Exploration of clay minerals in mitigating fish-killing algal blooms

Fish-killing algal blooms increasingly threaten global aquaculture operations through the production of a wide variety of potent fish-killing toxins (ichthyotoxins). Effective mitigation strategies are therefore much needed. This thesis work explored the potential of clay minerals to not only remove harmful algal cells through flocculation, but instead focused on their ichthyotoxin adsorptive properties. Our experimental work explored 14 different clays (bentonite, kaolin, Korean loess, Phoslock\(^{TM}\), zeolite), tested against 7 fish-killing algae (Alexandrium spp., Chattonella marina, Cochlodinium polykrikoides, Heterosigma akashiwo, Karlodinium veneficum, Karenia mikimotoi, Prymnesium parvum). Ichthyotoxicity was assessed with the gill cell line RTgill-W1 assay. Extensive screening of different clays against the haptophyte Prymnesium parvum showed that removal of ichthyotoxicity was clay type and pH specific. At pH 9, only bentonite clays could completely remove ichthyotoxicity. However, other clays, such as Phoslock\(^{TM}\) that worked well (57% removal) at pH 7, exacerbated ichthyotoxicity at pH 9 (by up to 30%), despite demonstrating high cell removal. We interpret this to be due to Prymnesium cell lysis caused by physical contact with clay, as well as pH induced changes in clay particle surface chemistry (e.g. zetapotential). Bentonite clay could completely eliminate P. parvum, K. mikimotoi and K. veneficum toxicity at clay loadings between 0.05-0.25 g L\(^{-1}\), with detailed additional experiments revealing high swelling (<20 mL/2g) bentonites of fine particle size (<5μm) to be best suited. While extracellular C. marina, H. akashiwo and Alexandrium spp. ichthyotoxins could only partially be removed, application of clay during cell lysis completely eliminated ichthyotoxicity. The likely reason appears to be the preferential adsorption of lipid peroxidation precursors (EPA, DHA) over highly toxic fatty acid aldehyde end products (deca- and heptadienal). Chattonella marina cell removal could be significantly improved through the addition of the flocculant polyaluminium chloride or synthetic amino-clays, which both proved excellent C. marina ichthyotoxin adsorbents. In the case of A. catenella, however, only bentonite clay proved suitable for ichthyotoxin removal purposes. Finally, in August 2015, we worked with Korean National Institute of Fisheries Science scientists on routine clay dispersal operations targeting finfish farm threatening Cochlodinium polykrikoides blooms off Namhae Island ($1.4M fish lost). Clay effectively reduced Cochlodinium cells to below levels considered dangerous to finfish aquaculture (<300 cells mL\(^{-1}\)), but exacerbated ichthyotoxicity by up to 32% compared to control sites. Simulated laboratory experiments with Korean loess confirmed that rapid (<5 min) Cochlodinium cell lysis after clay treatment caused increased ichthyotoxicity. Identical, finely ground (<30 μm) clay (not used in field) also generated significant cell lysis, but instead was capable of completely eliminating ichthyotoxicity. Laboratory manipulations with various Korean clay application regimes (pulsed additions, turbulence), indicated that only Cochlodinium concentration significantly influenced cell removal efficiency (higher removal at lower algal densities). Ground clay consistently outperformed conventional Korean loess (up to 20% higher cell removal). These findings highlight the potential of clay for ichthyotoxin adsorption, albeit HAB species, clay and pH specific, and point towards fine-tuning current practices for improved treatment efficacy.