Modelled effects of ambient UV radiation on a natural Antarctic marine microbial community

Abstract

Ozone depletion over Antarctica has enhanced ultraviolet-B radiation (UVBR, 280 to 320 nm wavelength). We measured the effect of ambient solar UV radiation on the biomass and species composition of phytoplankton, protozoa, bacteria and dissolved organic carbon (DOC) in natural microbial assemblages from Antarctic coastal waters. Results were modelled to determine the features of the irradiance responsible for changes in the biomass of these microbial components and responses of individual phytoplankton taxa. Model results showed that changes in phytoplankton biomass were primarily due to dose rate, indicating that their UV-induced mortality resulted from the equilibrium between damage and repair. However, there was considerable variability between individual species in their response to dose and dose rate. Changes in protozoan biomass were mainly due to dose and were likely due to community-level, trophodynamic interactions. UV radiation did not measurably affect bacterial biomass, but resulted in increasing concentrations of DOC. We found a threshold of erythemal irradiance of 28 mW m -2 , approximating peak noon-time irradiance at 3.6 m depth near the summer solstice in Antarctic coastal waters, below which no change in the community structure was observed, but above which phytoplankton mortality and protozoan biomass increased. Our results indicate that enhanced UVB radiation in Antarctic waters increases phytoplankton mortality and causes changes in the structure, function and composition of the microbial community that are likely to return more photoassimilated carbon to the atmosphere. KEY WORDS: Antarctic · Model · UV · Ozone · Marine microbes Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 42: [75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90] 2006 pervasive, with molecular and cellular impacts on autotrophs flowing, via the food web, to cause changes on the ecosystem level (Vincent & Roy 1993). Most research on the effect of UV on marine microbes has focussed on its impact on phytoplankton (Davidson 1998). However, Bothwell et al. (1994) first showed that studies of UV-induced effects on single species or trophic levels could not be used to predict the impacts in natural communities, as they did not incorporate the effect of UV on trophic interactions. Recent studies show that UVB also directly impacts the production, growth, survival and species composition of bacteria, viruses and protozoa , Mostajir et al. 2000, Vernet 2000, increasing the likelihood that ecosystem-level changes will result from Antarctic ozone depletion. However, few studies have addressed UV-induced changes in the dynamics of natural marine microbial assemblages (Keller et al. 1997a,b, Wickham & Carstens 1998, Mostajir et al. 1999) and findings vary greatly. To our knowledge, no studies have been conducted in Antarctic marine waters other than those we have performed (Davidson & van der Heijden 2000, Davidson & Belbin 2002.Models can be used to better unde...

Temporal changes in effects of ambient UV radiation on natural communities of Antarctic marine protists

“…Limited evidence suggests this alters the balance between UV-B-induced photodamage and PAR-and UV-A-induced repair (Smith et al 1992, Vincent & Roy 1993, Holm-Hansen et al 1997. Furthermore, our finding of significant inhibition of protist taxa by UV-B radiation in January agrees with a modelling study by Nunez et al (2006) that showed changes in the structure and function of a diatomdominated community at Davis Station below an ozone threshold of 300 DU. However, our use of a broadband sensor to measure erythemal UV radiation precluded our ability to detect changes in the ratio of UV-B radiation to other wavelengths.…”

Effects of ultraviolet radiation on growth, cell death and the standing stock of Antarctic phytoplankton

“…The few available experiments carried out with natural phytoplankton populations grown under UVR ambient conditions also demonstrate growth inhibition (Davidson & Belbin 2002, Nunez et al 2006. Here we show that ambient UVR not only suppresses Antarctic phytoplankton growth but also induces important cell death.…”

“…Although sufficient irradiance is essential for phytoplankton to grow, mesocosm experiments conducted with natural coastal Antarctic plankton (Agustí et al 2009) showed that Antarctic phytoplankton communities are either light limited or light stressed when exposed to high irradiances, suggesting the role of UVR as a potential inhibitor of phytoplankton growth and biomass (Agustí et al 2009). There have been only few studies examining the impact of increased UVBR levels on the growth of Antarctic phytoplankton populations in situ, with most studies performed with isolated Antarctic phytoplankton growing in cultures and exposed to artificial light (Davidson et al 1994, Davidson & Belbin 2002, Nunez et al 2006. Moreover, the effect of UVR on phytoplankton cell mortality has not yet been tested.…”

Photosynthetic performance, DNA damage and repair in gametes of the endemic Antarctic brown alga Ascoseira mirabilis exposed to ultraviolet radiation

“…Concern about global environmental changes, especially the increase in UV-B radiation due to ozone depletion, makes it necessary to study stress physiology on primary producers. In Antarctica, research efforts to evaluate the impact of this phenomenon have mostly focused on phytoplankton (Meador et al 2002;Nunez et al 2006). However, phytoplankton does not represent the only significant photoautotroph component of the aquatic environment susceptible to elevated UV-B.…”