Dynamics of Microbial Abundance in Unvegetated and Seagrass Habitats: A Case Study

Abstract

Seagrass meadows are recognized for their ecological importance, yet their influence on microbial community structure remains insufficiently characterized. This study examined the effects of seagrass presence on microbial assemblages in a subtropical coastal environment by comparing seagrass habitats to adjacent unvegetated sediments. Microbial abundances, including viruses, bacteria, picophytoplankton (Synechococcus spp. and picoeukaryotes), and heterotrophic nanoflagellates, were quantified using flow cytometry. Viral concentrations were significantly higher in seagrass treatments (2.4–9.2 × 10⁶ viruses mL⁻¹) than in controls (0.6–2.0 × 10⁶ viruses mL⁻¹), while bacterial abundances were slightly lower in seagrass treatments (5.1–16.0 × 10⁵ cells mL⁻¹) than in controls (7.9–16.6 × 10⁵ cells mL⁻¹). As a result, the virus-to-bacteria ratio (VBR) was significantly elevated in seagrass habitats, suggesting enhanced viral regulation of bacterial populations. Additionally, picophytoplankton and heterotrophic nanoflagellates increased in seagrass incubations, with strong correlations indicating that nanoflagellates are likely major grazers of picophytoplankton. These results highlight the role of seagrass habitats in modulating microbial interactions and emphasize the need to consider habitat-specific characteristics when evaluating microbial dynamics and biogeochemical processes in coastal systems.

1. Introduction

Coastal seagrass ecosystems are among the most important natural carbon sinks, as they remove carbon from the atmosphere and sequester it in organic matter in sediments, across a variety of habitats around the world [1]. Seagrasses also contribute substantially to the production of dissolved organic carbon (DOC) through root exudation, as well as through the leaching and decomposition of detritus [2]. These highly productive meadows provide essential ecosystem services, such as nutrient cycling and carbon storage, vital to the functioning of marine ecosystems [3,4].

Seagrass meadows create distinct physicochemical conditions across sediments, leaf surfaces, and overlying water, potentially influencing local microbial communities through alterations in hydrodynamics, DOC, and nutrient availability. As primary decomposers and mediators of organic matter transformation, heterotrophic bacteria play a vital role in linking detritus to higher trophic levels within aquatic food webs. The productivity of seagrasses, along with associated epiphytes, microphytobenthos, benthic macroalgae, and terrestrial inputs, contributes a diverse array of organic matter to seagrass ecosystems. These sources likely influence the bacterial carbon demand in such environments [5]. Substantial bacterial production has been reported in seagrass-dominated systems, supported by consumption of DOC derived from seagrass photosynthesis and plant debris [2]. However, DOC released by seagrasses tends to be more refractory than that released by algae. According to Vähätalo and Sundergaard [6], only 28% of seagrass-derived DOC is consumed by bacteria, compared to the more labile DOC from macroalgae and microalgae. Despite this lower lability, microbial activity has been observed to be significantly elevated in seagrass-associated waters compared to unvegetated areas [7], with bacterial abundances up to ten times greater in seagrass beds [8]. These findings suggest that the seagrass environment can provide favorable conditions for bacterial growth despite the lower bioavailability of their DOC.

Picophytoplankton, organisms typically smaller than 2–3 µm in diameter, are a ubiquitous and taxonomically diverse component of aquatic ecosystems. Picophytoplankton consist of picocyanobacteria (such as Prochlorococcus spp. and Synechococcus spp.) and picoeukaryotes, and they account for 26–56% of global phytoplankton biomass [9] and about 50% of all ocean primary productivity [10]. Some studies suggest a lower abundance of Synechococcus spp. in seagrass systems compared to other marine habitats [11], possibly due to grazing pressure [12] or physical trapping by seagrass leaves, leading to negative net growth rates of Synechococcus spp. in the water [13]. The high removal of natural picophytoplankton by seagrass leaves may contribute to the low biomass of picophytoplankton despite high growth rates in the study region. These observations underscore the importance of investigating the interactions between seagrass and picophytoplankton to better understand biogeochemical processes in coastal systems.

Both autochthonous and allochthonous factors can influence the structure and dynamic of autotrophic and heterotrophic microbial communities in aquatic ecosystems. The dynamics of picoplankton are governed not only by abiotic factors such as nutrient availability, light, and temperature but also by biotic processes including natural mortality, predation, and viral lysis. Despite research efforts directed at exploring various aspects of seagrass-associated microbial communities, including the diversity and abundance of bacteria and eukaryotes [5,14,15], our understanding of other key microbial components, such as viruses, remains limited. Luna et al. [16] reported a significant increase in virus abundances along with a significant increase in virus-to-bacteria ratios (VBR) in the sediments surrounding the seagrass Posidonia oceanica compared to the sediments in unvegetated areas. Although viruses do not directly infect seagrasses, they significantly contribute to bacterial mortality in sediments [17], which can alter the composition of sediment bacterial communities and therefore indirectly influence sediment chemistry and seagrass health. Viral lysis of bacterial cells also contributes to the DOC pool within seagrass sediments, with implications for microbial carbon cycling [18]. These findings suggest that viruses may play a substantial role in carbon transformation processes relevant to climate mitigation and blue carbon management. In addition to viruses, heterotrophic nanoflagellates are abundant in seagrass ecosystems, particularly on leaf surfaces [19], where they graze on epiphytic bacteria [20]. These protists not only influence bacterial abundance through grazing but also shape bacterial community composition [21], indicating their ecological significance within seagrass-associated microbial networks. Generally, there is little information on the temporal variations in viruses, picophytoplankton, and nanoflagellate grazers in systems with seagrass.

Many seagrass habitats can be found in Taiwan; however, the Penghu Islands are particularly significant for seagrass disturbance. There is generally limited information available about seagrass systems, and little attention has been paid to the microbial communities within the seagrass ecosystem at Penghu. During the study, we examined whether changes in viral, bacterial, Synechococcus spp., picoeukaryotic, and nanoflagellate abundances were associated with seagrass recolonization. We compared benthic chambers with and without seagrass to evaluate the influence of seagrass presence on microbial community dynamics. In particular, we hypothesized that the abundance of microbial communities would be significantly impacted by seagrass presence and that there would be a significant difference between microbial communities in seawater with and without seagrass habitats. The results of this study contribute to the understanding of the microbial mechanisms involved in the effects of seagrass ecosystems and highlight the need to focus on the microbial communities associated with seagrass habitats.

2. Materials and Methods

2.1. Study Site and Sampling Methods

The Penghu Islands are a group of 92 islets and islands located within the Taiwan Strait. The region is influenced by warm currents flowing northward year-round, while during winter, northeasterly monsoons drive cold currents northwestward, weakening the warm current’s intensity. An investigation of coastal waters off the island of Penghu (25°37.07′ N, 119°31.58′ E) was conducted for this study (Figure 1A). Water temperatures vary seasonally in the area from 12 °C in winter to 35 °C in summer, according to data recorded by temperature loggers (unpublished). The seagrass meadows in this area consist primarily of Halodule uninervis and Halophila ovalis, and cover levels range from 20% to 90%. In the shallow waters ranging from 1.7 m to 4.4 m depth, these seagrasses remain submerged throughout the day. Most of the sediment is composed of sand and is primarily composed of carbonate minerals.

Sampling was conducted in April 2024. Surface seawater and sediment samples, including seagrass, were collected to conduct two ex situ enclosure experiments using benthic chambers. The chambers (10 cm diameter × 50 cm length) were gently inserted 20 cm into the seabed, one in a seagrass habitat and the other in unvegetated sediment (Figure 1B,C). This design allowed us to differentiate between the seagrass system (sediment + seagrass + overlying water) and the control system (sediment + overlying water) with respect to microbial abundances (viruses, bacteria, Synechococcus spp., picoeukaryotes, and heterotrophic nanoflagellates). Studies using this approach have been conducted in coastal ecosystems and estuaries to assess nutrient concentrations and nutrient metabolism [22].

Airtight Plexiglas lids were fitted with sampling ports at the inflow and outflow of each chamber (Figure 2). Following deployment, chambers were transported to the incubation site within one hour and placed in 100 L acrylic tanks filled with site water, which had been collected near the sampling area. The tanks were maintained at in situ temperature using a chiller, and exposed to natural sunlight to simulate ambient light conditions [23]. Gentle water mixing was achieved using a magnetic stirrer controlled by a rotating magnet placed beneath each tank. A stir bar, suspended approximately 10 cm above the sediment surface, was rotated at 30 rpm to avoid disturbing the sediment structure while ensuring water homogeneity. Chambers were pre-incubated for six hours to allow stabilization of concentration gradients, after which they were sealed with Plexiglas lids for a 30 min equilibration period prior to sampling. The incubation lasted 36 h. Water samples were taken every 2 h from midnight to noon on 12 and 13 April for microbial analysis. From noon on 12 April to midnight on 13 April, the sediment core was left unsealed to allow gas exchange with the atmosphere. This natural aeration process helped minimize potential bottle effects during the incubation. A significant proportion of the seagrass species in the collected cores for ex situ incubation were H. uninervis and H. ovalis. There were between 20 and 40 shoots per 0.008 m² for H. uninervis, while there were between 2 and 20 shoots per 0.008 m² for H. ovalis. Using a YSI ProDSS multiparameter water quality checker, temperature and salinity were measured, and DO was measured with a thermo DO probe. Additionally, spectrophotometry was used to determine chlorophyll a (Chl a). On top of the incubation tank, a SQ-420X Smart Quantum Sensor was used to measure photosynthetically active radiation (PAR) levels.

2.2. Flow Cytometric Analyses

The abundances of viruses, bacteria, Synechococcus spp., picoeukaryotes, and nanoflagellates were collected from the six chambers (three of seagrass habitat and three of unvegetated sediment) and quantified using a CytoFLEX S flow cytometer (Beckman Coulter, Brea, CA, USA). Triplicate samples of viruses and bacteria were analyzed. Samples were thawed, diluted 1:10 in 0.2 μm filtered TE buffer (10 mM Tris, 1 mM EDTA), and stained with SYBR Green I (1:500 dilution; Molecular Probes, Eugene, OR, USA) at 80 °C for 10 min [24]. Fluorescent microspheres (1 μm diameter; Molecular Probes) were added to each sample at a final concentration of approximately 10⁵ beads mL⁻¹ to standardize measurements [25]. The sheath fluid was a phosphate-buffered saline solution (PBS). Each sample was analyzed for forward-angle light scatter (FSC), side-angle light scatter (SSC), and green fluorescence (SYBR Green-I). Synechococcus spp., picoeukaryotes, and pigmented nanoflagellates (PNF) were distinguished based on their pigment autofluorescence and FSC properties. Heterotrophic nanoflagellates (HNF) were identified according to the method of Christaki et al. [26], based on their green and red fluorescence signals in combination with SSC characteristics. To discriminate picophytoplankton (<2 μm) from nanophytoplankton (2–20 μm), 2 μm fluorescent beads were used to establish FSC thresholds. FCM was triggered by fluorescent particles only, ensuring specificity in cell detection. The analysis followed a modified protocol for HNF detection derived from standard bacterial FCM techniques. High fluorescence and FSC values enabled the differentiation of HNF by lowering detector voltage settings for SSC and green fluorescence to reduce background noise.

2.3. Data Analysis

The values are presented as mean ± standard deviation (n = 3). Data analyses were performed to compare seagrass and control treatments with the t-test. The significance of the regression analysis was tested using analysis of variance (ANOVA). STATISTICA 7.0 software was used to perform all statistical operations. A probability value of <0.05 was considered significant.

3. Results

3.1. Temporal Variability of Temperature, Salinity, and DO

Figure 3 presents the temporal variation of environmental parameters in the seagrass and control treatments during the two-day ex situ core incubation. Temperature ranged from 22 to 29 °C in both treatments (Figure 3A), whereas salinity levels ranged from 35 to 36 (Figure 3B). As shown in Figure 3C, saturation levels of DO in seagrass were more variable than those in controls (t-test, p < 0.05), with values ranging from 54% to 224% and 92% to 123%, respectively. In both treatments, DO saturation levels differed by time of day, with lower values at night and higher values during the day. A comparison of the Chl a concentrations between seagrass and non-seagrass chambers revealed that seagrass had higher values than non-seagrass chambers (t-test, p < 0.05) (Figure 3D). The average PAR was measured at 953 μmol m⁻² s⁻¹ on the first day of incubation, increasing slightly to 1026 μmol m⁻² s⁻¹ on the second day.

3.2. Temporal Variability of Microbial Community Abundance

Microbial community dynamics were monitored over a 36 h period following exposure to seagrass and control treatments, with triplicate subsamples analyzed for viral, bacterial, Synechococcus spp., picoeukaryotic, and nanoflagellate abundances. The flow cytometric counts for each group are shown in Figure 4. In this study, viral abundance was significantly higher (t-test, p < 0.05) in seagrass treatments (2.4–9.2 × 10⁶ viruses mL⁻¹) compared to the control (0.6–2.0 × 10⁶ viruses mL⁻¹) during the study period (Figure 4A). Furthermore, bacterial abundance in the seagrass treatment ranged from 5.1 × 10⁵ to 16.0 × 10⁵ cells mL⁻¹, whereas in the control treatment, bacterial abundance ranged from 7.9 × 10⁵ to 16.6 × 10⁵ cells mL⁻¹. Bacterial abundance was relatively low in the seagrass treatment (t-test, p < 0.05) (Figure 4B). Consequently, VBR was markedly elevated in the seagrass treatment compared to the control throughout the incubation period (t-test, p < 0.05; Figure 4C).

Synechococcus spp. dominated the abundance of picophytoplankton by making up about 80% of the total picophytoplankton community (Figure 4D). A significant difference in Synechococcus spp. and picoeukaryotic abundance was found between the seagrass and control treatments (t-test, p < 0.05) (Figure 4D,E). For HNF, peak abundances were observed during daylight hours in the seagrass treatment, ranging from 1.3 × 10³ to 2.3 × 10³ cells mL⁻¹ (Figure 4F). However, there were no significant differences between the seagrass and control groups with respect to the time course of the HNF abundance (t-test, p > 0.05) (Figure 4F).

Regression analysis was conducted to assess the relationships between microbial groups in seagrass and control treatments (Figure 5). In the seagrass treatment, viral abundance exhibited a significant positive correlation with bacterial abundance (r² = 0.58, p < 0.05; Figure 5A). No such relationship was observed in the control group (Figure 5A). There was no clear correlation between HNF and bacterial abundance in either treatment (Figure 5B). In contrast, HNF abundance was positively correlated with Synechococcus spp. in both the seagrass (r² = 0.31) and control (r² = 0.28) treatments, with stronger trends in the seagrass samples (Figure 5C). Additionally, HNF abundance displayed a weaker but positive correlation with picoeukaryotic abundance in the seagrass treatment (r² = 0.18; Figure 5D). No clear relationships were observed between HNF and picoeukaryotes in the control samples.

3.3. Increased Ratio of Microbial Community in the Seagrass Incubation

To assess the relative changes in microbial abundances, we calculated the increase in ratio between seagrass and non-seagrass chambers as (seagrass − control)/control (Figure 6). Viral abundance showed a pronounced increase in the seagrass treatment, with the highest value occurring at 10:30 am on April 12, reaching approximately an 11-fold increase compared to the control (Figure 6A). In contrast, bacterial abundance was consistently lower in the seagrass chambers, with relative reductions ranging from −0.02 to −0.49 (Figure 6B). VBR in the seagrass treatment displayed elevated values particularly during the midnight to early morning period (Figure 6C). The relative abundances of Synechococcus spp. and picoeukaryotes increased significantly in the seagrass treatment, particularly from early morning through midday on April 12 (Figure 6D,E). Additionally, HNF abundance fluctuated over time, with the increased ratio ranging from −0.58 to 2.03 across the sampling period (Figure 6F).

A significant positive linear correlation was observed between the ratio of bacterial abundance in the seagrass treatment and viral abundance compared to the control treatment (r² = 0.20, p < 0.05, Figure 7A). There was no significant relationship between HNF abundance and the increased ratio of bacterial abundance (Figure 7B). In contrast, the increased ratios of Synechococcus spp. and picoeukaryotes were both significantly and positively correlated with HNF abundance (r² = 0.25 and r² = 0.56, respectively; p < 0.05; Figure 7C,D).

4. Discussion

Coastal seagrasses support a range of marine keystone species at varying trophic levels, making them a critical component of coastal ecosystems [27,28]. In addition to their role in carbon sequestration and sediment stabilization, seagrass meadows produce considerable amounts of organic matter that support diverse microbial communities [29]. A key component of these communities is heterotrophic bacteria, which perform the functions of decomposers and facilitate the transfer of organic matter from detrital deposits to higher trophic levels. Because most seagrass-derived primary production is not directly consumed by herbivores, microbial processing is essential for converting it into bioavailable forms [5]. Organic matter in seagrass ecosystems originates not only from the seagrass itself but also from epiphytes, benthic microalgae, macroalgae, and allochthonous inputs, creating a complex pool of potential substrates for bacterial growth [5].

This study found significantly lower bacterial abundance in the seagrass treatment compared to the unvegetated control (Figure 4B), which contrasts with our initial hypothesis. We had expected that seagrass-derived DOC would enhance bacterial growth due to its continuous release during photosynthesis. This suggests that the concentration or bioavailability of seagrass-derived DOC may be insufficient to support bacterial proliferation under the incubation conditions. Supporting this, previous work in Florida Bay showed that bacterial communities preferentially utilize labile organic matter enriched in carbon, nitrogen, and phosphorus, and that phytoplankton production, rather than seagrass biomass, was more closely associated with bacterial activity and extracellular enzyme production [5]. Thus, while seagrasses contribute to the DOC pool, their inputs may be less readily bioavailable than those from phytoplankton, which could explain the relatively lower bacterial abundances observed in our seagrass incubations. Because DOC concentrations and composition were not measured in this study, future work should explore their role in shaping bacterial activity in seagrass systems. Additionally, microbial community composition likely varies between vegetated and unvegetated habitats. Studies of the seagrass phyllosphere have demonstrated that leaf surfaces harbor rich and diverse microbial communities distinct from those in the rhizosphere and the surrounding water column [30]. Seagrass ecosystems, including sediments, seagrass leaves, and seawater, support microbial groups responsible for metabolizing different kinds of organic matter and nutrients. These functional differences may have contributed to the observed variation in microbial abundance between treatments.

Mishra et al. [31] previously reported that bacterial abundance was highest in sediments, followed by seagrass leaves and the water column. Similar spatial patterns of bacterial distribution have been observed in other seagrass meadows as well [32]. The relatively low bacterial abundance in the water column may, in part, reflect nutrient limitation in oligotrophic environments, where nutrient availability is a major factor controlling bacterial growth [33]. The findings of Mishra et al. [31] further suggest that seagrass ecosystems support a closed loop of heterotrophic bacterial activity, with similar active bacterial communities found across sediments, seagrass surfaces, and the water column. It is believed that this loop is caused by the nutrient limitation in these oligotrophic waters, and the bacteria benefit from being in a closed loop as they have the opportunity to acquire nutrients, cycle them, and maintain ecosystem productivity.

To our knowledge, this is the first study to document differences in viral abundance between vegetated and unvegetated treatments at this site. Viral abundance was significantly higher in the seagrass treatment compared to the control. Identifying temporal and spatial patterns in viral abundance, along with their relationships to other microbial groups, is essential for understanding the ecological roles of viruses in aquatic systems [34]. In this study, a positive correlation between viral and bacterial abundance was observed in the seagrass treatment, implying that a large proportion of the viral population likely consists of bacteriophages. It is possible that viruses play a significant role in regulating the abundance of bacteria in waters associated with seagrasses. To further assess this interaction, we calculated VBR, which can provide insight into viral production and infection dynamics [35]. The seagrass treatment exhibited a significantly higher VBR than the control (Figure 4C), implying elevated viral activity. This aligns with previous studies suggesting that higher VBR values may be indicative of increased infection rates or greater viral release per infected host cell [36]. Together, these findings highlight the need for deeper investigations into plant–microbe–virus interactions in oligotrophic coastal systems, where tightly coupled microbial loops may be central to ecosystem function and resilience.

Seagrass presence increases the physical complexity of soft-sediment environments, creating localized environmental conditions that differ markedly from adjacent unvegetated areas. As highly productive ecosystems, seagrass meadows support a wide range of biological interactions and have been the focus of extensive research on their associations with various marine organisms [27,37]. In marine environments, Synechococcus spp. constitute the largest proportion of prokaryotic picophytoplankton, contributing substantially to primary production. A significant portion of phytoplankton biomass is contributed by Synechococcus spp. in some systems (>50%) [38]. Additionally, the conceptual framework emphasizes competition among phytoplankton and seagrasses for resources. The competition for inorganic nutrients and light was fierce between Synechococcus spp. and seagrass as autotrophs. Indeed, several studies have reported reduced Synechococcus spp. abundance in seagrass ecosystems relative to open waters, likely due to shading or nutrient competition [11,39]. Our findings differ from those of previous studies, including Li et al. [11] and Hamisi et al. [38], which reported lower Synechococcus spp. and picoeukaryotic abundances in seagrass habitats. In our study, we observed significantly higher abundances of both groups in the seagrass treatment compared to the control (Figure 4D,E). A possible explanation is that the increased viral abundance observed in the seagrass treatment (Figure 4A) may have led to enhanced viral lysis of heterotrophic bacteria, promoting the regeneration of ammonium and other nutrients. Viral lysis is known to contribute substantially to nutrient recycling in marine systems and has been shown to support phytoplankton growth under nutrient-limited conditions [18,40]. Specifically, Synechococcus spp. have been observed to benefit from virus-mediated nutrient regeneration in both oligotrophic oceanic and coastal environments [40]. A previous study suggested that the growth of Synechococcus spp. may depend on the regeneration of nutrients from the lysis of heterotrophic bacteria [40], and that interactions among phytoplankton, bacteria, and viruses are central to microbial loop dynamics and biogeochemical cycling in aquatic ecosystems [18,41]. Despite the importance of viral lysis, future research will need to determine the amount of nutrients released and regeneration fluxes during viral lysis.

Furthermore, there is an alternative explanation that we cannot exclude: nutrient recycling in the ocean serves as a link between phytoplankton and heterotrophic bacteria. In the case of phytoplankton and bacteria, the death of these cells releases DOC, which is a rich source of free and combined amino acids [42] that are taken up and metabolized by bacteria. When the carbon-to-nitrogen ratio (C:N) of DOC is low relative to bacterial nutritional requirements, bacteria deaminate organic compounds and release ammonium [43] in order to acquire carbon for energy and growth [44]. Ammonium release has been shown to promote phytoplankton growth [45].

Previous studies have provided valuable insights into the complex interactions between major primary producers in mesotidal coastal lagoons, namely phytoplankton and seagrass environments [12]. While phytoplankton typically exhibit high intrinsic growth potential, their standing biomass is often regulated by top-down controls, particularly grazing pressure. In these systems, Synechococcus spp. and picoeukaryotes are especially susceptible to such regulation. In the present study, the abundances of Synechococcus spp., picoeukaryotes, and HNF were measured in the seagrass treatment, and significant relationships among these groups were observed (Figure 4C, D). HNF abundance appeared closely coupled to that of picophytoplankton, suggesting that nanoflagellates are key grazers of Synechococcus spp. and picoeukaryotes in seagrass-associated waters. In addition to biotic interactions such as grazing, seagrass leaves may also act as physical traps for picophytoplankton, leading to negative net population growth in their presence. Although picophytoplankton may exhibit high growth rates, their biomass can remain low in seagrass systems due to removal through entrapment by seagrass surfaces [46]. An important loss process in controlling phytoplankton biomass and sediment loads in shallow waters may be the trapping of phytoplankton from the water column by epifaunal suspension feeders and phagotrophs on the leaves. Consequently, it appears that seagrass canopies play a significant role in the transfer of carbon from the water column to the benthic region in shallow coastal ecosystems, since they may capture most of the planktonic primary production. This study demonstrates the complex dynamic between seagrass and microbial communities and emphasizes the need for a better understanding of the underlying processes in ecosystems. In seagrass habitats, studies examining changes in microbial communities over short-term timescales are rare. A coordinated approach that includes multiple sample types, such as seawater, sediment, and seagrass, within this timeframe would be highly valuable for capturing the full scope of microbial interactions and responses.

5. Conclusions

In summary, this study demonstrated clear differences in microbial community dynamics between seagrass and unvegetated (control) treatments. Notably, bacterial abundance was lower in the seagrass treatment, while viral abundance was significantly higher, suggesting that viruses may play a key role in regulating bacterial populations through enhanced lysis in seagrass environments. This elevated viral activity likely contributed to nutrient regeneration, which in turn supported increased growth of Synechococcus spp. and picoeukaryotes. Moreover, HNF abundance was closely linked to picophytoplankton dynamics, indicating that HNF are likely the primary grazers of picophytoplankton in seagrass habitats. These findings highlight the importance of seagrass meadows in modulating microbial interactions and nutrient cycling, and they address a critical gap in our understanding of how vegetated and unvegetated habitats contribute to microbial ecosystem functioning in subtropical coastal regions.