Can toxin warfare against fungal parasitism influence short-term Dolichospermum bloom dynamics? - A field observation.

Cyanobacterial blooms often consist of numerous co-existing cyanobacterial species, with predominant taxa dynamically varying intra-annually. Parasitism by fungi (chytrids) has come into focus as an important factor driving short-term bloom dynamics. Using microscopic analysis, Illumina sequencing and cyanobacterial toxin analyses, we monitored the seasonal succession of Dolichospermum blooms in a reservoir along with environmental parameters. We identified two consecutive Dolichospermum blooms that were characterized by a straight and a coiled morphotype, separated by a complete bloom collapse. Phylotyping provided evidence for three putative Dolichospermum amplicon sequence variants (ASVs); i.e. Dolichospermum1 & 2 in the first bloom (straight filaments) and Dolichospermum3 in the second bloom (coiled filaments). Morphotype succession as well as total filament concentration did not correlate with any of the measured environmental parameters. Fungal parasitism by the chytrid Rhizosiphon crassum occurred in straight Dolichospermum filaments only. Coiled filaments showed no infection despite ambient presence of chytrids, deduced from fungal ASVs, throughout the entire observation period. Toxin concentrations (microcystins (MCs) and anabaenopeptins) correlated significantly with the abundance of the straight Dolichospermum morphotype. Enhanced cyanotoxin biosynthesis in the straight Dolichospermum morphotype, interpreted as a defensive reaction to fungal parasitism, appeared to come at the expense of lowered competitiveness with the co-occurring coiled morphotype. Our findings support the hypothesis that selective parasitism by chytrids is an important factor driving short-term morphotype and toxin dynamics within cyanobacterial blooms.

Rearing Temperature and Fatty Acid Supplementation Jointly Affect Lipid Fluorescence Polarization and Heat Tolerance in Daphnia.

The homeoviscous adaptation hypothesis states that the relative abundance of polyunsaturated fatty acids (PUFAs) in membrane phospholipids of ectothermic organisms decreases with increasing temperatures to maintain vital membrane properties. We reared Daphnia magna at 15°, 20°, and 25°C and increasing dietary concentrations of the long-chain PUFA eicosapentaenoic acid (EPA) to test the hypothesis that the well-documented increase in heat tolerance of high-temperature-reared Daphnia is due to a reduction in body PUFA concentrations. Heat tolerance was assessed by measuring the time to immobility at a lethally high temperature (Timm at 37°C), and whole body lipid fluorescence polarization (FP) was used as an estimate of membrane fluidity. At all rearing temperatures, EPA supplementation resulted in an increase in the relative abundance of EPA in body tissues, but only at 15° and 25°C did this result in a decrease in heat tolerance, and only at 20°C was this associated with an increase in membrane fluidity (i.e., decrease in FP). Overall, however, the degree of tissue fatty acid unsaturation correlated well with heat tolerance and FP. Our results support the homeoviscous adaptation hypothesis by showing that cold-reared Daphnia accumulate PUFAs within their body tissues and thus are more susceptible to heat than hot-reared Daphnia accumulating fewer PUFAs. However, our data also point out that further studies are required that elucidate the complex relationships between PUFA supply, membrane fluidity, and heat tolerance in ectotherms.