Diversity, optimal conditions and tolerance of planktonic cyanobacteria to different environmental conditions in Chile, South America

Abstract: Phytoplankton samples from 23 Chilean freshwater bodies (lotic and lentic) monitored by the General Water Directorate and by the EULA Center were examined to identify the diversity, optimal conditions and tolerance of planktonic cyanobacteria to different conditions. We selected ecosystems between 34 - 41° S latitude with different environmental conditions (trophic status) and human intervention in the watershed (land usage). Cyanobacteria and overall phytoplankton were identified and their abundance and biovolume were determined using the Utermöhl
method. In seven lakes or rivers with visible blooms, samples were identified using a polyphasic approach, in which morphological, ecological and molecular markers (rRNA loci) were integrated.
The optima and tolerance of each cyanobacterial genus at different conditions was calculated by weighted average regression (WA). The relationship among the genera and the environmental characteristics (water temperature, pH, transparency, dissolved oxygen, nitrogen and total phosphorus) were evaluated by pRDA. The results showed that cyanobacteria were found in all lakes, although they were rare (less than 5% abundance) at the southern limit. In total, 15 genera were identified morphologically, 6 Chroococcales (Aphanocapsa, Aphanothece,
Gomphosphaeria, Limnococcus, Merismospedia, Microcystis), 1 Spirulinales (Spirulina), 3 Synechococcales (Coelomoron, Snowella, Pseudanabaena), 4 Nostocales (Anabaenopsis, Aphanizomenon, Dolichospermum, Nodularia) and 1 Oscillatoriales (Oscillatoria). The same genotype (16S rRNA) of Microcystis aeruginosa was distributed in the eutrophic ecosystems between 34 and 37°S and formed dense blooms. Blooms of picocyanobacteria (e.g. Cyanobium)
and other filamentous cyanobacteria, such as Anabaenopsis elenkinii and Aphanizomenon ovalisporum,were also observed. In general, cyanobacteria correlated positively with eutrophic ecosystems located at lower latitudes (34 - 37°S), with a total P concentration (min - max: 0.3 - 1.9 mg L-1), and where the river basin was urbanized or used for monoculture tree plantations. The low nutrient concentration limited cyanobacterial abundance in oligotrophic systems. The
comparison between ecosystems of different latitudes can be an indicator of the long-term status of cyanobacteria (species diversity and abundance) in the context of global climate change. In the Southern Hemisphere, climate change projections indicate warming, together with reduced precipitation, and a higher incidence of extreme events (periods of rain and drought). If the nutrient conditions increase, genera such as Microcystis, Aphanocapsa, Aphanothece,
Aphanizomenon could be favored, together with a higher abundances of Dolichospermum.