Modulation of the bacterial response to spectral solar radiation by algae and limiting nutrients

Abstract

SUMMARY 1. The response of bacterial production (measured as [3H]TdR incorporation rate) to spectral solar radiation was quantified experimentally in an oligotrophic high‐mountain lake over 2 years. Bacterial responses were consistent: ultraviolet‐B (UVB) was harmful, whereas ultraviolet‐A (UVA) + photosynthetically active radiation (PAR) and PAR enhanced bacterial activity. Full sunlight exerted a net stimulatory effect on bacterial activity in mid‐summer but a net inhibitory effect towards the end of the ice‐free period. 2. Experiments were undertaken to examine whether the bacterial response pattern depended on the presence of algae and/or was modulated by the availability of a limiting inorganic nutrient (phosphorus, P). In the absence of algae, [3H]TdR incorporation rates were significantly lower than when algae were present under all light treatments, and the consistent bacterial response was lost. This suggests that the bacterial response to spectral solar radiation depends on fresh‐C released from algae, which determines the net stimulatory outcome of damage and repair in mid‐summer. 3. In the absence of algae, UVB radiation inhibited bacteria when they were strongly P‐deficient (mean values of N : P ratio: 46.1), whereas it exerted no direct effect on bacterial activity when they were not P‐limited. 4. P‐enrichment of lake water markedly altered the response of bacteria to spectral solar radiation at the end of ice‐free period, when bacteria were strongly P‐deficient. Phosphorus enrichment suppressed the inhibitory effect of full sunlight that was observed in October, both in whole lake water (i.e. including algae) and in the absence of algae. This indicates that the bacterial P‐deficiency, measured as the cellular N : P ratio, was partly responsible for the net inhibitory effect of full sunlight, implying a high bacterial vulnerability to UVB.

Phosphorus inputs unmask negative effects of ultraviolet radiation on algae in a high mountain lake

“…This ecosystem experiences frequent inputs of atmospheric Saharan dust containing high P levels, with a mean molar TN : TP ratio in total dust deposition ranging from 10 to 50 (Morales-Baquero et al, 2006). A highly characteristic trait of the pelagic structure is an absence of autotrophic picoplankton Medina-Sánchez et al, 2002, 2004Carrillo et al, 2006;Delgado-Molina, in preparation). Dominant algae are Chrysophyceae (mainly Chromulina nevadensis) and Chlorophyceae (mainly Dyctiosphaerium chloralloides).…”

Phytoplankton are more tolerant to UV than ­bacteria and viruses in the northern South China Sea

“…Previous studies showed that P-limited phytoplankton were often less resistant to UVR inhibitory effects (Doyle et al 2005). Medina-Sánchez et al (2002) also reported that UVB radiation inhibited bacteria when they were strongly P-deficient, whereas UVB exerted no direct effect on bacterial activity when they were not P-limited. A recent study showed that phosphorus amendment and UVR exposure had a synergistic negative effect on phytoplankton (Carrillo et al 2008).…”

Long term picoplankton dynamics in a warm-monomictic, tropical high altitude lake

“…Intensity of fluorescence signal varied along with the sampled layer depth: low signal coinciding with low numbers of APP found, e.g., in August 2001, might reflect APP damage by UV radiation (Bertoni & Callieri 1999;Medina-Sánchez et al 2002). On the other hand, APP observed around the limit of the euphotic zone were much brighter, which could be connected with changes of photosynthetic pigment concentrations in low-light conditions (Callieri & Stockner 2002).…”

“…APP number-increase at the end of such blooms in surface waters could be explained as a result of succession / competition with these filamentous cyanobacteria (Callieri & Stockner 2002). Upon exhausted nutrients and possible UV inhibition in a high altitude lake (Bertoni & Callieri 1999;Medina-Sánchez et al 2002), the cyanobacteria rapidly died and settled down without an apparent effect on TPP development. Also Escobar et al (1999) showed that particulate biogenic carbon originated from Nodularia sp.…”