The interaction of phytoplankton and bacteria in a high mountain lake: Importance of the spectral composition of solar radiation

Abstract

The role of spectral composition of solar radiation has seldom been considered as a critical factor controlling the algae-bacteria relationship. A coupled algae-bacteria relationship mediated by C released from algae was observed during a 2-yr period (1996)(1997) in an oligotrophic high mountain lake, except at upper depths. The intensity of photosynthetically active radiation was negatively related to primary production, and the highest percentages of excretion of organic carbon (%EOC) from algae were found at upper depths of the water column. The effect of different spectral regions of solar radiation on the algae-bacteria relationship was assessed by in situ experiments, in which the exposure, tracer uptake by target organisms, and interactions among abiotic and biotic factors were simultaneous. Primary production was ultraviolet radiation (UVR) inhibited by 33-83% depending on depth and date, with ultraviolet-A radiation (UVA) exerting the main effect. EOC and %EOC yielded highest values under UVR exposure. Sunlight affected bacterial production (BP) only at upper depths. UVB inhibited BP by 39-82%, whereas UVA ϩ photosynthetic active radiation (PAR) and PAR enhanced BP three-to fourfold. Full sunlight increased BP 2.5-fold in midsummer but inhibited it (37%) in the late open-water period. The percentage of photosynthetic exudates assimilated by bacteria, and photosynthetic carbon use efficiency by bacteria, showed a similar pattern to that of BP. The experimental results support our hypothesis that increased organic C release from UV-stressed algae stimulates bacterial growth if the bacteria are relatively well adapted to sunlight, determining a coupled algae-bacteria relationship. Thus, sunlight may play a key role as the underlying abiotic factor that regulates algae-bacteria interaction in shallow and clear-water ecosystems.The degradation of Earth's ozone layer, one of the main causes of global climate change, has allowed an increase in solar ultraviolet-B (UVB) radiation fluxes on the Earth's surface (Crutzen 1992) and aquatic ecosystems (Karentz et al. 1994; Häder 1997). The pelagic planktonic community functions through a web of energy and nutrient exchanges, mediated by a diverse array of producers and consumers that ultimately depend on the energy supplied by sunlight. Thus, alterations in spectral composition of solar radiation can modify the structure (Vinebrooke and Leavitt 1999) and functioning of the pelagic food web. In order to understand the C cycle in aquatic ecosystems, we need to explore the 1 To whom correspondence should be addressed. Present address: Departamento de Biología Animal y Ecología, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain (pcl@ugr.es). AcknowledgmentsWe are grateful to R. Sommaruga for his useful criticism on an earlier draft of this study and for the spectral UV measurements and DOC determinations, and to F. Figueroa for making the air UV radiation measurements available. We also thank I. Reche and Morales-Baquero for their comments on me...

Effect of solar ultraviolet radiation on bacterio- and phytoplankton activity in a large coral reef lagoon (southwest New Caledonia)

“…Alternatively, Cullen & Neale (1994) suggested that a relatively weak UV-B effect was observed because UV-A inhibition was dominant and could hide the UV-B stress, in the same way that an excess of PAR could eventually hide the UV-A stress. To conclude, we agree with Carrillo et al (2002) that the non-effect of UV-B is more likely due to the rapid extinction of UV-B in the water column (Fig. 4) than to a potential resistance of phytoplankton.…”

“…These scales were coherent with those given in the literature . For example, PP was UV-A inhibited by 33 to 83% depending on depth and date in a high mountain lake (Carrillo et al 2002). Concerning bacteria, Visser et al (2002) showed that BP inhibition by sunlight is cumulative during surface incubation (up to 80% relative to dark sample) and detectable down to a depth of 10 m in tropical coastal waters.…”

“…De Mora et al 2000, Helbling et al 2001, Xenopoulos & Schindler 2003. The predominance of UV-A effect in comparison to UV-B on the phytoplankton is widely documented in other tropical or subtropical ecosystems (Piazena et al 2002), polar (Villafañe et al 1995) and temperate marine ecosystems (Behrenfeld et al 1993, Helbling et al 2001), but also in lakes , Carrillo et al 2002. In fact, a wide range of interspecific UV-R tolerance in phytoplankton has been reported (Gieskes & Buma 1997, De Mora et al 2000.…”

“…Bacteria are able to take up fresh carbon in the dark with greater efficiency than under full sunlight (Carrillo et al 2002). This could be related to UV-R inhibition of bacterial growth or to a decrease of bioavailability of carbon released by phytoplankton due to photo-transformation processes.…”

Phenology of high-elevation pelagic bacteria: the roles of meteorologic variability, catchment inputs and thermal stratification in structuring communities

“…As microbial community dissimilarity increased with increasing distance of physical (depth) separation during stratified periods (Figure 4), these results suggest sensitive community responses to depth differences in environmental conditions, such as water temperature or light. Although not measured in this study, previous research in high-elevation lakes has emphasized the important function of solar radiation in structuring bacterial communities, with particular phylotypes adapted to the high flux of ultraviolet radiation found in shallow environments (Carrillo et al, 2002;Warnecke et al, 2005).…”