Response of a coastal Baltic Sea diatom-dominated phytoplankton community to experimental heat shock and changing salinity

Phytoplankton Community Response to Environmental Factors along a Salinity Gradient in a Seagoing River, Tianjin, China

A river-estuary ecosystem usually features a distinct salinity gradient and a complex water environment, so it is enormously valuable to study the response mechanism of living organisms to multiple abiotic factors under salinity stress. Phytoplankton, as an important part of aquatic microorganisms, has always been of concern for its crucial place in the aquatic ecosystem. In this study, phytoplankton data and 18 abiotic factors collected from 15 stations in Duliujian River, a seagoing river, were investigated in different seasons. The results showed that the river studied was of a Cyanophyta-dominant type. Salinity (SAL) was the key control factor for phytoplankton species richness, while water temperature (WT) was critical not only for species richness, but also community diversity, and the abundance and biomass of dominant species. Apart from WT, the abundance and biomass of dominant species were also driven by total nitrogen (TN), nitrate (NO₃^-), pH, and water transparency (SD). Moreover, total dissolved phosphorus (TDP), pH, and chemical oxygen demand (COD) were crucial for community diversity and evenness. The bloom of dominant species positively associated with TDP led to lower diversity and evenness in autumn. In addition, when available nitrogen was limited, Pseudoanabaena sp. could obtain a competitive advantage through the N₂ fixation function. Increased available nitrogen concentration could favor the abundance of Chlorella vulgaris to resist the negative effect of WT. The results show that Oscillatoria limosa could serve as an indicator of organic contamination, and nutrient-concentration control must be effective to inhibit Microcystis bloom. This could help managers to formulate conservation measures.

Air-dispersed aquatic microorganisms show establishment and growth preferences in different freshwater colonisation habitats

We attempted to mimic aeolian ecosystems to examine how filters posed by regional characteristics can influence the establishment and growth of airborne microcolonisers of a common air source. Using a natural single source of aerosols we applied a combined microscopy and high-throughput sequencing approach to examine the diversity, settling and growth potential of air-dispersed microbes in water containers representing newly formed aquatic colonisation habitats of different trophic states and salinity. Heterotrophic microeukaryotes were favoured as initial settlers when nutrients were low, while autotrophs rapidly proliferated in the high-nutrient containers, possibly due to favourable germinating conditions for their preferred mode of dispersal with resting spores. Following settling of colonisers, we investigated two contrasting hypotheses: if the different water colonisation habitats harboured the same microbial communities after establishment and growth periods, this would point towards a selection of best-fit cosmopolitan colonisers, regardless of habitat-specific characteristics. Alternatively, community dissimilarities after the growth period would suggest a selection of settlers due to bottom-up controls combined with priority effects. Both analyses suggested that the structure of the microbial communities in the different colonisation habitats were driven by nutrient content and salinity, showing clustering to similar bottom-up forces and dissimilarities in significantly different colonisation habitats.

Dissolved inorganic carbon determines the abundance of microbial primary producers and primary production in Tibetan Plateau lakes

Climate change globally accelerates the shrinkage of inland lakes, resulting in increases in both water salinity and dissolved inorganic carbon (DIC). The increases of salinity and DIC generate contrasting effects on microbial primary producers and primary production, however, their combined effects remain unclear in aquatic ecosystems. We hypothesized that increased DIC mitigates the constraints of enhanced salinity on microbial primary producers and primary production. To test this, we employed isotope labeling and molecular methods to explore primary production and four dominant types of microbial primary producers (form IA, IB, IC and ID) in lakes on the Tibetan Plateau. Results showed that DIC was positively correlated with the abundance of the form IAB and ID microbial primary producers and primary production (all P < 0.001) and offset salinity constraints. Structural equation models elucidated that DIC substantially enhanced primary production by stimulating the abundance of form ID microbial primary producers. The abundance of form ID primary producers explained more variations (14.6%) of primary production than form IAB (6%) and physicochemical factors (6.8%). Diatoms (form ID) played a determinant role in primary production in the lakes by adapting to high DIC and high salinity. Our findings suggest that inland lakes may support higher primary productivity in future climate change scenarios.