Marine picoplankton populations are influenced by bottom-up control (resource availability, temperature) and top-down processes (e.g. grazing, viral lysis); however, seasonal variability in the bottom-up and top-down controls of picoplankton remains unclear, especially in cold-season studies, which remain scarce due to sampling challenges. To gain insight into the role of bottom-up and top-down control as determinants of picoplankton population abundance in the cold season, we examined bacterial, Synechococcus spp., and picoeukaryotic phytoplankton growth and mortality in a subtropical coastal region during the cold season (<25 degrees C) using modified dilution experiments. Results showed bacterial and Synechococcus spp. production rates were positively correlated with temperature, with growth rates ranging from 0.02 to 4.91 d(-1) (bacteria) and 0.29 to 6.28 d(-1) (Synechococcus spp.). Nanoflagellate grazing accounted for 43-80% of bacterial and 64-448% of Synechococcus spp. production, with carbon losses ranging from 2.07 to 18.82 mg C m(-3) d(-1) (bacteria) and from 1.94 to 92.93 mg C m(-3) d(-1) (Synechococcus spp.), while viral lysis was minimal for both but more significant for picoeukaryotic phytoplankton. Moreover, the 3 microbial groups exhibited distinct patterns: (1) Bacterial production was nearly balanced by total mortality (grazing and viral lysis) across all temperatures; (2) Synechococcus spp. contributed significantly to picophytoplankton production but declined sharply below 20 degrees C as grazing exceeded production; (3) Picoeukaryotic phytoplankton production surpassed Synechococcus spp. below 20 degrees C. These findings underscore temperature-driven microbial production and grazing-dominated mortality, suggesting reduced viral lysis due to increased lysogeny.
Browning and eutrophication threaten freshwater biodiversity. However, the independent and joint effects of browning and nutrient enrichment on algal communities remain poorly known. Our aim was to investigate such effects on benthic diatoms using a field experiment with freshwater microcosms including 5 levels of water color (clear to humic) and 2 levels of nutrients (control and nutrient enrichment). We also compared diatom communities in the experimental pools to those of surrounding natural ecosystems. We found that the experimental pools had low species richness, most probably due to dispersal limitation, showing no clear response to the independent or combined effects of water color and nutrients. Water color was significantly associated with community composition, but only in the nutrient control pools on the first sampling occasion. Nutrients were not significantly related to community composition. The communities in the experimental pools and surrounding natural waterbodies were highly dissimilar. The low diversity in the experimental pools suggests that environmental conditions and dispersal limitation may jointly limit the number of species. The fact that richness did not show a clear response to color or nutrients suggests that tolerant species were dominant in the pools. The results further imply that nutrient enrichment may weaken the effect of browning, potentially leading to diatom community homogenization. Our findings indicate that the diatoms colonizing new habitats may possibly originate from the surrounding terrestrial habitats, in addition to aquatic ecosystems. Our study highlights the importance of investigating multiple environmental pressures on algal communities.
The pervasive use of herbicides in aquatic environments has heightened the need to assess their specific effects on photosynthetic organisms such as microalgae and macrophytes. In freshwater ecosystems, species interactions, nutrient availability, and pollutants such as paraquat can significantly alter community dynamics. We investigated the effect of paraquat on allelopathic interactions between Chlorella sorokiniana and Lemna minor. In monocultures, C. sorokiniana exhibited increased growth and chlorophyll content at lower paraquat concentrations (0.01-1 mg l-1). However, higher concentrations (10 and 50 mg l-1) triggered oxidative stress characterized by elevated peroxidase (POD) activity and lipid peroxidation (malondialdehyde, MDA). Similarly, L. minor showed reduced chlorophyll content and biomass at higher paraquat doses, whereas its antioxidant activity increased at sublethal concentrations. Co-culturing of the 2 species intensified oxidative stress responses, particularly when paraquat was present, with both species exhibiting heightened hydrogen peroxide (H2O2) levels and altered chlorophyll synthesis. Co-cultivation with C. sorokiniana enhanced the chlorophyll production by L. minor in the absence of paraquat, indicating potential synergistic interactions. These findings reveal that paraquat exposure disrupts the physiology of individual species and alters allelopathic interactions, with potential implications for ecosystem structure and function. Understanding these complex dynamics is essential for developing strategies to mitigate the ecological risks posed by herbicide contamination of aquatic environments.
Microcystis blooms frequently occur in eutrophic freshwater bodies, where epiphytic bacteria interact with Microcystis colonies through cooperative or competitive relationships. Declining colonies are sedentary and are always found in the lower water layer, whereas healthy, mobile colonies reside exclusively in the upper layer. In this study, we compared the composition of epiphytic bacterial communities associated with Microcystis colonies in the upper and lower water layers of an aquaculture pond. A dominant Flavobacterium strain was isolated from the lower epiphytic bacterial community, and its colony-degrading potential and carbohydrate-active enzyme (CAZyme)-encoding genes were analyzed. The results showed significant differences in community composition between the upper and lower epiphytic bacterial communities. Compared to the upper epiphytic bacterial community, the relative abundance of Flavobacterium in the lower community increased by 691%, whereas that of Cetobacterium decreased by 92.84%. The dominant Flavobacterium strain isolated from the lower community was identified as F. columnare TN-1. When Microcystis colonies were treated with F. columnare TN-1 for 12 h, both the size of individual colonies and the extracellular polysaccharide (EPS) content per cell significantly decreased, indicating that F. columnare TN-1 could degrade Microcystis EPS and lead to colony disaggregation. Genomic analysis revealed that F. columnare TN-1 harbors 274 CAZyme-encoding genes, representing 8.5% of its total gene repertoire, including glycoside hydrolase and polysaccharide lyase family members implicated in EPS degradation.
Salt marshes have a high capacity to stabilize organic carbon in their soils due to their high primary production and the prevailing anoxic conditions. However, halophytes transport oxygen to their rhizosphere, creating a heterogeneous environment where oxic, microoxic and anoxic micro-niches coexist. Understanding which mechanisms, beyond anoxia, protect organic matter (OM) from microbial consumption is crucial for explaining its persistence in the soil. We assessed the potential of soil microbial communities to consume carbon sources and to identify compounds that are preserved by mechanisms other than anoxia. Soil samples were collected at several depths, up to 89 cm, in 3 distinct halophyte communities and bare sediments in the Ria de Aveiro (Portugal). EcoPlatesTM microbial incubations and Fourier-transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR) were successfully applied to explore OM microbial mineralization (38 samples). The soil samples showed a high spatial homogeneity in microbial potential consumption, regarding both depth and halophyte species. The high potential aerobic consumption of all samples showed that, in the case of soil erosion, aerobic degradation of the accumulated soil OM may begin with the existent microbial community. Two of the carbon sources that were not consumed were likely embedded within larger molecules-d-galacturonic acids in hemicellulose and 4-hydroxybenzoic acids in phenolic compounds. The latter may have been accumulated, thereby contributing to the role of salt marshes as carbon sinks. Our study shows the potential of microbial incubations coupled with FTIR to greatly increase our understanding of OM dynamics in salt marsh soils by exploring the accumulation of specific compounds.
