Special Issue on Discovery and Research on Aquatic Microorganisms

The Special Issue entitled “Discovery and Research on Aquatic Microorganisms” wished to improve our knowledge on microorganisms living in aquatic environments [...]

Environmental concerns about the effects of effluents from wastewater treatment plants in tourist areas of the Alps: toxicity in aquatic microorganisms

Are the effluents of wastewater treatment plants in high mountains of concern for aquatic biodiversity? To answer this question, we carried out an experimental study testing the short-term toxicity of some Pharmaceutical Active Compounds (PhACs) in the effluents of a plant in a mountain valley of the Italian Alps sampled during the high tourist season (i.e., the ski season) when PhACs contamination is higher. We used different tools, taking as a model the bacterium Aliivibrio fischeri: the “whole-mixture approach” (Microtox test), “component-based approach”, predictive models “Concentration Addition (CA)”, “Independent Action (IA)”, and Combination Index (CI)”. We investigated the nature of interactions potentially occurring among seven selected PhACs (clarithromycin, naproxen, acetaminophen (paracetamol), ibuprofen, diclofenac, carbamazepine, and amoxicillin). This study showed that anti-inflammatory ibuprofen and diclofenac have higher short-term toxicity (IC50 <100 mg L-1) for A. fischeri compared with antibiotics, whose toxic effects are expected to become visible in the long term. Furthermore, based on the CI method, the seven PhACs seem not to interact in a synergistic or antagonistic way, but the final effect of their mixture seems to be equal to the sum of their individual effects. Notwithstanding the high tourist pressure, the Microtox test reported an overall toxicity of only 21%, which drops to 7% in the receiving water body, the Vermigliana stream. These values, besides the predictions by CA and IA, are not alarming per se, i.e., the treated effluent of the plant in the period of maximum tourist pressure can be considered no harmful to aquatic microorganisms. However, based on other studies highlighting negative effects of the diluted treated effluent of the same plant on macroinvertebrate community structure, we suggest that other model organisms be considered, including algae, insects, and fish, to assess the real ecological risk to wildlife of an effluent. The experimental tests on A. fischeri are useful for fast, preliminary information on the level of risk for freshwater ecosystems, but they should be combined with field studies and experiments on the wild populations to increase the ecological realism.

Agricultural pressures impair trophic link between aquatic microorganisms and invertebrates

Decadal declines in aquatic ecosystem health prompted monitoring efforts and studies on effects of human practices on aquatic biodiversity, yet a consideration of ecological processes and trophic linkages is increasingly required to develop an in-depth understanding of aquatic food webs and its vulnerability to human activities. Here, we test in laboratory incubations using natural organic matter whether agricultural practices have an effect on two interacting ecological processes (i.e., decomposition and invertebrate growth) as the relevant temporal components of the trophic linkage between aquatic microbial communities and aquatic invertebrates. We further assess whether these altered trophic interactions are visible on ecologically relevant scales. We observed clear patterns in agricultural constraints on microbial decomposition, which coincided with reduced invertebrate growth and an unexpected increase in invertebrate consumption of organic matter. Similar differences in invertebrate length depending on land use were observed in our field survey, thereby providing important clues on the relevance and vulnerability of interdependent processes that can serve to improve future forays in monitoring ecosystem health.

The microbial dimension of submarine groundwater discharge: current challenges and future directions

Despite the relevance of submarine groundwater discharge (SGD) for ocean biogeochemistry, the microbial dimension of SGD remains poorly understood. SGD can influence marine microbial communities through supplying chemical compounds and microorganisms, and in turn, microbes at the land-ocean transition zone determine the chemistry of the groundwater reaching the ocean. However, compared to inland groundwater, little is known about microbial communities in coastal aquifers. Here we review the state-of-the-art of the microbial dimension of SGD, with emphasis on prokaryotes, and identify current challenges and future directions. Main challenges include improving the diversity description of groundwater microbiota, characterized by ultra-small, inactive and novel taxa, and by high ratios of sediment-attached versus free-living cells. Studies should explore microbial dynamics and their role in chemical cycles in coastal aquifers, the bidirectional dispersal of groundwater and seawater microorganisms, and marine bacterioplankton responses to SGD. This will require combining sequencing methods, visualization, and linking taxonomy to activity, but also considering the entire groundwater-marine continuum. Interactions between traditionally independent disciplines (e.g., hydrogeology, microbial ecology) are needed to frame the study of terrestrial and aquatic microorganisms beyond the limits of their presumed habitats, and to foster our understanding of SGD processes and their influence in coastal biogeochemical cycles.

Cyanobacterial blooms contribute to the diversity of antibiotic-resistance genes in aquatic ecosystems

Cyanobacterial blooms are a global ecological problem that directly threatens human health and crop safety. Cyanobacteria have toxic effects on aquatic microorganisms, which could drive the selection for resistance genes. The effect of cyanobacterial blooms on the dispersal and abundance of antibiotic-resistance genes (ARGs) of concern to human health remains poorly known. We herein investigated the effect of cyanobacterial blooms on ARG composition in Lake Taihu, China. The numbers and relative abundances of total ARGs increased obviously during a Planktothrix bloom. More pathogenic microorganisms were present during this bloom than during a Planktothrix bloom or during the non-bloom period. Microcosmic experiments using additional aquatic ecosystems (an urban river and Lake West) found that a coculture of Microcystis aeruginosa and Planktothrix agardhii increased the richness of the bacterial community, because its phycosphere provided a richer microniche for bacterial colonization and growth. Antibiotic-resistance bacteria were naturally in a rich position, successfully increasing the momentum for the emergence and spread of ARGs. These results demonstrate that cyanobacterial blooms are a crucial driver of ARG diffusion and enrichment in freshwater, thus providing a reference for the ecology and evolution of ARGs and ARBs and for better assessing and managing water quality.

Auxotrophic interactions: a stabilizing attribute of aquatic microbial communities?

ABSTRACT Auxotrophy, or an organism's requirement for an exogenous source of an organic molecule, is widespread throughout species and ecosystems. Auxotrophy can result in obligate interactions between organisms, influencing ecosystem structure and community composition. We explore how auxotrophy-induced interactions between aquatic microorganisms affect microbial community structure and stability. While some studies have documented auxotrophy in aquatic microorganisms, these studies are not widespread, and we therefore do not know the full extent of auxotrophic interactions in aquatic environments. Current theoretical and experimental work suggests that auxotrophy links microbial community members through a complex web of metabolic dependencies. We discuss the proposed ways in which auxotrophy may enhance or undermine the stability of aquatic microbial communities, highlighting areas where our limited understanding of these interactions prevents us from being able to predict the ecological implications of auxotrophy. Finally, we examine an example of auxotrophy in harmful algal blooms to place this often theoretical discussion in a field context where auxotrophy may have implications for the development and robustness of algal bloom communities. We seek to draw attention to the relationship between auxotrophy and community stability in an effort to encourage further field and theoretical work that explores the underlying principles of microbial interactions.

Magnetosomes could be protective shields against metal stress in magnetotactic bacteria

Magnetotactic bacteria are aquatic microorganisms with the ability to biomineralise membrane-enclosed magnetic nanoparticles, called magnetosomes. These magnetosomes are arranged into a chain that behaves as a magnetic compass, allowing the bacteria to align in and navigate along the Earth’s magnetic field lines. According to the magneto-aerotactic hypothesis, the purpose of producing magnetosomes is to provide the bacteria with a more efficient movement within the stratified water column, in search of the optimal positions that satisfy their nutritional requirements. However, magnetosomes could have other physiological roles, as proposed in this work. Here we analyse the role of magnetosomes in the tolerance of Magnetospirillum gryphiswaldense MSR-1 to transition metals (Co, Mn, Ni, Zn, Cu). By exposing bacterial populations with and without magnetosomes to increasing concentrations of metals in the growth medium, we observe that the tolerance is significantly higher when bacteria have magnetosomes. The resistance mechanisms triggered in magnetosome-bearing bacteria under metal stress have been investigated by means of x-ray absorption near edge spectroscopy (XANES). XANES experiments were performed both on magnetosomes isolated from the bacteria and on the whole bacteria, aimed to assess whether bacteria use magnetosomes as metal storages, or whether they incorporate the excess metal in other cell compartments. Our findings reveal that the tolerance mechanisms are metal-specific: Mn, Zn and Cu are incorporated in both the magnetosomes and other cell compartments; Co is only incorporated in the magnetosomes, and Ni is incorporated in other cell compartments. In the case of Co, Zn and Mn, the metal is integrated in the magnetosome magnetite mineral core.