Elevated pH Conditions Associated With Microcystis spp. Blooms Decrease Viability of the Cultured Diatom Fragilaria crotonensis and Natural Diatoms in Lake Erie

<jats:p>Cyanobacterial Harmful Algal Blooms (CyanoHABs) commonly increase water column pH to alkaline levels ≥9.2, and to as high as 11. This elevated pH has been suggested to confer a competitive advantage to cyanobacteria such as <jats:italic>Microcystis aeruginosa</jats:italic>. Yet, there is limited information regarding the restrictive effects bloom-induced pH levels may impose on this cyanobacterium’s competitors. Due to the pH-dependency of biosilicification processes, diatoms (which seasonally both precede and proceed <jats:italic>Microcystis</jats:italic> blooms in many fresh waters) may be unable to synthesize frustules at these pH levels. We assessed the effects of pH on the ecologically relevant diatom <jats:italic>Fragilaria crotonensis in vitro</jats:italic>, and on a Lake Erie diatom community <jats:italic>in situ</jats:italic>. <jats:italic>In vitro</jats:italic> assays revealed <jats:italic>F. crotonensis</jats:italic> monocultures exhibited lower growth rates and abundances when cultivated at a starting pH of 9.2 in comparison to pH 7.7. The suppressed growth trends in <jats:italic>F. crotonensis</jats:italic> were exacerbated when co-cultured with <jats:italic>M. aeruginosa</jats:italic> at pH conditions and cell densities that simulated a cyanobacteria bloom. Estimates demonstrated a significant decrease in silica (Si) deposition at alkaline pH in both <jats:italic>in vitro F. crotonensis</jats:italic> cultures and <jats:italic>in situ</jats:italic> Lake Erie diatom assemblages, after as little as 48 h of alkaline pH-exposure. These observations indicate elevated pH negatively affected growth rate and diatom silica deposition; in total providing a competitive disadvantage for diatoms. Our observations demonstrate pH likely plays a significant role in bloom succession, creating a potential to prolong summer <jats:italic>Microcystis</jats:italic> blooms and constrain diatom fall resurgence.</jats:p>