The effects of salinity and N : P on N‐rich toxins by both an N‐fixing and non‐N‐fixing cyanobacteria

Abstract

Freshwater ecosystems are experiencing increased salinization. Adaptive management of harmful algal blooms (HABs) contributes to eutrophication/salinization interactions through the hydrologic transport of blooms to coastal environments. We examined how nutrients and salinity interact to affect growth, elemental composition, and cyanotoxin production/release in two common HAB genera. Microcystis aeruginosa (non‐nitrogen [N]‐fixer and microcystin‐LR producer [MC‐LR]) and Aphanizomenon flos‐aquae (N‐fixer and cylindrospermopsin producer [CYN]) were grown in N : phosphorus (N : P) 4 and 50 (by atom) for 21 and 33 d, respectively, then dosed with a salinity gradient (0–10.5 g L−1). Both total MC‐LR and CYN were correlated with particulate N. We found Microcystis MC‐LR production and release was affected by salinity only in the N : P 50 treatment. However, Aphanizomenon CYN production and release was affected by salinity regardless of N availability. Our results highlight how cyanotoxin production and release across the freshwater–marine continuum are controlled by ecophysiological differences between N‐acquisition traits.

Diazotrophy modulates cyanobacteria stoichiometry through functional traits that determine bloom magnitude and toxin production

“…Particulate cylindrospermopsin was lyophilized using a Virtis SP Scientific benchtop pro freeze dryer (SP Scientific) for 48 h and stored at −80°C until analyzed. An isotope dilution liquid chromatography triple quadrupole mass spectrometry method was used for cylindrospermopsin analysis as previously described (Haddad et al 2019) and applied (Osburn et al 2022). Samples were spiked with cylindrospermopsin‐ 15 N 5 before extracting particulate cylindrospermopsin in 1 mL of 25 : 75 acetonitrile : water.…”

“…The exact energetic and elemental costs of synthesizing the multifunctional enzyme complex are unknown, but it likely exceeds that of a cyanotoxin molecule. However, cyanotoxin quotas are often correlated to the resource N : P supply (Wagner et al 2019; Brandenburg et al 2020; Osburn et al 2022). Like many secondary metabolites, cyanotoxins have a variety of elemental compositions, which can be categorized as either N‐rich, or C‐rich/N‐poor relative to typical phytoplankton stoichiometric ratios such as the Redfield ratio (C : N 6.6).…”

“…Nutrient supply can alter the concentration and cell quotas of cylindrospermopsin; however, there is conflicting information on what nutrient regimes increase, decrease, or do not affect cylindrospermopsin production. For example, Raphidiopsis produced more cylindrospermopsin when grown on exclusively atmospheric N compared to combined N sources (Saker and Neilan 2001), whereas Aphanizomenon grown on atmospheric N had lower concentrations and cell quotas of cylindrospermopsin compared to high combined N conditions (Preußel et al 2014; Osburn et al 2022). Conflicting cylindrospermopsin responses also occur in P‐limited conditions with Aphanizomenon increasing cylindrospermopsin concentrations and quotas (Preußel et al 2014) or decreasing cylindrospermopsin concentrations (Bácsi et al 2006).…”

“…Finally, we predicted that N‐fixation would alter light‐harvesting and cyanotoxin relationships through changes in N allocation, with N‐fixing populations allocating less N to cylindrospermopsin quotas, and more N to light‐harvesting pigments compared to non‐N‐fixing populations. We tested this hypothesis using batch culture Aphanizomenon flos‐aquae (PCC 7905) experiments, which is known to be diazotrophic and produce cylindrospermopsin (Osburn et al 2022).…”

Documenting the impacts of increasing salinity in freshwater and coastal ecosystems: Introduction to the special issue

“…Mobilian et al (2020) conducted a large‐scale, multiyear, field manipulation to address how in situ microbial community diversity and carbon cycling activity are more affected by the consistent press of seawater intrusion (i.e., by long‐term sea‐level rise—than by pulses of high salinity due to episodic seawater intrusion). Osburn et al (2022) employed a laboratory experiment to understand the implications of what may happen at the mouth of coastal estuaries when toxic cyanobacterial blooms, such as those that occur in upstream eutrophic reservoirs, encounter salinity when they are flushed downstream to the ocean. Their results indicate that such practices may magnify the harmful effects of cyanobacterial blooms when they mix with ocean water.…”