Oceanic high-nitrate, low-chlorophyll environments have been highlighted for potential large-scale iron fertilizations to help mitigate global climate change. Controversy surrounds these initiatives, both in the degree of carbon removal and magnitude of ecosystem impacts. Previous open ocean enrichment experiments have shown that iron additions stimulate growth of the toxigenic diatom genus Pseudonitzschia . Most Pseudonitzschia species in coastal waters produce the neurotoxin domoic acid (DA), with their blooms causing detrimental marine ecosystem impacts, but oceanic Pseudonitzschia species are considered nontoxic. Here we demonstrate that the sparse oceanic Pseudonitzschia community at the high-nitrate, low-chlorophyll Ocean Station PAPA (50° N, 145° W) produces approximately 200 pg DA L ⁻¹ in response to iron addition, that DA alters phytoplankton community structure to benefit Pseudonitzschia , and that oceanic cell isolates are toxic. Given the negative effects of DA in coastal food webs, these findings raise serious concern over the net benefit and sustainability of large-scale iron fertilizations.

The ichthyotoxic red tide organism Heterosigma akashiwo (Raphidophyceae) has been associated with fish kill events within the aquaculture industry for many years. The precise toxicological mechanism involved in these fish kills is unclear; however, much research attention has focused on the production of reactive oxygen species (ROS) by these toxic algae. In this study, we investigated the production of hydrogen peroxide (H₂O₂) by isolates of H. akashiwo and the nontoxic chlorophyte Tetraselmis apiculata. Subsequently, we tested those concentrations of H2O2 on vertebrate cell lines and the invertebrate Artemia salina (brine shrimp) to investigate mortality. Net production rates for the H. akashiwo isolates ranged from 0.46 to 7.89 pmol H₂O₂ min⁻¹ (10⁴ cells)⁻¹ while obtaining maximum concentrations between 0.14 and 0.91 µM H₂O₂. Conversely, T. apiculata produced only 0.03 pmol H₂O₂ min^±1 (10⁴ cells with a maximum level on 0.04 µM. However, toxic effects on UMRߚ106 and HEKߚ293 cells were only induced by acute and protracted exposure to concentrations of H₂O₂ >= 0.1 mM. Additionally, significant mortality of A. salina in the presence or absence of ferric and ferrous iron was induced by H₂O₂ levels >= 1 mM. Iron is a redox metal that reduces H₂O₂ to hydroxy radicals. These data collectively indicate that production of H₂O₂ by multiple isolates of H. akashiwo is orders of magnitude less than that required for mortality of either the vertebrate cell lines or the invertebrate A. salina. Other nonichthyotoxic roles for extracellular ROS are proposed.

Cyanobacterial blooms pose a significant threat to water security, with anthropogenic forcing being implicated as a key driver behind the recent upsurge and global expansion of cyanobacteria in modern times. The potential effects of land‐use alterations and climate change can lead to complicated, less‐predictable scenarios in cyanobacterial management, especially when forecasting cyanobacterial toxin risks. There is a growing need for further investigations into the specific stressors that stimulate cyanobacterial toxins, as well as resolving the uncertainty surrounding the historical or contemporary nature of cyanobacterial‐associated risks. To address this gap, we employed a paleolimnological approach to reconstruct cyanobacterial abundance and microcystin‐producing potential in temperate lakes situated along a human impact gradient. We identified breakpoints (i.e., points of abrupt change) in these time series and examined the impact of landscape and climatic properties on their occurrence. Our findings indicate that lakes subject to greater human influence exhibited an earlier onset of cyanobacterial biomass by 40 years compared to less‐impacted lakes, with land‐use change emerging as the dominant predictor. Moreover, microcystin‐producing potential increased in both high‐ and low‐impact lakes around the 1980s, with climate warming being the primary driver. Our findings chronicle the importance of climate change in increasing the risk of toxigenic cyanobacteria in freshwater resources.

Management of cyanobacteria has become an increasingly complex venture. Cyanobacteria risks have amplified as society moves forward in an era of accelerated global changes. The cyanobacteria management "pendulum" has progressively shifted from prevention to mitigation, with management considerations often put forth after bloom formation. A universal system (i.e., one-size-fits-all management) fails to provide a management path forward due to the inherent complexities of each lake. A tailored management plan is needed: the right species at the right time in the right place (i.e., the three Rs). The three Rs represent a customizable management strategy that is flexible and informed by advances in scientific understanding to lower cyanobacteria-associated risks. Identifying thresholds in risk tolerance, where thresholds are defined by community collectives, is essential to frame cyanobacteria management targets and to decide on what management interventions are warranted.