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

Abstract

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 Microcystis aeruginosa. 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 Microcystis 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 Fragilaria crotonensis in vitro, and on a Lake Erie diatom community in situ. In vitro assays revealed F. crotonensis 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 F. crotonensis were exacerbated when co-cultured with M. aeruginosa 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 in vitro F. crotonensis cultures and in situ 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 Microcystis blooms and constrain diatom fall resurgence.

Effect of Culture pH on Properties of Exopolymeric Substances from Synechococcus PCC7942: Implications for Carbonate Precipitation

“…Large EPS aggregates with neutral buoyancy, referred to as transparent exopolymeric particles (TEP), were produced in an artificial marine Synechococcus bloom under nutrient limitation. Alkaline conditions, similar to those in our culture grown in non-buffered medium, are typically associated with picoplankton blooms [52] and support a less dense, water-soluble dispersed EPS structure [38]. These conditions are attributed to the onset of TEP formation in coastal waters and lakes [53,140].…”

“…In contrast, EPS excretion in pelagic cyanobacteria is predominantly associated with nutrient limitation, excessive light conditions in the photic zone, and increases in salinity during transport from freshwater to estuaries [34,[48][49][50]. High rates of photosynthesis in dense picoplankton populations notably increase the environmental pH [51,52]. Soluble EPS produced by the cyanobacterial picoplankton are a precursor for transparent exopolymeric particles (TEP) [53,54].…”

Assessment of surface waters quality of small water reservoirs in agricultural landscape

“…Dense bloom of cyanobacteria in the WRs of the cluster 2 was accompanied by high contents of chlorophyll a, phycocyanin and phycoerythrin and also by increased pH above 9. According to Zepernick et al (2021), this increase in pH provides a competitive advantage to cyanobacteria, such as e.g. Microcystis aeruginosa.…”

Draft Genome Sequence of the Freshwater Diatom Fragilaria crotonensis SAG 28.96

“… Fragilaria crotonensis is broadly distributed in freshwater systems, including both oligotrophic and hypereutrophic lakes, and serves as a biological indicator of eutrophication ( 1 – 5 ). F. crotonensis is an important member of Lake Erie’s phytoplankton because it has historically bloomed in summer ( 6 ) and remains a dominant member seasonally ( 7 – 11 ). To facilitate diatom-focused omics studies of Lake Erie and other lakes, we report the assembled and annotated F. crotonensis SAG 28.96 genome.…”

“…Nonaxenic unialgal cultures of F. crotonensis SAG 28.96 (Culture Collection of Algae at the University of Göttingen, Göttingen, Germany) were cultured and collected as reported previously ( 8 ). DNA was extracted using standard phenol-chloroform methods with ethanol precipitation ( 12 ) and was quantified using the Qubit double-stranded DNA (dsDNA) HS assay kit (Invitrogen).…”