Constant and fluctuating high temperatures interact with Saharan dust leading to contrasting effects on aquatic microbes over time.

Mediterranean lakes are facing heightened exposure to multiple stressors, such as intensified Saharan dust deposition, temperature increases and fluctuations linked to heatwaves. However, the combined impact of dust and water temperature on the microbial community in freshwater ecosystems remains underexplored. To assess the interactive effect of dust deposition and temperature on aquatic microbes (heterotrophic bacteria and phytoplankton), a combination of field mesocosm experiments covering a dust gradient (five levels, 0-320 mg L-1), and paired laboratory microcosms with increased temperature at two levels (constant and fluctuating high temperature) were conducted in a high mountain lake in the Spanish Sierra Nevada, at three points in time throughout the ice-free period. Heterotrophic bacterial production (HBP) increased with dust load regardless of the temperature regime. However, temperature regime affected the magnitude and nature of the interactive Dust×T effect on HBP. Specifically, constant and fluctuating high temperature showed opposing interactive effects in the short term that became additive over time. The relationships between HBP and predictor variables (soluble reactive phosphorus (SRP), excreted organic carbon (EOC), and heterotrophic bacterial abundance (HBA)), coupled with an evaluation of the mechanistic variable photosynthetic carbon use efficiency by bacteria (%CUEb), revealed that bacteria depended on primary production in nearly all treatments when dust was added. The %CUEb increased with dust load in the control temperature treatment, but it was highest at intermediate dust loads under both constant and fluctuating high temperatures. Overall, our results suggest that while dust addition alone strengthens algae-bacteria coupling, high temperatures lead to decoupling in the long term at intermediate dust loads, potentially impacting ecosystem function.

Thresholds and interactive effects of BPA-gradient and temperature on life history traits of Daphnia magna.

Bisphenol A (BPA), a synthetic organic compound widely used in the production of plastics, is recognized as an emerging contaminant because of its toxicity and the potential risks associated with bioaccumulation in organisms. Despite potential environmental hazards, there is a lack of studies examining BPA toxicity mechanisms and its potential impact on various trophic levels, with even fewer exploring whether global stressors such as temperature can affect the toxicity of BPA in organisms. Our aim was to assess the combined impact of BPA and varying temperature regimes on life-history traits in Daphnia magna. Our results revealed a significant impact of BPA on the growth, reproduction, and accumulated moulting of D. magna, with adverse effects primarily associated with the assimilation of BPA in algae rather than the BPA present in the medium, pointing to a trophic transfer mechanism. The interactive effect between BPA and temperature demonstrated a slight stimulatory effect of low BPA level on D. magna growth rate under warming constant conditions, but an inhibitory under warming fluctuating temperatures. Additionally, a BPA threshold was identified, below which growth became temperature-dependent. This study emphasizes the crucial role of considering temperature in predicting how toxins may affect Daphnia within aquatic food webs.

Uneven response of phytoplankton-bacteria coupling under Saharan dust pulse and ultraviolet radiation in the south-western Mediterranean Sea.

The microbial carbon (C) flux in the ocean is a key functional process governed by the excretion of organic carbon by phytoplankton (EOC) and heterotrophic bacterial carbon demand (BCD). Ultraviolet radiation (UVR) levels in upper mixed layers and increasing atmospheric dust deposition from arid regions may alter the degree of coupling in the phytoplankton-bacteria relationship (measured as BCD:EOC ratio) with consequences for the C-flux through these compartments in marine oligotrophic ecosystem. Firstly, we performed a field study across the south-western (SW) Mediterranean Sea to assess the degree of coupling (BCD:EOC) and how it may be related to metabolic balance (total primary production: community respiration; PPT:CR). Secondly, we conducted a microcosm experiment in two contrasting areas (heterotrophic nearshore and autotrophic open sea) to test the impact of UVR and dust interaction on microbial C flux. In the field study, we found that BCD was not satisfied by EOC (i.e., BCD:EOC >1; uncoupled phytoplankton-bacteria relationship). BCD:EOC ratio was negatively related to PPT:CR ratio across the SW Mediterranean Sea. A spatial pattern emerged, i.e. in autotrophic open sea stations uncoupling was less severe (BCD:EOC ranged 1-2), whereas heterotrophic nearshore stations uncoupling was more severe (BCD:EOC > 2). In the experimental study, in the seawater both enriched with dust and under UVR, BCD:EOC ratio decreased by stimulating autotrophic processes (particulate primary production (PPP) and EOC) in the heterotrophic nearshore area, whereas BCD:EOC increased by stimulating heterotrophic processes [heterotrophic bacterial production (HBP), bacterial growth efficiency (BGE), bacterial respiration (BR)] in the autotrophic open sea. Our results show that this spatial pattern could be reversed under future UVR × Dust scenario. Overall, the impact of greater dust deposition and higher UVR levels will alter the phytoplankton-bacteria C-flux with consequences for the productivity of both communities, their standing stocks, and ultimately, the ecosystem's metabolic balance at the sea surface.

The human connection: First evidence of microplastics in remote high mountain lakes of Sierra Nevada, Spain.

Microplastics have become one of the most serious global threats to animal and human health. While their presence has been documented in all Earth water ecosystems, including remote mountain lakes, the observation that the abundance of microplastics is largely different across nearby lakes has rarely been examined. As part of a citizen science initiative, this study analyzed for the first time the abundance of microplastics in the surface of 35 glacial lakes of Sierra Nevada National Park in Southern Spain with the objective of determining the local factors that control their abundance. First, we described the shape, size, color and nature of microplastics. Second, we tested whether the number of microplastics differed between basins and analyzed environmental and morphometrical features of lakes affecting their abundance. We found that microplastics were common in most lakes, with a maximum abundance of 21.3 particles per liter that akin to some of the most microplastic polluted lakes worldwide. Fragments were the predominant shape (59.7%) followed by fibers (38.8%) and very scarce spheres (1.5%). Microplastics were observed for all size-fractions, but the abundance of particles <45 µm was higher, what advocates for the use of low pore-size filters to prevent underestimation of microplastics. While the mean abundance of microplastics did not differ among basins, their quantity was related to the presence of meadows surrounding the lakes. This result indicates that while atmospheric transport of microsplastics may equally reach all basins, differences in microplastics among nearby-lakes has an anthropic origin caused by mountaineers who find lakes with ample meadows much more attractive to visit relative to barren lakes. The staggering number in these remote lakes, headwaters of rivers that feed drinking reservoirs, is a major concern that warrants further investigation and the strict compliance with waste management laws to reduce the harmful impacts of microplastic contamination.

Divergent nucleic acid allocation in juvenile insects of different metamorphosis modes.

Nucleic acids help clarify variation in species richness of insects having different metamorphosis modes, a biological conundrum. Here we analyse nucleic acid contents of 639 specimens of aquatic insects collected from four high mountain streams of Sierra Nevada in southern Spain to test whether the allocation to RNA or DNA content differs during ontogeny between juvenile insects undergoing direct (hemimetabolous) or indirect (holometabolous) metamorphosis. The results show that RNA content as a function of body mass was negatively correlated to insect body length in four out of six and three out of six of the holometabolan and hemimetabolan taxa, respectively. Although no significant differences in RNA content were found between holometabolans and hemimetabolans, the significant interaction between body length and metamorphosis mode for RNA and RNA:DNA indicates a strong ontogenetic component to RNA allocation. In addition, our finding of lower DNA content in holometabolans relative to hemimetabolans agree with the analysis of empirical genome data in aquatic and terrestrial insects, and extend to this class of arthropods the "growth rate-genome size-nutrient limitation" hypothesis that differences in allocation between RNA and DNA may reflect fundamental evolutionary trade-off of life-history strategies associated with high growth rates (and RNA content) in holometabolans at the expense of diminished genome sizes.

Housekeeping in the Hydrosphere: Microbial Cooking, Cleaning, and Control under Stress.

Who's cooking, who's cleaning, and who's got the remote control within the waters blanketing Earth? Anatomically tiny, numerically dominant microbes are the crucial "homemakers" of the watery household. Phytoplankton's culinary abilities enable them to create food by absorbing sunlight to fix carbon and release oxygen, making microbial autotrophs top-chefs in the aquatic kitchen. However, they are not the only bioengineers that balance this complex household. Ubiquitous heterotrophic microbes including prokaryotic bacteria and archaea (both "bacteria" henceforth), eukaryotic protists, and viruses, recycle organic matter and make inorganic nutrients available to primary producers. Grazing protists compete with viruses for bacterial biomass, whereas mixotrophic protists produce new organic matter as well as consume microbial biomass. When viruses press remote-control buttons, by modifying host genomes or lysing them, the outcome can reverberate throughout the microbial community and beyond. Despite recognition of the vital role of microbes in biosphere housekeeping, impacts of anthropogenic stressors and climate change on their biodiversity, evolution, and ecological function remain poorly understood. How trillions of the smallest organisms in Earth's largest ecosystem respond will be hugely consequential. By making the study of ecology personal, the "housekeeping" perspective can provide better insights into changing ecosystem structure and function at all scales.

Interplay between resistance and resilience governs the stability of a freshwater microbial food web under multiple stressors.

Energy (photosynthetically active [PAR] and ultraviolet [UVR] radiation) and matter (organic and inorganic nutrients) fluxes regulate the ecosystem's stability. However, the mechanisms underpinning the potential interplay between resistance and resilience to shifts in nutrient inputs and UVR are poorly understood. To assess how the UVR × nutrients interaction alters ecosystem stability, we exposed in situ a microbial food web from an oligotrophic ecosystem to: (1) two light (UVR + PAR and PAR), and (2) four nutrient (ambient concentrations, phosphorus [P], carbon [C] and C × P addition) treatments for three weeks. During this period, we quantified the community composition and biomass, sestonic P and C:P ratio, primary [PP] and bacterial [BP] production, community [CR] and bacterial [BR] respiration, excreted organic carbon [EOC], as well as the commensalistic phytoplankton-bacteria interaction (i.e. bacterial carbon demand [BCD]:EOC ratio) and the metabolic balance of the ecosystem (i.e. [PP:R] ratio). The stability of all response variables under the four environmental scenarios tested (i.e. UVR, UVR × C, UVR × P, and UVR × C × P) was quantified by means of the resistance and resilience indexes. The microbial community was dominated by phototrophs during the experimental period regardless of the treatment considered. The most complex scenario, i.e. UVR × C × P, decreased the resistance for all variables, except for BR and the PP:R ratio. Despite that PP:R ratio showed the highest resistance under such scenario, it was >1 in all environmental scenarios (i.e. net autotrophic), except under the UVR × C interaction, where, concomitant with increased resilience, the balance shifted towards net heterotrophy (PP:R < 1). Under the UVR × C × P scenario, the metabolic balance of the ecosystem proved strongly resistant due mainly to high resistance of bacterial respiration and a firm stability of the commensalistic interaction. Our results evidence that the high resilience of phototrophs (favoring their predominance over mixo- and heterotrophs) may lead to the maintenance of the autotrophic nature and carbon (C) sink capacity of the ecosystem.

Climate-driven shifts in algal-bacterial interaction of high-mountain lakes in two years spanning a decade.

Algal-bacterial interactions include mutualism, commensalism, and predation. However, how multiple environmental conditions that regulate the strength and prevalence of a given interaction remains unclear. Here, we test the hypothesis that the prevailing algal-bacterial interaction shifted in two years (2005 versus 2015), due to increased temperature (T) and Saharan dust depositions in high-mountain lakes of Sierra Nevada (S Spain). Our results support the starting hypothesis that the nature of the prevailing algal-bacterial interaction shifted from a bacterivory control exerted by algae to commensalism, coinciding with a higher air and water T as well as the lower ratio sestonic nitrogen (N): phosphorous (P), related to greater aerosol inputs. Projected global change conditions in Mediterranean region could decline the functional diversity and alter the role of mixotrophy as a carbon (C) by-pass in the microbial food web, reducing the biomass-transfer efficiency up the web by increasing the number of trophic links.

Growth impacts of Saharan dust, mineral nutrients, and CO2 on a planktonic herbivore in southern Mediterranean lakes.

Rising levels of CO2 can boost plant biomass but reduce its quality as a food source for herbivores. However, significant uncertainties remain as to the degree to which the effect is modulated by other environmental factors and the underlying processes causing these responses in nature. To address these questions, we carried out CO2-manipulation experiments using natural seston from three lakes under nutrient-enriched conditions (mimicking eutrophication and atmospheric dust-input processes) as a food source for the planktonic Daphnia pulicaria. Contrary to expectations, there were no single effects of rising CO2 on herbivorous growth. Instead, synergistic CO2 × nutrient interactions indicated that CO2 did not support higher zooplankton growth rates unless supplemented with dust or inorganic nutrients (nitrogen, N; phosphorus, P) in two of three studied lakes. The overall positive correlation between zooplankton growth and seston carbon (C), but not seston C:P, suggested that this was a food quantity-mediated response. In addition, we found that this correlation improved when the data were grouped according to the nutrient treatments, and that the response was largest for dust. The synergistic CO2 × nutrient effects reported here imply that the effects of rising CO2 levels on herbivorous growth may be strongly influenced by eutrophication processes and the increase in dust deposition predicted for the Mediterranean region.

Predominant Non-additive Effects of Multiple Stressors on Autotroph C:N:P Ratios Propagate in Freshwater and Marine Food Webs.

A continuing challenge for scientists is to understand how multiple interactive stressor factors affect biological interactions, and subsequently, ecosystems-in ways not easily predicted by single factor studies. In this review, we have compiled and analyzed available research on how multiple stressor pairs composed of temperature (T), light (L), ultraviolet radiation (UVR), nutrients (Nut), carbon dioxide (CO2), dissolved organic carbon (DOC), and salinity (S) impact the stoichiometry of autotrophs which in turn shapes the nature of their ecological interactions within lower trophic levels in streams, lakes and oceans. Our analysis from 66 studies with 320 observations of 11 stressor pairs, demonstrated that non-additive responses predominate across aquatic ecosystems and their net interactive effect depends on the stressor pair at play. Across systems, there was a prevalence of antagonism in freshwater (60-67% vs. 47% in marine systems) compared to marine systems where synergism was more common (49% vs. 33-40% in freshwaters). While the lack of data impeded comparisons among all of the paired stressors, we found pronounced system differences for the L × Nut interactions. For this interaction, our data for C:P and N:P is consistent with the initial hypothesis that the interaction was primarily synergistic in the oceans, but not for C:N. Our study found a wide range of variability in the net effects of the interactions in freshwater systems, with some observations supporting antagonism, and others synergism. Our results suggest that the nature of the stressor pairs interactions on C:N:P ratios regulates the "continuum" commensalistic-competitive-predatory relationship between algae and bacteria and the food chain efficiency at the algae-herbivore interface. Overall, the scarce number of studies with even more fewer replications in each study that are available for freshwater systems have prevented a more detailed, insightful analysis. Our findings highlighting the preponderance of antagonistic and synergistic effects of stressor interactions in aquatic ecosystems-effects that play key roles in the functioning of feedback loops in the biosphere-also stress the need for further studies evaluating the interactive effects of multiple stressors in a rapidly changing world facing a confluence of tipping points.

Rising nutrient-pulse frequency and high UVR strengthen microbial interactions.

Solar radiation and nutrient pulses regulate the ecosystem's functioning. However, little is known about how a greater frequency of pulsed nutrients under high ultraviolet radiation (UVR) levels, as expected in the near future, could alter the responses and interaction between primary producers and decomposers. In this report, we demonstrate through a mesocosm study in lake La Caldera (Spain) that a repeated (press) compared to a one-time (pulse) schedule under UVR prompted higher increases in primary (PP) than in bacterial production (BP) coupled with a replacement of photoautotrophs by mixotrophic nanoflagellates (MNFs). The mechanism underlying these amplified phytoplanktonic responses was a dual control by MNFs on bacteria through the excretion of organic carbon and an increased top-down control by bacterivory. We also show across a 6-year whole-lake study that the changes from photoautotrophs to MNFs were related mainly to the frequency of pulsed nutrients (e.g. desert dust inputs). Our results underscore how an improved understanding of the interaction between chronic and stochastic environmental factors is critical for predicting ongoing changes in ecosystem functioning and its responses to climatically driven changes.

Saharan dust inputs and high UVR levels jointly alter the metabolic balance of marine oligotrophic ecosystems.

The metabolic balance of the most extensive bioma on the Earth is a controversial topic of the global-change research. High ultraviolet radiation (UVR) levels by the shoaling of upper mixed layers and increasing atmospheric dust deposition from arid regions may unpredictably alter the metabolic state of marine oligotrophic ecosystems. We performed an observational study across the south-western (SW) Mediterranean Sea to assess the planktonic metabolic balance and a microcosm experiment in two contrasting areas, heterotrophic nearshore and autotrophic open sea, to test whether a combined UVR × dust impact could alter their metabolic balance at mid-term scales. We show that the metabolic state of oligotrophic areas geographically varies and that the joint impact of UVR and dust inputs prompted a strong change towards autotrophic metabolism. We propose that this metabolic response could be accentuated with the global change as remote-sensing evidence shows increasing intensities, frequencies and number of dust events together with variations in the surface UVR fluxes on SW Mediterranean Sea. Overall, these findings suggest that the enhancement of the net carbon budget under a combined UVR and dust inputs impact could contribute to boost the biological pump, reinforcing the role of the oligotrophic marine ecosystems as CO2 sinks.

Shifts in food quality for herbivorous consumer growth: multiple golden means in the life history.

Consumer growth can be affected by imbalances between the nutrient content of the consumer and its food resource. Although ontogenetic-driven changes in animal composition are well documented, their potential consequences for the organism's sensitivity to food quality constraints have remained elusive. Here we show that the potential growth response of the copepod Mixodiaptomus laciniatus (as %RNA and RNA:DNA ratio) to the natural gradient of seston carbon (C) : nutrient ratio is unimodal and stage specific. Solution of the equation given by the first derivative function provided the optimum C : nutrient ratio for maximum stage-specific growth, which increased during ontogeny. The peakedness of the function indicated that animal vulnerability to suboptimal food quality decreased as juveniles reached adulthood. Consistent with these results, a field experiment demonstrated that potential consumer growth responded to variations in seston C: phosphorus ratio, and that early life stages were particularly vulnerable to suboptimal food quality.

Nucleic acid content in crustacean zooplankton: bridging metabolic and stoichiometric predictions.

Metabolic and stoichiometric theories of ecology have provided broad complementary principles to understand ecosystem processes across different levels of biological organization. We tested several of their cornerstone hypotheses by measuring the nucleic acid (NA) and phosphorus (P) content of crustacean zooplankton species in 22 high mountain lakes (Sierra Nevada and the Pyrenees mountains, Spain). The P-allocation hypothesis (PAH) proposes that the genome size is smaller in cladocerans than in copepods as a result of selection for fast growth towards P-allocation from DNA to RNA under P limitation. Consistent with the PAH, the RNA:DNA ratio was >8-fold higher in cladocerans than in copepods, although 'fast-growth' cladocerans did not always exhibit higher RNA and lower DNA contents in comparison to 'slow-growth' copepods. We also showed strong associations among growth rate, RNA, and total P content supporting the growth rate hypothesis, which predicts that fast-growing organisms have high P content because of the preferential allocation to P-rich ribosomal RNA. In addition, we found that ontogenetic variability in NA content of the copepod Mixodiaptomus laciniatus (intra- and interstage variability) was comparable to the interspecific variability across other zooplankton species. Further, according to the metabolic theory of ecology, temperature should enhance growth rate and hence RNA demands. RNA content in zooplankton was correlated with temperature, but the relationships were nutrient-dependent, with a positive correlation in nutrient-rich ecosystems and a negative one in those with scarce nutrients. Overall our results illustrate the mechanistic connections among organismal NA content, growth rate, nutrients and temperature, contributing to the conceptual unification of metabolic and stoichiometric theories.

Maximum in the middle: nonlinear response of microbial plankton to ultraviolet radiation and phosphorus.

The responses of heterotrophic microbial food webs (HMFW) to the joint action of abiotic stressors related to global change have been studied in an oligotrophic high-mountain lake. A 2×5 factorial design field experiment performed with large mesocosms for >2 months was used to quantify the dynamics of the entire HMFW (bacteria, heterotrophic nanoflagellates, ciliates, and viruses) after an experimental P-enrichment gradient which approximated or surpassed current atmospheric P pulses in the presence vs. absence of ultraviolet radiation. HMFW underwent a mid-term (<20 days) acute development following a noticeable unimodal response to P enrichment, which peaked at intermediate P-enrichment levels and, unexpectedly, was more accentuated under ultraviolet radiation. However, after depletion of dissolved inorganic P, the HMFW collapsed and was outcompeted by a low-diversity autotrophic compartment, which constrained the development of HMFW and caused a significant loss of functional biodiversity. The dynamics and relationships among variables, and the response patterns found, suggest the importance of biotic interactions (predation/parasitism and competition) in restricting HMFW development, in contrast to the role of abiotic factors as main drivers of autotrophic compartment. The response of HMFW may contribute to ecosystem resilience by favoring the maintenance of the peculiar paths of energy and nutrient-mobilization in these pristine ecosystems, which are vulnerable to threats by the joint action of abiotic stressors related to global change.

Patterns of resource limitation of bacteria along a trophic gradient in Mediterranean inland waters.

The nature of the resource that limits heterotrophic bacteria, i.e. mineral nutrients or carbon (C), has consequences for biogeochemical cycles in aquatic ecosystems. Our aim was to identify the resource [C or phosphorus (P)] that mainly limits bacteria in a set of 31 Mediterranean inland water ecosystems spanning a wide trophic range. We followed an intersystem observational approach with three complementary perspectives, comparing the bacterial demand with the resource supply in terms of both the quantity (demand : supply ratio for C and P) and quality (C : P ratio of demand and supply), and assessing the relative strength of each resource in controlling bacterial production. The trophic gradient revealed a shift in the main limiting resource for bacteria, from C at the oligotrophic end (typically high-mountain, low-productivity lakes) to mainly P at the eutrophic end (typically nonmountain, high-productivity lakes). The patterns of resource limitation of bacteria found here may be related to the autotrophic nature of most of the studied ecosystems linked to a Mediterranean climate regime as representative of lakes with low inputs of allocthonous C. These patterns are consistent with the theoretical approaches and may potentially shape the contribution of this type of ecosystems to biogeochemical cycles.

Does microorganism stoichiometry predict microbial food web interactions after a phosphorus pulse?

Knowledge of variations in microbial food web interactions resulting from atmospheric nutrient loads is crucial to improve our understanding of aquatic food web structure in pristine ecosystems. Three experiments mimicking atmospheric inputs at different nitrogen/phosphorus (N/P) ratios were performed in situ covering the seasonal biological succession of the pelagic zone in a high-mountain Spanish lake. In all experiments, abundance, biomass, algal cell biovolume, P-incorporation rates, P-cell quota, and N/P ratio of algae strongly responded to P-enrichment, whereas heterotrophic bacteria remained relatively unchanged. Ciliates were severely restricted when a strong algal exploitation of the available P (bloom growth or storage strategies) led to transient (mid-ice-free experiment) or chronic (late ice-free experiment) P-deficiencies in bacteria. In contrast, maximum development of ciliates was reached when bacteria remained P-rich (N/P < 20) and algae approached Redfield proportions (N/P approximately 16). Evidence of a higher P-incorporation rate supports the proposition that algae and bacteria shifted from a mainly commensalistic-mutualistic to a competitive relationship for the available P when bacterial P-deficiency increased, as reflected by their unbalanced N/P ratio (N/P > 20-24). Hence, the bacterial N/P ratio proved be a key factor to understand the algae-bacteria relationship and microbial food web development. This study not only demonstrates the interdependence of life history strategies, stoichiometric nutrient content, and growth but also supports the use of bacterial N/P thresholds for diagnosing ciliate development, a little-studied aspect worthy of further attention.