Virus-to-prokaryote ratio in the Salar de Huasco and different ecosystems of the Southern hemisphere and its relationship with physicochemical and biological parameters

The virus-to-prokaryote ratio (VPR) has been used in many ecosystems to study the relationship between viruses and their hosts. While high VPR values indicate a high rate of prokaryotes' cell lysis, low values are interpreted as a decrease in or absence of viral activity. Salar de Huasco is a high-altitude wetland characterized by a rich microbial diversity associated with aquatic sites like springs, ponds, streams and a lagoon with variable physicochemical conditions. Samples from two ponds, Poza Rosada (PR) and Poza Verde (PV), were analyzed by epifluorescence microscopy to determine variability of viral and prokaryotic abundance and to calculate the VPR in a dry season. In addition, to put Salar de Huasco results into perspective, a compilation of research articles on viral and prokaryotic abundance, VPR, and metadata from various Southern hemisphere ecosystems was revised. The ecosystems were grouped into six categories: high-altitude wetlands, Pacific, Atlantic, Indian, and Southern Oceans and Antarctic lakes. Salar de Huasco ponds recorded similar VPR values (an average of 7.4 and 1.7 at PR and PV, respectively), ranging from 3.22 to 15.99 in PR. The VPR variability was associated with VA and chlorophyll a, when considering all data available for this ecosystem. In general, high-altitude wetlands recorded the highest VPR average (53.22 ± 95.09), followed by the Oceans, Southern (21.91 ± 25.72), Atlantic (19.57 ± 15.77) and Indian (13.43 ± 16.12), then Antarctic lakes (11.37 ± 15.82) and the Pacific Ocean (6.34 ± 3.79). Physicochemical variables, i.e., temperature, conductivity, nutrients (nitrate, ammonium, and phosphate) and chlorophyll a as a biological variable, were found to drive the VPR in the ecosystems analyzed. Thus, the viral activity in the Wetland followed similar trends of previous reports based on larger sets of metadata analyses. In total, this study highlights the importance of including viruses as a biological variable to study microbial temporal dynamics in wetlands considering their crucial role in the carbon budgets of these understudied ecosystems in the southern hemisphere.

Contrasting land-uses in two small river basins impact the colored dissolved organic matter concentration and carbonate system along a river-coastal ocean continuum

Human activities have led to an increase in land use change, with effects on the structure and functioning of ecosystems. The impact of contrasting land uses along river basins on the concentration of colored dissolved organic matter (CDOM) reaching the coastal zone, and its relationship with the carbonate system of the adjacent coastal ocean, is poorly known. To understand the relationship between land use change, CDOM and its influence on the carbonate system, two watersheds with contrasting land uses in southern Chile were studied. The samples were collected at eight stations between river and adjacent coastal areas, during three sampling campaigns in the austral summer and spring. Chemical and biological samples were analyzed in the laboratory according to standard protocols. Landsat 8 satellite images of the study area were used for identification and supervised classification using remote sensing tools. The Yaldad River basin showed 82% of native forest and the Colu River basin around 38% of grassland (agriculture). Low total alkalinity (A_T) and Dissolved Inorganic Carbon (DIC), but high CDOM proportions were typically observed in freshwater. A higher CDOM and humic-like compounds concentration was observed along the river-coastal ocean continuum in the Yaldad basin, characterized by a predominance of native forests. In contrast, nutrient concentrations, A_T and DIC, were higher in the Colu area. Low CaCO₃ saturation state (Ω_Ar < 2) and even undersaturation conditions were observed at the coastal ocean at Yaldad. A strong negative correlation between A_T, DIC and Ω_Ar with CDOM/fDOM, suggested the influence of terrestrial material on the seawater carbon chemistry. Our results provide robust evidence that land uses in river basins can influence CDOM/fDOM proportion and its influence on the carbonate chemistry of the adjacent coastal, with potential implications for the shellfish farming activity in this region.

Greenhouse gases and biogeochemical diel fluctuations in a high-altitude wetland

The landscapes of high-altitude wetland ecosystems are characterized by different kinds of aquatic sites, including ponds holding conspicuous microbial life. Here, we examined a representative pond of the wetland landscape for dynamics of greenhouse gases, and their association with other relevant biogeochemical conditions including diel shifts of microbial communities' structure and activity over two consecutive days. Satellite image analysis indicates that the area of ponds cover 238 of 381.3 Ha (i.e., 62.4%), representing a significant landscape in this wetland. Solar radiation, wind velocity and temperature varied daily and between the days sampled, influencing the biogeochemical dynamics in the pond, shifting the pond reservoir of inorganic versus dissolved organic nitrogen/phosphorus bioavailability, between day 1 and day 2. Day 2 was characterized by high dissolved organic nitrogen/phosphorus and N₂O accumulation. CH₄ presented a positive excess showing maxima at hours of high radiation during both days. The microbial community in the sediment was diverse and enriched in keystone active groups potentially related with GHG recycling including bacteria and archaea, such as Cyanobacteria, Verrucomicrobia, Rhodobacterales and Nanoarchaeaota (Woesearchaeia). Archaea account for the microbial community composition changes between both days and for the secondary productivity in the water measured during day 2. The results indicate that an intense recycling of organic matter occurs in the pond systems and that the activity of the microbial community is correlated with the availability of nutrients. Together, the above results indicate a net sink of CO₂ and N₂O, which has also been reported for other natural and artificial ponds. Overall, our two-day fluctuation study in a representative pond of a high-altitude wetland aquatic landscape indicates the need to explore in more detail the short-term besides the long-term biogeochemical variability in arid ecosystems of the Andes plateau, where wetlands are hotspots of life currently under high anthropogenic pressure.

Greenhouse gas cycling by the plastisphere: The sleeper issue of plastic pollution

Plastic is an allochthonous material to marine ecosystems but is rapidly colonized by marine microbial communities, with an as yet unclear contribution to biogeochemical cycles. In this study, we investigated the influence of an active microbial community grown on microplastic particles (the plastisphere) on CO₂ and N₂O recycling and its potential role in greenhouse gas inventories and air-sea exchange. Microplastics were collected during two cruises (Cimar 21 and FIP Montes Submarinos) from the surface layer (5 m depth) from several contrasting trophic regions of the South Pacific Ocean, i.e., from a transition zone off the eutrophic coastal upwelling of Chile, to a mesotrophic transition area of oceanic seamounts and, finally, to an oligotrophic zone in the South Pacific Subtropical Gyre. . Experiments were carried out onboard to evaluate CO₂ and N₂O production/consumption by the plastisphere. The active microbial community and its specific quantification were determined for Cimar 21 using iTag 16 S rRNA. The experiments showed that the plastisphere generally contributed to CO₂ and N₂O production/consumption, with rates ranging from -20.5 (consumption) to +4.5 (production) μmol/m²/d. The seamounts and the transition zone presented the highest production/consumption rates. The experiments performed in the two seamount stations showed that production and consumption of CO₂ were related to the environmental nutrient concentration. Both stations presented N₂O consumption that was associated with the high nitrogen deficit of the subantarctic water mass. The transition zone presented CO₂ and N₂O production in a plastisphere dominated by heterotrophic communities. The plastisphere in oligotrophic waters was diverse and active. The experiments, however, presented low or no production of greenhouse gases. Our results show a contribution of CO₂ and N₂O to the global gas surface inventories and air-sea exchange is lower than 1% of the global sources. These results highlight different critical impacts of plastic pollution on the environment that have, until now, not been considered.