What Triggers the Annual Cycle of Cyanobacterium Oscillatoria sp. in an Extreme Environmental Sulfide-Rich Spa?

A seasonal cycle of sulfide, nitrate, phosphate, ammonium, chlorophyll a (Chl a) and Oscillatoria sp. abundance (<100 mu m), as well as the relative contribution of taxonomic phytoplanktonic groups (cyanobacteria, green algae, cryptomonads, diatoms and dinoflagellates) to total Chl a were measured by fluorometric measurements at La Hedionda sulfide-rich spa (southern Spain). Fluorometry determined that cyanobacteria Chl a concentration correlated positively with the abundance of Oscillatoria sp. Aggregates at 45-100 mu m equivalent spherical diameter (ESD) and was used as an indicator of Oscillatoria sp. Abundance, including for aggregates <45 and >100 mu m (ESD). In addition, air temperature, radiation and precipitation were downloaded from meteorological databases. In agreement with the meteorological annual cycle observed in air temperature, radiation and precipitation, sulfide concentration at La Hedionda Spa shows an annual cycle with concentrations around 40 mu M in winter and up to 200 mu M in the dry summer period. Phytoplankton composition was dominated by cyanobacteria (mainly Oscillatoria sp.), but other groups were also represented (green algae, cryptomonads, diatoms and dinoflagellates), although they remained constant throughout the year (median Chl a < 0.2 mu g L-1). Cyanobacteria, in contrast, showed an annual cycle with a significantly higher median in summer (Chl a = 1.6 mu g L-1) than in winter (Chl a = 0.4 mu g L-1). No linear relationship between nutrients and cyanobacteria concentration was observed, but an optimum curve of cyanobacteria concentration to sulfide concentration was fitted through a general additive model (GAM). The four-fold increase of cyanobacteria concentration under exposition of an elevated sulfide concentration can be due to higher growth rates at elevated sulfide concentrations reported for an Oscillatoria sp. strain isolated during the same annual cycle at La Hedionda and we suggest that the selective agent, sulfide, positively triggers Oscillatoria sp. proliferation in summer. According to our findings, the Oscillatoria sp. population of La Hedionda not only is sulfide-resistant, but requires sulfide in its optimal niche.