Mixotrophic phytoplankton is enhanced by UV radiation in a low altitude, P-limited Mediterranean lake

Abstract. An indirect effect of global warming is a reduction in the depth of the upper mixed layer (UML) causing organisms to be exposed to higher levels of ultraviolet (UVR, 280-400 nm) and photosynthetically active radiation (PAR, 400-700 nm). This can affect primary and bacterial production as well as the commensalistic phytoplankton-bacteria relationship. The combined effects of UVR and reduction in the depth of the UML were assessed on variables related to the metabolism of phytoplankton and bacteria, during in situ experiments performed with natural pico-and nanoplankton communities from two oligotrophic lakes with contrasting UVR transparency (high-UVR versus low-UVR waters) of southern Spain. The negative UVR effects on epilimnetic primary production (PP) and on heterotrophic bacterial production (HBP), intensified under increased stratification, were higher in the low-UVR than in the high-UVR lake, and stronger on the phytoplanktonic than on the heterotrophic bacterial communities. Under UVR and increased stratification, the commensalistic phytoplankton-bacteria relationship was strengthened in the high-UVR lake where excretion of organic carbon (EOC) rates exceeded the bacterial carbon demand (BCD; i.e., BCD : EOC(%) ratio < 100). This did not occur in the low-UVR lake (i.e., BCD : EOC(%) ratio > 100). The greater UVR damage to phytoplankton and bacteria and the weakening of their commensalistic interaction found in the low-UVR lake indicates that these ecosystems would be especially vulnerable to UVR and increased stratification as stressors related to global climate change. Thus, our findings may have important implications for the carbon cycle in oligotrophic lakes of the Mediterranean region.

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“…Therefore, this lake is hereafter called the "low-UVR" lake. The autotrophic community is composed of pico-and nanoplankton (Rojo et al, 2012).…”

Section: Model Ecosystemsmentioning

confidence: 99%

Synergistic effects of UVR and simulated stratification on commensalistic phytoplankton–bacteria relationship in two optically contrasting oligotrophic Mediterranean lakes

et al. 2015

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“…We carried out the experiment from 11 to 28 July 2009. This timespan is sufficient to monitor variations in growth of populations from bacteria to small microzooplankton (Gillooly 2000, Rojo et al 2012. We decided on this duration of experiment based on a trade-off between providing enough time to allow changes in density of all populations and minimizing enclosure bias or effects caused by enclosures limitation (Stewart et al 2013).…”

Section: Study Site and Experimental Designmentioning

confidence: 99%

“…Dissolved organic C (DOC) content ranges from 1.5 to 3.5 mg C/L. More information on the environmental conditions of the lake on the days of the experiment was published by Rojo et al (2012).…”

Section: Study Site and Experimental Designmentioning

confidence: 99%

“…The magnitude of UVR effects on community structure depends on the taxonomic composition of the community. For example, UVR negatively affects diatoms with more intensity than cyanobacteria and has a greater effect on cladocerans than copepods (Jiang and Qiu 2005, Hansson and Hylander 2009, Rojo et al 2012). An increase in P concentration in aquatic ecosystems, a eutrophication process, changes community composition in ways that follow well-known patterns of substitution of some species for others (Reynolds 1984).…”

mentioning

confidence: 99%

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Are the small-sized plankton communities of oligotrophic ecosystems resilient to UVR and P pulses?

et al. 2017

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Evaluating how environmental stressor interactions influence ecosystem structure and functioning is critical to understanding the response of ecosystems to global change. We exposed a species-poor planktonic community to P pulses in the absence and presence of solar ultraviolet radiation (UVR). We used a field-mesocosm study in an oligotrophic mid-altitude lake to test the hypothesis that interaction between these factors affects the community's diversity and composition, but not its biomass-size spectrum, making this community resilient in terms of its C-transfer function. Our findings show that P pulses and UVR affected the relative abundance of different planktonic populations. The abundance of phytoplankton was enhanced strongly by P pulses and secondarily by UVR. The UVR Â P interaction affected only the smallest autotrophic species. Chlorococcal abundance increased, whereas chroococcal cyanobacteria decreased. However, UVR had a much more pronounced effect than P on the composition of microcrustaceans, and both factors interactively affected their biomass. Hence, the biomass-size spectrum was not resilient to the UVR Â P pulse interaction. The steepness of the slope increased under P-pulse conditions because zooplankton were not able to grow concomitantly with phytoplankton, as confirmed by the low zooplankton to phytoplankton biomass ratio (Z∶P). The observed planktonic changes under new foreseeable conditions that include an increase in UVR fluxes and P inputs might decouple the C cycle in inland oligotrophic lakes in the Mediterranean region.

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“…Furthermore, it has been shown that UVR directly influences the elemental composition of phytoplankton by decreasing the sestonic C:P and N:P ratios (Rojo et al. , Carrillo et al. ).…”

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confidence: 99%

Mixotrophic trade‐off under warming and UVR in a marine and a freshwater alga

González-Olalla

Medina‐Sánchez

Carrillo

2019

Journal of Phycology

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Mixotrophic protists combine phagotrophy and phototrophy within a single cell. Greater phagotrophic activity could reinforce the bypass of carbon (C) flux through the bacteria‐mixotroph link and thus lead to a more efficient transfer of C and other nutrients to the top of the trophic web. Determining how foreseeable changes in temperature and UVR affect mixotrophic trade‐offs in favor of one or the other nutritional strategy, along the mixotrophic gradient, is key to understanding the fate of carbon and mineral nutrients in the aquatic ecosystem. Our two main hypotheses were: (i) that increased warming and UVR will divert metabolism toward phagotrophy, and (ii) that the magnitude of this shift will vary according to the organism's position along the mixotrophic gradient. To test these hypotheses, we used two protists (Isochrysis galbana and Chromulina sp.) located in different positions on the mixotrophic gradient, subjecting them to the action of temperature and of UVR and their interaction. Our results showed that the joint action of these two factors increased the primary production:bacterivory ratio and stoichiometric values (N:P ratio) close to Redfield's ratio. Therefore, temperature and UVR shifted the metabolism of both organisms toward greater phototrophy regardless of the original position of the organism on the mixotrophic gradient. Weaker phagotrophic activity could cause a less efficient transfer of C to the top of trophic webs.

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Mixotrophic phytoplankton is enhanced by UV radiation in a low altitude, P-limited Mediterranean lake

Cited by 10 publications

References 56 publications

Synergistic effects of UVR and simulated stratification on commensalistic phytoplankton–bacteria relationship in two optically contrasting oligotrophic Mediterranean lakes

Synergistic effects of UVR and simulated stratification on commensalistic phytoplankton–bacteria relationship in two optically contrasting oligotrophic Mediterranean lakes

Are the small-sized plankton communities of oligotrophic ecosystems resilient to UVR and P pulses?

Mixotrophic trade‐off under warming and UVR in a marine and a freshwater alga

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