate contained 211 g/m 2 /yr C. 165 We fit the exponential model to the LOI and TOC datasets (Equation 1, Figure 3) with 166 the assumption that there is little to no accumulation of OM or C from biofilms without sediment 167 deposition, as without burial the labile OM from biofilms will decompose and will not to 168 contribute to OM/C accumulation. As accumulation rates increase, the rates of C production 169 increase decreases ( Figure 3B, 3C). For LOI, we found that the maximum amount of OM 170 accumulated, , was 534 g/m 2 /yr. In terms of TOC, was determined to be 201 g/m 2 /yr

The potential for biofilm carbon accumulation
This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.

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The carbon accretion rates (CAR) from this study are comparable with those observed in 175 marshes worldwide. We found rates of 100-200 g/m 2 /yr C with moderate to high accretion rates, 176 while worldwide rates for marshes range from 100-300 g/m 2 /yr C, depending on the latitude and   195 Another explanation for the increase in OC accumulation with sedimentation rate is that 196 sedimentation could provide additional space (volume) that the biofilms are able to fill as they This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.  Our results suggest that sedimentation may constantly "reset" the biofilm age and allow it to 218 grow as in the early stage of development, allowing for the production of more carbon and This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.

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High rates of carbon accumulation have been related to high mineral suspended sediment

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Although our results suggest that a constant level of organic carbon accumulation can be reached 232 for arbitrary high sedimentation rates, this is likely not the case. We expect that there is a 233 sedimentation maximum which the biofilms would not be able to recover from (Grotzinger and

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Knoll 1999), thus limiting its ability to accumulate carbon. Ultimately, at some deposition rate, This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.

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An unexpected result of this experiment was that the biofilms were incredibly resilient 244 and able to grow despite large sedimentation rates. Following each sedimentation event, the 245 biofilms colonized the new sediment-water interface very quickly, within 24-48 hours. Indeed,

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PAM fluorescence (Figure 2A) did not decrease following the sedimentation events, even though 247 these measurements were taken 24-48 hours following such an event.
The mineral sedimentation 248 rates tested in this experiment exceed most sedimentation rates for coastlines worldwide and 249 were done episodically. As the biofilms were able to grow in these extreme conditions, biofilms 250 in nature would likely be able to withstand normal sedimentation, as well as sedimentation from 251 storm events. can also be an important source of C in tidal flats. There are substantial data gaps in our This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.
12 possible that these systems may play a large role in coastal carbon storage (Lovelock and Duarte 267   2019). As there is no vascular vegetation, the primary autochthonous C in tidal flats is biofilms.

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Thus, quantifying the amount of C in tidal flats from biofilms will improve our understanding of 269 this potential carbon sink.  as what is typically measured in salt marshes. While this carbon is labile and may not be stored 308 on a centennial to millennial timescale, it likely plays an important role in the carbon cycle in the 309 marsh.

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All analyses validate our hypothesis that higher sedimentation rates increase biofilm C 311 accumulation. A sedimentation threshold above which biofilms cease to grow and to accumulate This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.  Colors represent the five different sedimentation rates (see Table 1). Fluorescence measurements 567 are consistent across treatments. Bed height measurements were corrected for the consolidation 568 of the initial bed over time. Vertical lines in panel B indicate when sediment was added to the 569 experiment.
This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage. This manuscript has been submitted for publication in Wetlands. This manuscript has not yet undergone peer review and is a preprint. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the 'Peer-reviewed Publication DOI' link on the right-hand side of this webpage.