Promotion of the dominance of filamentous bloom-forming cyanobacteria by environmental factors

AGUILERA ANABELLA; RICARDO ECHENIQUE; GRACIELA SALERNO; LUIS AUBRIOT

Mar del Plata

Congreso; X Congreso Argentino de Microbiología General SAMIGE; 2014

Raphidiopsis mediterranea and Planktothrix agardhii are planktonic, filamentous cyanobacteria belonging to the Nostocales and Oscillatoriales, respectively. Both are of great concern due to their ability to produce toxins harmful to humans and animals. In eutrophic freshwater lakes, these two bloom-forming cyanobacteria occur regularly, but the causes that lead to the dominance of either species are still unknown. it is not completely known what leads to the dominance of either species. Studies have pointed out that P. agardhii is a resilient, shade-tolerant species, favored by turbid conditions with elevated phosphorus concentrations, while the abundance of R. mediterranea has been related to less turbid warmer waters. The aim of the present work was to investigate the response of P. agardhii and R. mediterranea to changes in light and phosphate availability. The approach was done in field and under controlled laboratory conditions. A part of the study was carried out in Los Patos shallow lake, a small eutrophic shallow lake placed in Buenos Aires province, Argentina (34º 50´ 44´´S, 57º 57´ 26´´W) where dense cyanobacterial blooms performed by both species have been observed during the past decades. In situ environmental measurements, nutrient and phytoplankton samples taken were fortnightly over two years (April 2012 to April 2014). In field, although R. mediterranea and P. agardhii co-occurred and dominated the phytoplankton during the study, the abundances of the two species were negatively related: R. mediterranea abundance increased when P. agardhii population decreased. Regression analyses showed that these species responded differently to environmental variables such as light and temperature. The second approach was carried out at the laboratory, investigating the coexistence of P. agardhii and R. mediterranea in co-cultures growing in MLA media. Two light intensities (40 and 80 µmol photon m-2 s-1, low and high light, respectively) and two phosphate concentrations (5 µM and 10 mM K2HPO4) were tested in batch cultures over 18 days. The variation in morphology, abundance and biomass of the two strains growing together were evaluated daily by optical density (750 nm) and trichome counting by optical microscope. Before starting the experiment, the co-culture was dominated by P agardhii, (83%). After 7 days, there was a shift in the dominances of these species under high light, which was more pronounced in the of 5 µM treatment. After 18 days, P. agardhii population collapsed and the co-culture was dominated completely by R. mediterranea, accounting for the 98% and 96% in the 5 µM and 10 mM treatment, respectively. Under low light, R. mediterranea population increased, but at the end of the experiment the species co-dominated in both phosphorus conditions. In regards to the morphology, most of the trichomes of R. mediteranea counted were straight. A higher proportion of coiled trichomes (approximately 15%) were observed under high light and 10 mM K2HPO4. These results supported the conclusion that both species have different environmental preferences. A better understanding of the environmental conditions that promote the dominance of R. mediterranea or P. agardhii is needed to characterize the niches of these two bloom-forming species, which will useful to improve water management practices.