The effects of solar radiation (PAR and UV) on secondary production and DNA structure of bacterioplankton from an upwelling zone in the Humboldt Current System (36şS)

Klaudia Liliana Hernández Rondón.
Doctorate Program in Oceanography
University of Concepción, 2006



The effect of solar radiation on bacterial metabolism and bacteria cell structure, expressed as bacterial secondary production (BSP) and DNA damage (thymidine dimmers formation), has not been evaluated yet in highly productive upwelling zones such as those of the Humboldt Current System (HCS). In this Thesis, the impact of solar radiation (in their visible spectrum PAR, 400-800 nm and ultraviolet UVR, 280-400 nm) was assessed on two natural bacterioplankton assemblages coming from water with different light history during spring and summer 2004-2005. Bacterioplankton samples coming from surface waters (depth = 0.5 m; with recent light history) and subsurface waters (depth = 80 m; no recent light history) from the continental shelf off south - central Chile were exposed to ambient solar radiation for 4-11 hours. The samples were incubated under three spectral radiation treatments: PAR (400-700 nm), PAR+UV-A (320-700 nm), and PAR+UVR (280-700 nm). BSP was estimated by 14C-leucine uptake (as precursor of protein synthesis) and [methyl-3H]-thymidine (analogous to DNA synthesis). The daily cycle of BSP, bacterial abundance, bacterial growth efficiency (BGE) and growth rates (µ), were used as indicators of the metabolic activity of the bacterioplankton from both depths. A differential response was observed in both bacterioplankton assemblages that was dependent on the wavelengtand predominant for PAR and highly variable in the UVR range. Furthermore, the metabolic activity of the bacterioplankton from surface waters (0.5m) was inhibited mainly in terms of protein synthesis followed by DNA synthesis. In the case of the subsurface waters assemblages (80 m), a differential response of the metabolic activity was obtained depending on the season. During springtime the BSP was enhanced (protein synthesis and DNA synthesis) whereas in summer it was inhibited. Using a molecular approach, the daily impact of PAR and UVR on bacterioplankton BSP (<0.7 µm), was assessed as well DNA damage induction and repair mechanisms (by Ciclobutil pyrimidine dimmers CPDS quantification) in both bacterioplankton assemblages with different photobiological history. The BSP and DNA damage of the bacterioplankton coming from surface and subsurface waters from this reacted to the irradiance reached during the day. The recovery effect during the afternoon and night could be due to the balance between damage and recovery when was exposed to a lower light intensity, and, to the efficient combination (mainly on bacterioplankton from surface waters) of repair mechanisms (photoexcision repair / darkness nucleotide excision repair). On the other hand, during the experiments no solar radiation effects on Chlorophyll a, and nutrients were detected in any of the two evaluated water types. Finally, the solar radiation associated with atmospheric variables is described using a stochastic climate model for the Concepción zone during the period 2003-2004. The results of the model suggest that cloudiness (CI) and meridian pseudostress winds (Wv), followed in a lower extent and out of phase due to the ozone variations, are the main factors determining the variability in the solar radiation cycle in Concepción.