Environmental Factors Impacting the Development of Toxic Cyanobacterial Proliferations in a Central Texas Reservoir.

Cyanobacterial harmful algal proliferations (cyanoHAPs) are increasingly associated with dog and livestock deaths when benthic mats break free of their substrate and float to the surface. Fatalities have been linked to neurotoxicosis from anatoxins, potent alkaloids produced by certain genera of filamentous cyanobacteria. After numerous reports of dog illnesses and deaths at a popular recreation site on Lady Bird Lake, Austin, Texas in late summer 2019, water and floating mat samples were collected from several sites along the reservoir. Water quality parameters were measured and mat samples were maintained for algal isolation and DNA identification. Samples were also analyzed for cyanobacterial toxins using LC-MS. Dihydroanatoxin-a was detected in mat materials from two of the four sites (0.6-133 ng/g wet weight) while water samples remained toxin-free over the course of the sampling period; no other cyanobacterial toxins were detected. DNA sequencing analysis of cyanobacterial isolates yielded a total of 11 genera, including Geitlerinema, Tyconema, Pseudanabaena, and Phormidium/Microcoleus, taxa known to produce anatoxins, including dihydroanatoxin, among other cyanotoxins. Analyses indicate that low daily upriver dam discharge, higher TP and NO3 concentrations, and day of the year were the main parameters associated with the presence of toxic floating cyanobacterial mats.

First Confirmed Case of Canine Mortality Due to Dihydroanatoxin-a in Central Texas, USA.

The frequency of dogs becoming ill or dying from accidental exposure to cyanotoxins, produced by cyanobacteria, is increasing throughout the United States. In January and February of 2021, two dogs died and five dogs became ill after swimming in Lake Travis, central Texas, USA; one deceased dog (C1) was subjected to pathological testing. Algal materials, sediment samples, zebra mussel viscera, periphyton from shells, as well as fluids and tissues from the digestive tract of C1 were investigated for the following cyanotoxins: anatoxin-a, homoanatoxin-a, dihydroanatoxin-a (dhATX), cylindrospermopsin, saxitoxin, and microcystins. Necropsy results of C1 indicated neurotoxicosis with significant levels of dhATX in the duodenum tissues (10.51 ng/g dry weight (DW)), jejunum tissue (6.076 ng/g DW), and stomach contents (974.88 ng/g DW). Algae collected near the site of C1's death contained levels of dhATX, ranging from 13 to 33 µg/g. By comparison, dhATX was detected at much lower concentrations in sediment samples (310.23 ng/g DW) and the periphyton on zebra mussel shells (38.45 ng/g DW). While dhATX was suspected in the deaths of canines from an event in Texas in 2019, this is the first report linking dhATX neurotoxicosis through pathological findings in Texas and potentially in the United States.

Dark adaptation and ability of pulse-amplitude modulated (PAM) fluorometry to identify nutrient limitation in the bloom-forming cyanobacterium, Microcystis aeruginosa (Kützing).

Harmful algal blooms in inland waters are widely linked to excess phosphorus (P) loading, but increasing evidence shows that their growth and formation can also be influenced by nitrogen (N) and iron (Fe). Deficiency in N, P, and Fe differentially affects cellular photosystems and is manifested as changes in photosynthetic yield (Fv/Fm). While Fv/Fm has been increasingly used as a rapid and convenient in situ gauge of nutrient deficiency, there are few rigorous comparisons of instrument sensitivity and ability to resolve specific nutrient stresses. This study evaluated the application of Fv/Fm to cyanobacteria using controlled experiments on a single isolate and tested three hypotheses: i) single Fv/Fm measurements taken with different PAM fluorometers can distinguish among limitation by different nutrients, ii) measurements of Fv/Fm made by the addition of DCMU are comparable to PAM fluorometers, and iii) dark adaptation is not necessary for reliable Fv/Fm measurements. We compared Fv/Fm taken from the bloom-forming Microcystis aeruginosa (UTEX LB 3037) grown in nutrient-replete treatment (R) and N-, P-, and Fe-limited treatments (LN, LP, LFe, respectively), using three pulse-amplitude modulated (PAM) fluorometers and the chemical photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and evaluated the effects of dark adaptation prior to PAM measurement. There were significant differences in Fv/Fm estimates among PAM fluorometers for light- versus dark-adapted cell suspensions over the whole experiment (21 days), which were all significantly higher than the DCMU-based measurements. However, dark adaptation had no effect on Fv/Fm when comparing PAM-based values across a single nutrient treatment. All Fv/Fm methods could distinguish LN and LP from R and LFe treatments but none were able to resolve LFe from R, or LN from LP cultures. These results indicated that for most PAM applications, dark adaptation is not necessary, and furthermore that single measurements of Fv/Fm do not provide a robust measurement of nutrient limitation in Microcystis aeruginosa UTEX LB 3037, and potentially other, common freshwater cyanobacteria.