Warming and CO2 effects under oligotrophication on temperate phytoplankton communities.

Eutrophication, global warming, and rising carbon dioxide (CO2) levels are the three most prevalent pressures impacting the biosphere. Despite their individual effects are well-known, it remains untested how oligotrophication (i.e. nutrients reduction) can alter the planktonic community responses to warming and elevated CO2 levels. Here, we performed an indoor mesocosm experiment to investigate the warming × CO2 interaction under a nutrient reduction scenario (40%) mediated by an in-lake management strategy (i.e. addition of a commercial solid-phase phosphorus sorbent -Phoslock®) on a natural freshwater plankton community. Biomass production increased under warming × CO2 relative to present-day conditions; however, a Phoslock®-mediated oligotrophication reduced such values by 30-70%. Conversely, the warming × CO2 × oligotrophication interaction stimulated the photosynthesis by 20% compared to ambient nutrient conditions, and matched with higher resource use efficiency (RUE) and nutrient demand. Surprisingly, at a group level, we found that the multi-stressors scenario increased the photosynthesis in eukaryotes by 25%, but greatly impaired in cyanobacteria (ca. -25%). This higher cyanobacterial sensitivity was coupled with a reduced light harvesting efficiency and compensation point. Since Phoslock®-induced oligotrophication unmasked a strong negative warming × CO2 effect on cyanobacteria, it becomes crucial to understand how the interplay between climate change and nutrient abatement actions may alter the, ecosystems functioning. With an integrative understanding of these processes, policy makers will design more appropriate management strategies to improve the ecological status of aquatic ecosystems without compromising their ecological attributes and functioning.

Diversity and antimicrobial potential in sea anemone and holothurian microbiomes.

Marine invertebrates, as holobionts, contain symbiotic bacteria that coevolve and develop antimicrobial substances. These symbiotic bacteria are an underexplored source of new bioactive molecules to face the emerging antibiotic resistance in pathogens. Here, we explored the antimicrobial activity of bacteria retrieved from the microbiota of two sea anemones (Anemonia sulcata, Actinia equina) and two holothurians (Holothuria tubulosa, Holothuria forskali). We tested the antimicrobial activity of the isolated bacteria against pathogens with interest for human health, agriculture and aquaculture. We isolated 27 strains with antibacterial activity and 12 of these isolates also showed antifungal activity. We taxonomically identified these strains being Bacillus and Vibrio species the most representative producers of antimicrobial substances. Microbiome species composition of the two sea anemones was similar between them but differed substantially of seawater bacteria. In contrast, microbiome species composition of the two holothurian species was different between them and in comparison with the bacteria in holothurian feces and seawater. In all the holobiont microbiomes Bacteroidetes was the predominant phylum. For each microbiome, we determined diversity and the rank-abundance dominance using five fitted models (null, pre-emption, log-Normal, Zipf and Zipf-Mandelbrot). The models with less evenness (i.e. Zipf and Zipf-Mandelblot) showed the best fits in all the microbiomes. Finally, we tracked (using the V4 hypervariable region of 16S rRNA gene) the relative abundance of these 27 isolates with antibacterial activity in the total pool of sequences obtained for the microbiome of each holobiont. Coincidences, although with extremely low frequencies, were detected only in the microbiome of H. forskali. This fact suggests that these isolated bacteria belong to the long tail of rare symbiotic bacteria. Therefore, more and more sophisticated culture techniques are necessary to explore this apparently vast pool of rare symbiontic bacteria and to determine their biotechnological potentiality.

Flamingos and drought as drivers of nutrients and microbial dynamics in a saline lake.

Waterbird aggregations and droughts affect nutrient and microbial dynamics in wetlands. We analysed the effects of high densities of flamingos on nutrients and microbial dynamics in a saline lake during a wet and a dry hydrological year, and explored the effects of guano on prokaryotic growth. Concentrations of dissolved organic carbon, total phosphorus and total nitrogen in the surface waters were 2-3 fold higher during the drought and were correlated with salinity. Flamingos stimulated prokaryotic heterotrophic production and triggered cascading effects on prokaryotic abundance, viruses and dissolved nitrogen. This stimulus of heterotrophic prokaryotes was associated with soluble phosphorus inputs from guano, and also from sediments. In the experiments, the specific growth rate and the carrying capacity were almost twice as high after guano addition than in the control treatments, and were coupled with soluble phosphorus assimilation. Flamingo guano was also rich in nitrogen. Dissolved N in lake water lagged behind the abundance of flamingos, but the causes of this lag are unclear. This study demonstrates that intense droughts could lead to increases in total nutrients in wetlands; however, microbial activity is likely constrained by the availability of soluble phosphorus, which appears to be more dependent on the abundance of waterbirds.