Mexican Euplotes euryhalinus Cyanobacteria-Grazing Vs. Possible Symbiont Acquisition

A comprehensive analysis was performed with the 2003-2019 data from an athalassohaline maar-crater lake in Alchichica, Mexico (18°10'N; 93°10' W, 2340 m a.s.l.). Ciliate numbers were obtained using Quantitative Protargol Stain; the molecular identification of Euplotes euryhalinus (Valbonesi & Luporini 1990) was proven. Ingested ciliate food was analysed in DAPI stained preparations, while feeding rates were based on an in situ ingestion of Fluorescently Labelled Bacteria (Sherrs´ FLB method), prepared from Synechococcus sp. During 2018, dilution experiments were performed with 20 μm screen-harvested ciliates and 0.2 μm filtered water with low dissolved oxygen, DO and enriched carbon dioxide content (“candle-jar” treatment). E. euryhalinus used to be found throughout the water thermal stratification-period with the anoxic hypolimnion. Surprisingly, most of the ciliate were located either around the oxycline and/or in the well illuminated layers including the surface itself. During the lake mixing, the ciliate was not recorded. E. euryhalinus was observed either with ingested minute diatoms (Cyclotella choctawhatcheeana), green algae (Oocystis parva, Monoraphidium minutum) or other chloroplast-bearing eukaryotes in different levels of pigment degradation that nullify the previous hypothesis of ciliate mixotrophy in the lake. Statistical tests are being conducted to explore the possibility of a direct relation to the nanophytoplankton peaks as a food source. However, the ciliates were not almost observed in the layers with abundant photosynthetic anoxygenic bacteria. We had to re-interpret previously measured feeding rates upon picocyanobacteria, Pcy, although the values were confirmed in recent experiments. Furthermore, published “picocyanobacteria in feeding vacuoles” were observed in vivo being ingested as microcolonies. This explained why observed filtration rates were not related directly to the Pcy numbers and why the Pcy uptakes were far beyond the ciliate growth needs. In both environmental samples and in the dilution experiments, Pcy / FLB were not included in observable vacuoles, which could be related to the known acquisition of endosymbiotic bacteria by Euplotes spp. It was confirmed that Pcy observed inside the ciliate cells were obtained through two different mechanisms: (i) ingestion of colonies placed in vacuoles and, as a hypothesis, (ii) filtration feeding / acquisition of potential symbionts towards the cytoplasm.