Effects of solar ultraviolet radiation on the periphyton community in lotic systems: comparison of attached algae and bacteria during their development

Abstract

The effects of solar ultraviolet radiation (UVR) on the development of a periphyton community were studied in an outdoor artificial stream apparatus. Algal biomass, species composition, and bacterial cell density were measured under full sunlight and non-UVR (photosynthetically active radiation [PAR]-only) conditions. Attachment of algae was detected on days 6-9. Although the chlorophyll-a concentration under non-UVR conditions was 2-4 times that under full sunlight (PAR + UVR) throughout the experiment, neither net algal growth rate nor species composition differed significantly between the two light conditions. The relative carotenoid pigment contents of attached algae in the PAR + UVR condition were 1.1-1.3 times those in the non-UVR condition. Rates of increase of bacterial cell densities under the PAR + UVR condition were depressed by solar UVR for the first few days, although there were no apparent differences in the rates of increase between the light conditions later in the experiment. The small effect of UVR on the development of this periphyton community may be attributable to low UV flux at this study site and to the experimental conditions under which the algae were kept: a high physiological state with high nutrient conditions. Attached bacteria and algae that colonize substrata first are likely to be sensitive to solar UVR, and the negative effects of UVR are mitigated by the development of a periphyton community.

Effects of UV Radiation on Wetland Periphyton: Algae, Bacteria, and Extracellular Polysaccharides

“…Under lower, natural UV exposure neither Hodoki (2005) nor the present study observed any difference in periphytic bacterial densities between +UV and -UV treatments. In contrast to the lack of any UV effect on total bacterial densities, greater proportions (-1 -6 %) of non-viable bacteria were consistently observed in the +UV treatment of the present study.…”

“…Furthermore, W exposure and concomitant reduction in the proportion of viable bacteria in wetland periphyton may also influence wetland ecological processes dependent on bacterial metabolism, such as whole-system carbon flow/turnover and nutrient cycling. As observed for algal biomass, periphytic AFDM did not differ between treatments, which is in concordance with prior research in artificial streams (Hodoki 2005). In contrast, Bothwell et al (1994) observed that UV exposure inhibited periphytic AFDM accrual for the first two weeks of colonization and growth on ceramic tiles in artificial streams; however, beyond that time period there was no difference in AFDM due to UV-mediated interactions with higher trophic levels.…”

“…A significant time x UV interaction was also detected (p = 0.03 I), most likely due to a large difference in EPS content on Day 3 1 (t-test p < 0.01, Fig. 5 DISCUSSION Several investigations of the effects of UV radiation by exclusion of ambient UV observed reduced chl a accrual within +UV treatments in lakes Leavit 1996, Francoeur andLowe 1998) and artificial streams (Bothwell et al 1994, Hodoki 2005. In contrast, other studies conducted in lakes (Higley et al 2001, Watkins et al 2001), a wetland (Kahn and Wetzel 1999), and a stream (Hill et al 1997) found no significant differences in periphytic algal biomass between fill ambient sunlight (+UV) and PAR-only (-UV) treatments.…”

“…Direct count microscopy with 4'-6-diamidino-2-phenylindole (DAPI) staining of wetland periphytic communities has shown significantly less bacterial biomass in +UV as compared with -UV treatments (Kahn and Wetzel 1999). In contrast, Hodoki (2005) observed no significant difference in bacterial biomass (DAPI direct counts) among +UV and -UV treatments in lotic periphyton. The limitation of the DAPI staining technique is that it does not account for cell viability, staining both living bacteria and non-viable (dead) bacterial cells that still contain nucleic acids.…”

“…Prior studies have observed reductions in algal photosynthesis and biomass accumulation in freshwater periphyton under full ambient light (+UV) as compared to -UV treatments (e.g., Vinebrooke and Leavitt 1996), whereas other studies have observed no affect of UV radiation (Hill et al 1997, Hodoki 2005. Shifts in algal species composition have also been observed in longer-term studies, with sensitivity to UV varying by taxon (Bothwell et al 1993, Vinebrooke and Leavitt 1996, Francoeur and Lowe 1998, Higley et al 2001).…”