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Effects of seagrass Zostera capricorni on sediment microbial processes
Abstract and Figures
The effect of the seagrass Zostera capricorni on sediment microbial processes was studied in a tank experiment, where vegetated and unvegetated control sediments were incubated in 10 and 50% of incident light. Leaf and root-rhizome biomass, shoot density, and leaf productivity were significantly higher when plants were incubated in 50 % than in 10 % of incident Light. Nitrogen fixation, sulphate reduction, and urea turnover in the Z. capricorni vegetated sediment were higher in the 50% than in the 10% light treatment and higher in the vegetated than in the unvegetated sediment. The stimulation of microbial processes in the Z, capricorni vegetated sediment took place in the rhizosphere, where nitrogen fixation and sulphate reduction in particular were stimulated. The sediment studies were supplemented by measurements of nitrogen fixation, sulphate reduction, and urea turnover by microorganisms associated with the roots and rhizomes of Z. capricorni. The rates of nitrogen fixation and sulphate reduction associated with root-rhizomes were up to 40- and 7-fold higher, respectively, than the highest respective sediment rates, whereas the root-rhizome associated urea turnover was lower than sediment rates. Nitrogen fixation and sulphate reduction associated with root-rhizomes could account for up to 39 and 4%, respectively, of the depth-integrated sediment rates. Nitrogen fixed by microorganisms associated with root-rhizomes could supply up to 65 % of the nitrogen needed for plant growth. Further, it was estimated that 8 to 18% of the carbon fixed by Z. capricorni was released to the sediment by the roots and rhizomes. Urea turnover was suggested to be an important intermediate in the gross production of ammonium, and a low net production of ammonium indicated rapid internal nitrogen cycling within the sediment.
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... Most of the other bacterial taxa were evenly distributed between the two sampling sites, with variations observed only within sample type. At the phylum level, while the Proteobacteria was dominant across all samples, other phyla showed differences: Cyanobacteria thrived in seawater and on leaf samples, probably favored by light and hydrodynamic conditions [40]; Desulfobacteria was abundant exclusively in the sediment and the rhizome/root samples, where they play a crucial role in sulfate reduction driven by root exudates, especially in seagrass meadows belowground [64]. Furthermore, taxa belonging to Bacteroidota were more abundant in the seawater and leaf samples than in the rhizomes/root and sediment samples, although they are generally found in seagrass meadows belowground as they are well-known decomposers of cellulose and chitin [60]. ...
The anchoring and mooring of boats mechanically damage Posidonia oceanica plants; however, no information is available on the effect of this kind of damage on the plant holobiont, i.e., on the associated bacterial and fungal communities. Indeed, bacterial communities are known to change under different plant stress conditions but the dynamics of seagrass-associated fungi remain largely unexplored. We used DNA metabarcoding to profile the bacterial and fungal colonizers of two nearby P. oceanica patches in the Villefranche-sur-Mer bay (France) differing by the amount of exposure to mechanical stress due to boat transit and anchoring. Bacterial communities showed a significant reduction in diversity with an increase in Vibrio sp. in the rhizome and root samples from the impacted site, where the accumulation of dead organic material favors opportunistic heterotrophs. Conversely, fungal communities showed increased diversity in the leaf samples from the impacted site, where a reduction in the dominant P. oceanica host-specific mutualistic endosymbiont, Posidoniomyces atricolor, was found. This change was probably due to the opening up of new colonizable niches for several fungal species. Although this study represents a preliminary assessment of the effect of mechanical stresses on P. oceanica-associated microbial communities, it further supports their putative use as a seagrass descriptor.
... Monitoring prokaryotic compositions in sediments, therefore, can be a key task in tracking coral health at the local level. Likewise, in seagrass meadows, which are vital for carbon sequestration (also known as "blue carbon" [79,80]), the rhizosphere microbiome plays a crucial role in maintaining meadow health, involving essential biochemical pathways such as nitrogen fixation, sulfate reduction, sulfide oxidation, and urea turnover [81][82][83]. Alterations in sedimentary chemical conditions, such as hypoxia and/ or enriched sulfide, often combined with other stressors such as high/low salinities or temperatures, can lead to large-scale die-offs of seagrass [84][85][86], which also affect prokaryote composition [83,87,88]. Prokaryotic communities have been shown to have the ability to recover relatively quickly after such disturbances due to their composition plasticity [19,89,90]. ...
Background
Marine benthic prokaryotic communities play crucial roles in material recycling within coastal environments, including coral reefs. Coastal sedimentary microbiomes are particularly important as potential reservoirs of symbiotic, beneficial, and pathogenic bacteria in coral reef environments, and therefore presumably play a core role in local ecosystem functioning. However, there is a lack of studies comparing different environments with multiple sites on the island scale, particularly studies focusing on prokaryotic communities, as previous investigations have focused mainly on a single site or on specific environmental conditions. In our study, we collected coastal sediments from seven sites around Okinawa Island, Japan, including three different benthic types; sandy bottoms, seagrass meadows, and hard substratum with living scleractinian corals. We then used metabarcoding to identify prokaryotic compositions and estimate enzymes encoded by genes to infer their functions.
Results
The results showed that the three substrata had significantly different prokaryotic compositions. Seagrass meadow sites exhibited significantly higher prokaryotic alpha-diversity compared to sandy bottom sites. ANCOM analysis revealed that multiple bacterial orders were differentially abundant within each substratum. At coral reef sites, putative disease- and thermal stress-related opportunistic bacteria such as Rhodobacterales , Verrucomicrobiales , and Cytophagales were comparatively abundant, while seagrass meadow sites abundantly harbored Desulfobacterales , Steroidobacterales and Chromatiales , which are common bacterial orders in seagrass meadows. According to our gene-coded enzyme analyses the numbers of differentially abundant enzymes were highest in coral reef sites. Notably, superoxide dismutase, an important enzyme for anti-oxidative stress in coral tissue, was abundant at coral sites. Our results provide a list of prokaryotes to look into in each substrate, and further emphasize the importance of considering the microbiome, especially when focusing on environmental conservation.
Conclusion
Our findings prove that prokaryotic metabarcoding is capable of capturing compositional differences and the diversity of microbial communities in three different environments. Furthermore, several taxa were suggested to be differentially more abundant in specific environments, and gene-coded enzymic compositions also showed possible differences in ecological functions. Further study, in combination with field observations and temporal sampling, is key to achieving a better understanding of the interactions between the local microbiome and the surrounding benthic community.
... There is most likely nutrient competition between seagrasses and pelagic primary producers to explain the negative growth rate of Synechococcus spp. According to previous studies, benthic microalgae and seagrasses obtained nutrients from sediment pore waters and the water column [39]. Seagrasses can also take up nutrients from sediment, which helps maintain high production rates in water with nutrient scarcity [40]. ...
There is no doubt that seagrass beds constitute one of the most productive ecosystems in shallow coastal waters. Despite this, picoplankton in seagrass ecosystems has received relatively little attention. The purpose of this study was to compare picoplankton growth and mortality rates between seagrass and unvegetated habitats using chamber incubations. We tested two main hypotheses: (i) incubation with seagrass would result in higher bacterial growth rates due to increased DOM release from seagrass photosynthesis, and (ii) Synechococcus spp. would be lower in the presence of seagrass due to competition for inorganic nutrients. Bacterial growth rates were higher in seagrass chambers (2.44 d–1) than in non-seagrass chambers (2.31 d−1), respectively, suggesting that organic carbon coming from the seagrass community may support bacterial production. Furthermore, the growth rate of Synechococcus spp. was significantly lower in the seagrass treatment than in the non-seagrass treatment, likely reflecting nutrient competition with the seagrass. Small-scale chambers proved to be a useful tool for studying the factors controlling spatial and temporal patterns of picoplankton across different habitats. Furthermore, future studies should examine picoplankton growth over a wider range of spatial scales in seagrass beds and adjacent unvegetated sediment.
... In silicate-rich coastal sediment (often observed in temperate seagrass ecosystems), nitrogen is often the limiting nutrient (47)(48)(49). Seagrass rhizosphere sediments can exhibit intense N 2 fixation (15,16,18,43,44,50), and heterotrophic, diazotrophic bacteria (especially N 2 -fixing SRB) have been hypothesized to thrive in symbiosis with seagrasses in the rhizosphere, based on carbon and nitrogen exchange (16). In the rhizosphere of Zostera noltii, root-and rhizome-asso ciated N 2 fixation rates were largely driven by SRB, reaching 40-to 140-fold higher rates than in the bulk sediment (BS) and accounting for ~30% of the fixed nitrogen in the rhizosphere (18). ...
Seagrasses can enhance nutrient mobilization in their rhizosphere via complex interactions with sediment redox conditions and microbial populations. Yet, limited knowledge exists on how seagrass-derived rhizosphere dynamics affect nitrogen cycling. Using optode and gel-sampler-based chemical imaging, we show that radial O 2 loss (ROL) from rhizomes and roots leads to the formation of redox gradients around below-ground tissues of seagrass ( Zostera marina ), which are co-localized with regions of high ammonium concentrations in the rhizosphere. Combining such chemical imaging with fine-scale sampling for microbial community and gene expression analyses indicated that multiple biogeochemical pathways and microbial players can lead to high ammonium concentration within the oxidized regions of the seagrass rhizosphere. Symbiotic N 2 -fixing bacteria ( Bradyrhizobium ) were particularly abundant and expressed the diazotroph functional marker gene nifH in Z. marina rhizosphere areas with high ammonium concentrations. Such an association between Z. marina and Bradyrhizobium can facilitate ammonium mobilization, the preferred nitrogen source for seagrasses, enhancing seagrass productivity within nitrogen-limited environments. ROL also caused strong gradients of sulfide at anoxic/oxic interfaces in rhizosphere areas, where we found enhanced nifH transcription by sulfate-reducing bacteria. Furthermore, we found a high abundance of methylotrophic and sulfide-oxidizing bacteria in rhizosphere areas, where O 2 was released from seagrass rhizomes and roots. These bacteria could play a beneficial role for the plants in terms of their methane and sulfide oxidation, as well as their formation of growth factors and phytohormones. ROL from below-ground tissues of seagrass, thus, seems crucial for ammonium production in the rhizosphere via stimulation of multiple diazotrophic associations.
IMPORTANCE
Seagrasses are important marine habitats providing several ecosystem services in coastal waters worldwide, such as enhancing marine biodiversity and mitigating climate change through efficient carbon sequestration. Notably, the fitness of seagrasses is affected by plant–microbe interactions. However, these microscale interactions are challenging to study and large knowledge gaps prevail. Our study shows that redox microgradients in the rhizosphere of seagrass select for a unique microbial community that can enhance the ammonium availability for seagrass. We provide first experimental evidence that Rhizobia , including the symbiotic N 2 -fixing bacteria Bradyrhizobium , can contribute to the bacterial ammonium production in the seagrass rhizosphere. The release of O 2 from rhizomes and roots also caused gradients of sulfide in rhizosphere areas with enhanced nifH transcription by sulfate-reducing bacteria. O 2 release from seagrass root systems thus seems crucial for ammonium production in the rhizosphere via stimulation of multiple diazotrophic associations.
... There is most likely nutrient competition between seagrasses and pelagic primary producers to explain the negative growth rate of Synechococcus sp. According to previous studies, benthic microalgae and seagrasses obtained nutrients from sediment pore waters and the water column [38]. Seagrasses can also take up nutrients from sediment, which helps maintain high production rates in water with nutrient scarcity [39]. ...
There is no doubt that seagrass beds constitute one of the most productive ecosystems in shallow coastal waters. Despite this, picoplankton in seagrass ecosystems has received relatively little attention. The purpose of this study was to compare picoplankton growth and mortality rates between seagrass and unvegetated habitats using chamber incubations. We tested two main hypotheses: (i) incubation with seagrass would result in higher bacterial growth rates due to increased DOM release from seagrass photosynthesis, and (ii) Synechococcus spp. would be lower in the presence of seagrass due to competition for inorganic nutrients. Our results showed bacterial growth rates of 2.31 and 2.44 d–1 without and with seagrass, respectively. In seagrass chambers, bacterial growth was higher than that in non-seagrass chambers, suggesting that organic carbon coming from the seagrass community may support bacterial production. Furthermore, we observed a negative Synechococcus spp. growth rate (-0.9 d–1) in the seagrass treatment, likely reflecting nutrient competition with the seagrass. Small-scale chambers proved to be a useful tool for studying the factors controlling spatial and temporal patterns of picoplankton across different habitats. Furthermore, future studies should examine picoplankton growth over a wider range of spatial scales in seagrass beds and adjacent unvegetated sediment.
... Seagrass rhizosphere sediments and surrounding seawaters are hotspots of nitrogen and sulfur cycles, processes that are governed by specific microorganisms, particularly bacteria [11,12]. These processes include nitrogen fixation and nitrification [13], ammonium production [14], sulfide oxidation, and sulfate reduction [4,13]. Research has demonstrated that fungi are crucial for the fitness and health of terrestrial plants and act as pathogens or endophytes, but the role of fungi in marine habitats has been poorly studied [15]. ...
Background
Seagrasses offer various ecosystem services and possess high levels of primary productivity. However, the development of mariculture has affected the homeostasis of seagrass meadow ecosystems. Plant-microbiome associations are essential for seagrasses health, but little is known about the role of environmental microbiomes and how they affect seagrass in a mariculture environment. In this study, we investigated the influence of mariculture on the rhizosphere and seawater microbiome surrounding Zostera marina and focused on the bacterial, eukaryotic, and fungal components in the composition, diversity, metabolism, and responses to mariculture-related environmental factors.
Results
Significant differences in the composition, richness, diversity, and internal relations of the bacterial community between the seawater and rhizosphere sediment surrounding Z. marina were observed, while differences in the eukaryotic and fungal communities were less significant. More complex bacterial and fungal co-occurrence networks were found in the seawater and rhizosphere sediment of the Saccharina japonica (SJ) and sea cucumber (SC) culture zones. The seawater in the SJ zone had higher levels of dissimilatory and assimilatory nitrate reduction, denitrification, and nitrogen fixation processes than the other three zones. The assimilatory sulfate reduction enzymes were higher in the rhizosphere sediments of the SJ zone than in the other three zones. Tetracycline, sulfonamide, and diaminopyrimidine resistance genes were enriched in the mariculture SJ and SC zones.
Conclusions
Our findings might contribute to a better understanding of the effects of mariculture on the seagrass and the meadow ecosystems and thus revealing their potential operating mechanisms. These insights may serve to raise awareness of the effects of human activities on natural ecosystems, regulation of antibiotic usage, and environmental restoration.
7tqXcmUUPbCQ9vMfWLp9EQVideo Abstract
Seagrass meadows are important marine ecosystems in coastal areas, offering ecological and economic services to the mankind. However, these ecosystems are facing declines due to climate changes and human activities. Rhizosphere-associated microbiomes play critical roles in the survival and adaptation of seagrasses. While prior studies have explored the general microbial communities and their roles in seagrass meadows, there is a gap in understanding the specific rhizosphere microbiomes of different seagrass species and their interdependent relationships. Our study analyzed the microbial community composition and their metabolism in the rhizosphere of Ruppia sinensis (RS), Zostera japonica (ZJ), and Zostera marina (ZM) obtained from the coastal area of Shandong, China, using high throughput and metagenome sequencing. We found that Rhodobacteraceae, Desulfocapsaceae, and Sulfurovaceae were enriched in RS, ZJ, and ZM samples, respectively, compared with the other two seagrass species, and the bacterial connections were decreased from RS to ZM and ZJ samples. The abundances of nirKS and norBC , mediating denitrification, were higher in RS samples with 2.38% ± 0.59% and 2.14% ± 0.24%, respectively. RS samples also showed a higher level of genes in assimilatory sulfate reduction but lower levels in dissimilatory sulfate reduction and oxidation, with a greater ability to convert sulfide into L-cysteine and acetate. Metagenome-assembled genomes from metagenome of RS rhizosphere had a higher diversity and were assigned to eight phyla. Our study could provide a typical project to analyze the bacterial community structures and metabolic functions in the rhizosphere microbiomes of different seagrasses.
IMPORTANCE
Seagrasses are indispensable in marine ecosystems, offering numerous critical services, with their health significantly influenced by associated rhizosphere microbiomes. Although studies have investigated the microbial communities and their ecological roles in seagrass meadows, the correlations between rhizosphere microbiome and seagrass species from a particular geographic region are limited. Some studies concentrated on the bacterial composition within the rhizosphere of various seagrasses, but the functional aspects of these microbiomes remain unexplored. Our research delves into this void, revealing that Ruppia sinensis , Zostera japonica , and Zostera marina host diverse bacterial community in the composition, connections, functions, and metabolism, such as nitrogen and sulfur metabolism. Our study revealed that seagrass species play an important role in shaping the rhizosphere microbiomes in an equivalent environment, emphasizing the importance of seagrass species in shaping the rhizosphere microbial communities.
Plant growth-promoting rhizobacteria (PGPR) play a crucial role in enhancing plant growth. However, investigations into the presence and function of PGPR in seagrass rhizospheres remain relatively limited. In this study, we isolated 45 strains of PGPR from Zostera marina rhizospheres and assessed their functional attributes. Additionally, we evaluated the performance of these candidate strains under varying environmental conditions, such as temperature, salinity, and pH. Out of the 45 analyzed strains, 6 were found to possess the ni f H gene; some strains also exhibited the ability to solubilize inorganic phosphorus, with dissolved phosphorus content ranging widely from 14.6 to 393.9 mg l ⁻¹ . Eleven strains demonstrated indole-3-acetic acid production, with yields spanning from 16.3 to 42.8 mg l ⁻¹ . Siderophores and ammoniated proteins were produced by 32 and 20 strains, respectively. Notably, 5 PGPR strains (F65, G84, G85, G86, and I109) displayed multiple growth-promoting properties along with strong adaptability to a wide range of physicochemical conditions. This study highlights the potential reservoir of PGPR in the eelgrass rhizosphere and provides significant implications for utilizing these bacteria to enhance the success rate of restoring degraded seagrass meadows.
Current rates of ocean warming are predicted to exacerbate ongoing declines in seagrass populations. Above-ground responses of seagrass to increasing temperatures have been studied from a direct physiological perspective while indirect effects, including changes to microbially-mediated below-ground processes, remain poorly understood. To test potential effects of increased temperature on seagrass growth and associated microbial communities, we sampled seagrass beds experiencing ambient and elevated water temperatures at Lake Macquarie, Australia. Sites with warmer water were associated with a plume from a power station discharge channel with temperatures analogous to conditions predicted by 2100 under current rates of ocean warming (+3°C). The microbial community composition in both sediments and leaf tissues varied significantly between warm and ambient water temperatures with higher relative abundances of putative sulphate-reducing bacteria such as Desulfocapsaceae, Desulfobulbaceae and Desulfosarcinaceae in sedimentary communities in warm water. Above-ground biomass and seagrass growth rates were greater at warm sites while below-ground biomass and detrital decomposition rates showed no difference suggesting potential buffering of temperature effects below-ground. These findings suggest a 3°C rise in temperate regions is unlikely to induce mortality in seagrass however, it may shift microbial communities towards more homogenous structure and composition.
At Groote Eylandt, values ranged from 16-47 mg N m-2 day-1 in a Syringodium isoetifolium community and from 13-19 mg N m-2 day-1 on a reef flat with Thalassia hemprichii. At Weipa, mean rates of nitrogen fixation were 25 mg N m-2 day -1 in an Enhalus acoroides community and 20 mg N m-2 day-1 on a mud bank below mangroves. About 5% of fixation was due to epiphytes on the seagrass leaves, and 8% of fixation associated with the S. isoetifolium community occurred in washed roots and rhizomes; the remainder was due mostly to bacteria in the sediment. Nitrogen fixation supplied 8-16% of the N requirements of the plants. -from Authors
Root respiration decreased across the transect from the intertidal site to the 2 subtidal stations of increased sediment anaerobiosis. Data collected in field and laboratory experiments in which the hydrogen sulphide concentration surrounding the roots and rhizomes was enhanced showed physiological adaptations characteristic of tolerance to anaerobiosis. Zostera marina is thus capable of responding to markedly different microenvironments. - from Authors
Submerged macrophytes (Isoetes lacustris, Littorella uniflora, Lobelia dortmanna, Potamogeton crispus, P. pectinatus, Sparganium simplex, Zostera marina) release oxygen from their roots to oxygen-depleted water both during illumination, and at lower rates, during darkness. During illumination less oxygen was released from roots of the marine macrophyte, Z. marina (0.04 mu g O 2 mg -1 plant d wt h -1) and from 4 stream macrophytes (0.14-0.44) than was released from 3 isoetid species from oligotrophic lakes (0.50-3.12). As percentages of the total oxygen release during illumination, root release was very low in Zostera (1%) and among stream macrophytes (2-4%), but quite high in the isoetids (28-100%) . Lobelia dortmanna had the highest rate of release from roots during illumination (3.12 mu g O 2 mg -1 h -1) which represented 100% of the total oxygen release. The ready exchange of oxygen across the roots of isoetids may play a role for the uptake of sediment carbon dioxide.-from Authors
Radial oxygen loss (ROL) was measured as a function of distance behind the tip for roots of the seagrass Halophila ovalis (R.Br.) Hook f. The effects of shoot illumination and leaf area on ROL were also examined as were the porosity and anatomy of the roots, rhizomes and petioles of H. ovalis. For plants taken from the Swan Canning Estuary, Western Australia, the porosities of roots, rhizomes and petioles were 15%, 27% and 17%, respectively. ROL from roots in an O2-free and saline medium was measured using root-sleeving cylindrical platinum O2 electrodes. The shoots were submerged in aerated seawater during the measurements. ROL was substantially higher when the shoots were exposed to saturating light and it decreased markedly in the dark. These findings, and experiments in which the leaves were excised, show that O2 lost radially from the roots was photosynthetically derived. Moreover, ROL showed a marked gradient along the root; ROL decreased from an average maximum value of 72 ng cm−2 min−1 at 0.5 cm behind the root tip to only 4 ng cm−2 min−1 at 3 cm, the most basal position tested. These data show that roots of H. ovalis contain `a barrier to ROL' in the more basal regions, an adaptation shown by other workers to enhance the growth of roots of wetland macrophytes into anaerobic sediments.
Marine nitrogen-fixing bacteria distributed in the eelgrass bed and seawater of Aburatsubo Inlet, Kanagawa, Japan were investigated using anaerobic and microaerobic enrichment culture methods. The present enrichment culture methods are simple and efficient for enumeration and isolation of nitrogen-fixing bacteria from marine environments. Mostprobable-number (MPN) values obtained for nitrogen-fixing bacteria ranged from 1.1×10(2) to 4.6×10(2)/ml for seawater, 4.0×10(4) to 4.3×10(5)/g wet wt for eelgrass-bed sediment, and 2.1 × 10(5) to 1.2 × 10(7)/g wet wt for eelgrass-root samples. More than 100 strains of halophilic, nitrogen-fixing bacteria belonging to the family Vibrionaceae were isolated from the MPN tubes. These isolates were roughly classified into seven groups on the basis of their physiological and biochemical characteristics. The majority of the isolates were assigned to the genusVibrio and one group to the genusPhotobacterium. However, there was also a group that could not be identified to the generic level. All isolates expressed nitrogen fixation activities under anaerobic conditions, and no organic growth factors were required for their activities.
Many variables must be considered in seeking to describe differences in population sizes for native aquatic bacterial populations. In this study of seagrass- and nearby plant-free sediments, seasonal effects on total bacterial counts were found to be highly significant, outweighing the significance of factors such as geographic variability, but on populations of a chosen Alteromonas sp., they were not significant at the 5% level. Summer counts for both populations were higher than those for winter; this result is likely to reflect the higher productivity of the host Zostera capricomi in summer months, resulting in the exudation of increased amounts of organic nutrients. The Alteromonas sp. occurred in greatest abundance (1.8% of the total population) at the seagrass sediment site from which it was originally isolated and formed up to 1.5% of the population in adjacent plant-free sediments. In fluorescent microscopy studies with labeled antibodies, the Alteromonas sp. was found to be ubiquitous in seagrass and plant-free sediments but was found closely associated in much higher numbers with seagrass root-rhizome tissue, suggesting a possible nutritional relationship between plant and bacterium. In associated trials of sediment preservation techniques, bacterial counts of replicate sediments preserved with glutaraldehyde (3% v/v) were higher than those obtained using Lugol's iodine or freezing.
Seasonal changes in net benthic C-mineralization were measured as sediment O2 uptake, as well as ΣCO2 flux from the sediment to the water column. Oxygen uptake varied between 26 and 181 mmol m−2 d−1 through the year, and ΣCO2 efflux varied between 17 and 132 mmol m−2 d−1. Both parameters were related to temperature, and the Q10 for O2 uptake and ΣCO2 efflux were 2.5 and 2.0, respectively. The ΣCO2/O2 ratio varied between 0.8 and 1.0 during most of the year. The ΣCO2/O2 ratio was, however, less than 0.8 in late summer and autumn, which might be due to reoxidation of accumulated reduced sulfur compounds and carbonate precipitation. Calcium carbonate deposition by molluscs seemed quantitatively important for the ΣCO2 efflux in September 1990 and August 1991. The ΣCO2/O2 ratio reached a maximum of 1.3 in mid-April during decomposition of benthic and sedimented microalgae. On an annual basis, all the oxidants which were reduced during anaerobic mineralization (e.g., Mn2+, Fe2+, HS−), apparently were reoxidized by O2, as illustrated by an annual ΣCO2/O2 ratio of 0.8. Benthic macrofauna stimulated C-mineralization, indicated by a positive relationship between O2 uptake and ΣCO2 efflux rates, respectively, and macrofauna biomass. Sediment chlorophyll-a + pheopigment content and C/N ratio served as good indicators both of algal sedimentation from the water column and of the presence of benthic microalgae. The sediment parameters, including the organic carbon and nitrogen content, C/N ratio, and chlorophyll-a + pheopigment content, clearly reflected the seasonal variation in sediment mineralization rates. There was a temporal phase displacement between input of organic matter and benthic C-mineralization, which resulted in a temporary build up of organic matter during periods with high primary production and/or low C-mineralization (spring, late autumn, winter). The accumulated organic matter was mineralized during periods when the actual supply of organic matter was insufficient (summer, early autumn).