ESSENTIAL AND SUBSTITUTABLE RESOURCE RATIO THEORY PREDICTS PHYTOPLANKTON COMPOSITION AND METABOLITES

Phytoplankton competition response to stoichiometric nutrient ratios has been shown to affect phytoplankton biomass, composition, and toxin concentrations. This study examines the effect of essential (total nitrogen [TN] and total phosphorus [TP]) and substitutable (nitrate [NO3-] and ammonium [NH4+]) resource ratios on phytoplankton community composition, metabolites, and microcystin cell quotas. In a 21-day, large-scale mesocosm experiment, six nutrient treatments created high (NO3- and NH4+) and low (NO3- + P, NH4+ + P, and P) TN:TP ratios with differing N forms, effectively driving P- or N- deficient conditions throughout the experiment. We predict that cyanobacteria will dominate when the TN:TP and NO3-:NH4+ molar ratio is low. Our findings indicate that cyanobacteria dominated low TN:TP treatments by the end of the experiment. On day 14, the low TN:TP (~18) and NO3-:NH4+ (~0.12) molar ratios favored Sphaerospermopsis sp. and Microcystis sp. leading to a sixfold increase in microcystin production, particularly LA and LF congeners, in the NH4+ + P treatment. In the NO3- + P and P treatments, the TN:TP molar ratio of 29-31 and NO3-:NH4+ ratio of 0.07-0.09 on day 21 led to dominance of diazotrophic and toxin producing species, with microcystin concentrations peaking on day 14. These P-rich conditions also upregulated the sxtA saxitoxin gene. Our findings emphasize that essential and substitutable ratios can affect phytoplankton community composition highlighting their pivotal role in effectively managing cyanobacterial dominance and toxin synthesis in aquatic ecosystems

Presentation Preference: Oral

Primary Presenter: Zohreh Mazaheri Kouhanestani, University of Missouri (zmfmf@missouri.edu)

Authors:

Zohreh Mazaheri Kouhanestani, University of Missouri  (zmfmf@missouri.edu)

Ted D. Harris, Kansas Biological Survey, University of Kansas  (ted.daniel.harris@gmail.com)

Jacob A. Cianci-Gaskill, Old Woman Creek National Estuarine Research Reserve, Ohio Department of Natural Resources  (jacob.cianci-gaskill@dnr.ohio.gov)

Laura Webb, U.S. Environmental Protection Agency Region 7  (webb.laura@epa.gov)

Regina Klepikow, U.S. Environmental Protection Agency Office of Research and Development  (klepikow.regina@epa.gov)

Heath Mash, U.S. Environmental Protection Agency Office of Research and Development  (mash.heath@epa.gov)

Rebecca L. North, University of Missouri  (northr@missouri.edu)