Resumo

Phytoplankton in the oceans represents a group of microorganisms which is of crucial importance in regulating aquatic food webs, biogeochemical cycles and the Earth’s climate. The species that compose the phytoplankton can be classified by size differentiation. Picophytoplankton (3-0.2 μm in diameter) includes cyanobacteria, represented by Synechococcus & Prochlorococcus, and photosynthetic picoeukaryotes (PPE) belonging to a wide range of algal classes. The distribution, abundance and composition of picophytoplankton communities are controlled principally by light and inorganic nutrients, normally displaying a fairly predictive annual cycle. At smaller time scales, picophytoplankton cellular division and growth are tightly coupled with the light cycle. In this work, we studied the effect of light regimes on the abundance of picophytoplanktonic organisms in a microcosm experiment. 10 Liter water samples were collected from El Quisco Bay, Chile, at 5m depth and kept in darkness during the transport to the laboratory. Four microcosms (10 L containers) were kept in a tank with flowing water simulating constant, near in situ conditions. Each of the microcosms was exposed to regular day/night cycles, with different light regimes (100%, 10%, 1% and 0% incident light) by shading the tanks with layers of black mesh. 10 mL samples were withdrawn daily from each microcosm for cell counting analysis by flow cytometry, determination of total in vivo chlorophyll fluorescence and epifluorescence microscopy. The light regime influenced the abundance of picophytoplanktonic populations. Photosynthetic eukaryotic picoplankton tend to be more sensitive to decreases in light intensity than the bacterial counterpart in microcosm experiment. The maximum chlorophyll fluorescence occurs at 10% of incidence light intensity and was not correlated with the abundance of the different picophytoplanktonic populations. Synechococcus is more resistant to light fluctuation than eukaryotic cells in our microcosm experiment. This is in agreement with the fact that marine Synechococcus have developed specific adaptations to cope more efficiently with gradients of nutrients and light quality than PPEs, that generally inhabit at deep chlorophyll maximum. This work was developed during the ECODIM (Ecology & Diversity of Marine Microorganisms) Postgraduate course 2012, in Las Cruces, Chile.