Mohovic B, Gianesella SMF, Laurion I, Roy S
UVB, phytoplankton, radiation, light treatments, mesocosm, Rimouski, Canada, Ubatuba, Brazil, 2 m3, 1.8 m3
Photoprotection against UV-B radiation (UVBR; 280-320 nm)was examined in natural phytoplankton communities from twocoastal environments at different latitudes: temperate Rimouski(Canada) and tropical Ubatuba (Brazil). Mesocosm experimentswere performed at these sites to examine the response ofphytoplankton to increases in UVBR that corresponded to localdepletions of 30% and 60% in atmospheric ozone levels (lowand high UVBR treatments, respectively). A fluorescencemethod using a pulse amplitude modulation fluorometer (Xe-PAM, Walz, Germany) with selective UV filters was used toestimate photoprotection, and these results were comparedwith an index of mycosporine-like amino acid (MAA) concentrationsdetermined using spectrophotometry of methanolextracts. The present study provided the first evidence, toour knowledge, of the suitability of this in vivo fluorescencemethod for the estimation of UV photoprotection efficiency innatural phytoplankton. No significant differences were foundfor most of the variables analyzed between the light treatmentsused at both sites, but differences were found between sitesthroughout the duration of the experiments. Vertical mixing,used to maintain cells in suspension, likely alleviated seriousUVBR-induced damage during both experiments by reducingthe length of time of exposure to the highest UVBR irradiancesat the surface. In Rimouski, this was the main factorminimizing the effects of treatment, because optical propertiesof the coastal seawater rapidly attenuated UVBR throughoutthe water column of the ca 2 m deep mesocosms. In thislocation, synthesis of MAAs and photoprotective pigmentslikely contributed to the observed phototolerance of phytoplanktonand, hence, to their growth; however, in a comparisonof the UVBR treatments, these variables showed no differences.In Ubatuba, where nutrient concentrations were significantlylower than those in Rimouski, light attenuation was less thanthat in Rimouski and UVBR reached the bottom of themesocosms. UVBR penetration and the forced vertical mixingof the cells, without the possibility of vertical migration belowthis photostress zone, resulted in photo-inhibition, becauseconfinement in the mesocosms forced cells to remain constantlyexposed to high levels of irradiance during the daytime. Hence,additional effects of UVBR were masked in this experiment,because cells were damaged too much and phytoplanktonpopulations were rapidly declining. There was also an overallpreservation of MAAs, in contrast with chlorophyll (Chl)degradation, in spite of the fact that this UV screening was notsufficient to counteract photo-inhibition, which suggests animportant role for these molecules, either in the overallphotoprotection strategy or in other physiological processes.Altogether, local water characteristics, namely attenuation,mixing, and nutrients concentration, can strongly modulate thephotoprotection strategies used by natural phytoplanktonpopulations in coastal environments.