Planktothrix in Sandusky Bay
Updated: Sep 15, 2020
Sandusky Bay is a shallow basin in the southern Lake Erie and experiences annual cyanobacterial harmful algal blooms (cyanoHABs). Located in a primarily agricultural watershed, Sandusky Bay receives high nitrogen (N) and Phosphorus (P) loading fromt the Sandusky River. Planktothrix agardhii is the dominant bloom-forming species and the main producer of toxins microcystins in Sandusky Bay. Just like Microcystis, Planktothrix is a non-N fixer and thrives on chemically reduced forms of nitrogen (ammonium and urea).
For the second chapter of my dissertation, I was interested in measuring the rate of ammonium cycling (assimilation and remineralization/regeneration) and comparing ammonium uptake kinetics (via Michaelis–Menten model) of a Planktothrix-dominated bloom in Sandusky Bay and Microcystis-dominated bloom in Maumee Bay (Western Lake Erie). By measuring internal regeneration rates we can determine how much N is recycled within the water column and compare these values to the external N loading from the river. The second part of this project was designed to answer the overarching question: Why does Planktothrix dominate in Sandusky Bay while Microcystis is the main cyanoHAB in the main basin of Lake Erie? Lastly, thanks to our collaborators at Bowling Green State University (Bullerjahn/McKay Labs), we were able to investigate Planktothrix N metabolic pathways using metatranscriptomics and pair them with our measured geochemical rates.
In our publication, we show that internal recycling of ammonium via regeneration/remineralization plays an important role during the late summer blooms in Sandusky Bay. When the ambient N concentrations and external loading decrease throughout the summer, the role of regeneration increases, and by the end of August regeneration is the main mechanism sustaining the Planktothrix blooms. At the beginning of the summer (June) extrapolated regeneration rates are two times greater than the external N loading from the Sandusky River. However, at the end of August, regeneration exceeds the river loading 1200 times. We also calculated that the equivalence of 77% of the annual river load is regenerated during the three summer months (June–August).
The Michaelis-Menten half saturation constant (Km) can help determine the affinity of a microbe for substrate. For example, microbes with low Km have high substrate affinity, which can be a competitive advantage in low N systems (they can quickly take up ammonium and get saturated fast). On the other hand, microbes with high Km have lower substrate affinity and can assimilate more ammonium before getting saturated; a competitive advantage in higher trophic systems. Our kinetic experiments in Sandusky and Maumee Bays show that a Planktothrix-dominated bloom has a much lower Km (higher affinity) for ammonium than the Microcystis-dominated bloom (higher Km, lower affinity). We speculated that under low (or undetectable) N conditions in Sandusky Bay, Planktothrix has a competitive advantage over Microcystis. In Maumee Bay, however, ambient N concentrations in the water column are higher and can favor Microcystis.
Metatranscriptome results showed that at the beginning of the summer (when ambient N is available) Planktothrix takes up N and stores it intracellularly (cyanophycin synthetase). However, late in the summer (when ambient N is depleted and undetectable) Planktothrix breaks down stored N (cyanophycinase) to help meet the metabolic needs. Planktothrix is also actively taking up ammonium throughout the summer (amt transcription), and the up-regulation of glutamine synthetase (glnA) reveal active mechanisms to acquire extracellular ammonium during N depletion.
Combined, our results show that, in late summer cyanoHABs in Sandusky Bay rely on regenerated N to support growth and toxin production and Planktothrix "wins" over Microcystis in Sandusky Bay due to the competitive advantage at low N concentrations (Michaelis-Menten experiments).
Hampel, J.J., McCarthy, M.J., Neudeck, M., Bullerjahn, G.S., McKay, R.M., and Newell, S.E., 2019. Ammonium recycling supports toxic Planktothrix blooms in Sandusky Bay, Lake Erie: evidence from stable isotope and metatranscriptome data. Harmful Algae, 81, pp.42–52. https://doi.org/10.1016/j.hal.2018.11.011