The aggregation of clay minerals and marine microalgal cells : physicochemical theory and implications for controlling harmful algal blooms

Abstract

In recent years, the use of clay minerals has emerged as one of the most promising strategies for directly controlling harmful algal blooms (HABs). Its principle is based on the mutual aggregation of algal cells and mineral particles, leading to the formation of large flocs that rapidly settle to the ocean floor. This work investigated the effectiveness of various domestic clays against a number of bloom-forming species from the United States. Twenty-five clays were tested against the dinoflagellate, Karenia brevis (formerly Gymnodinium breve), and the chrysophyte, Aureococcus anophagefferens. In general, the highest removal efficiencies (RE > 90% at 0.25 g rl of clay) against K. brevis were found using montmorillonite, bentonite and phosphatic clays (i.e. a product of phosphate mining containing large amounts of montmorillonite). The RE of phosphatic clays remained high (> 80%) even at 0.03 g rl. Kaolinite and zeolite were mostly ineffective against K. brevis. Removal with clay exceeded those for alum, polyaluminum chloride (PAC) and several other polymeric flocculants by a factor of two. However, the combination of phosphatic clay and PAC (at 5 mg rl) decreased the amount of clay needed to maintain 80% RE by one order of magnitude. Cell viability and recovery remained high when clay loading stayed below 0.03 g rl with or without resuspension ofthe sediment. However, cell mortality approached 100% with 0.50 g rl even with daily resuspension. Between 0.10 and 0.25 g rl, K. brevis survival and recovery depended on the interplay of clay loading, the frequency of resuspension, and duration of contact prior to the first resuspension event. For A. anophagefferens, the RE did not exceed 40% for any clay at 0.25 g rl even in combination with coagulants and flocculants. The highest removal was achieved by thoroughly mixing the clay slurry (e.g. phosphatic clay) into the cell culture.The RE by phosphatic clay varied significantly in a survey consisting of 17 different species from five algal classes. Moreover, the removal trends varied substantially with increasing cell concentration. For example, cell removal increased with increasing clay loading and cell concentration for K. brevis. However, RE dropped below 70% when cell concentration was < 1000 cell ml-1 for clay loadings up to 0.50 g rl. This suggested that a critical number of organisms should be present for clays to remain effective. Similarly, enhanced removal with increasing cell concentration was also found in Akashiwo sanguinea (formerly Gymnodinium sanguineum), Heterosigma akashiwo and Heterocapsa triquetra. In the six remaining species, RE initially increased then decreased, or RE remained constant as more cells were treated. The removal pattern among the species at comparable cell numbers did not correlate with the cross-sectional area (R 2 = 0.23), swimming speed (R 2 = 0.04), or a type of cell covering (i.e. theca, silica frustule).3 However, when the total collision frequency coefficients were calculated (including collisions due to cell motilit...

Mitigating Fish-Killing Prymnesium parvum Algal Blooms in Aquaculture Ponds with Clay: The Importance of pH and Clay Type

“…This indicates the release of intracellular toxins. Physical contact between clay particles and algal cells has previously been demonstrated to cause lysis in a range of harmful algal species, ranging from fragile raphidophytes to armored dinoflagellates [19][20][21][22]. Observed higher particle counts in the lower size range (<4 μm) that neither corresponded to P. parvum cells nor clay particles support this suggestion, as these smaller-sized particles likely represent cell fragments and/or aggregates of cell fragments and clay particles.…”

“…Later work, focusing on the clay-mediated removal of Karenia brevis dinoflagellate cells and Microcystis aeruginosa cyanobacterial toxins, reported evidence of direct adsorption of water-soluble, extracellular ichthyotoxins to bentonite-type clays [17,18]. This latter property of clay deserves special attention, since algal cell lysis in response to collisions with clay particles has previously been reported [19][20][21][22]; a process that could greatly amplify ichthyotoxic effects through the release of intracellular ichthyotoxins [23,24]. The potential of clays to adsorb P. parvum ichthyotoxins and the factors governing adsorption remain largely unknown.…”

Physiological responses of Microcystis aeruginosa NIES-843 (cyanobacterium) under the stress of chitosan modified kaolinite (CMK) loading

“…These results indicated that cell mortality of M. aeruginosa occurred after flocculated by CMK. Physical contact between the algal cells and clays was proposed as the main reason for the death of algal cells since no cytotoxic substances was detected releasing from the clay (Sengco 2001). In the present study, we also tested the effect of chitosan and kaolinite separately on cell viability of M. aeruginosa NIES-843, and no of them could cause the lethal effect on the Microcystis cells as CMK loading at corresponding levels of 80 and 160 mg/l respectively (data not shown).…”

“…A few studies, towards cell viability and recovery possibility after flocculation, indicated that clay flocculation could cause cell mortality of some red tide forming algae (Shirota 1989;Bae et al 1998;Sengco 2001), but mechanism underlying cell mortality is not fully understood. The principle of clay flocculation is based on the mutual aggregation of algal cells with mineral particles, leading to the formation of large flocs that rapidly settle into the floor of water column.…”

“…The principle of clay flocculation is based on the mutual aggregation of algal cells with mineral particles, leading to the formation of large flocs that rapidly settle into the floor of water column. Aluminum released from clays was proposed to be the reason for the cell mortality during flocculation (Shirota 1989), but Sengco (2001) suggested that direct physical contact rather than aluminum release, was responsible for the cell mortality. The above conclusions were drawn from physics and chemical views, but no study has been hitherto performed to investigate the physiological responses of HABs to clay flocculation, and such information about the physiological responses is certainly useful in explaining the mechanism for cell mortality of HABs during clay flocculation.…”