Brevetoxin Dynamics and Bioavailability from Floc Following PAC-Modified Clay Treatment of Karenia brevis Blooms

Abstract

Harmful algal blooms (HABs) caused by the dinoflagellate Karenia brevis present serious ecological and public health concerns due to the production of brevetoxins (BTX). Clay flocculation and sedimentation of cells, particularly with polyaluminum chloride (PAC)-modified clays, is a promising HAB mitigation approach. This study evaluated the efficacy of Modified Clay-II (MCII), a PAC-modified kaolinite clay, in reducing K. brevis cell abundance in mesocosm experiments and examined the bioavailability of BTX potentially released from settled floc back into the water column and sediment over the first 72 h after treatment. Additionally, we quantified trace metals in benthic clams (Mercenaria mercenaria) exposed to the floc post-treatment to assess metal accumulation and potential toxicological effects from MCII application. MCII treatment (0.2 g/L) resulted in a 91% reduction in K. brevis cell density and a 50% decrease in waterborne brevetoxins after 5 h. Brevetoxins accumulated in sediment post-flocculation, with BTX-B5 emerging as the dominant congener. Clams exposed to MCII-treated floc showed comparable tissue BTX levels to controls and significantly elevated aluminum concentrations, though without mortality. The aluminum accumulations in this study do not raise concerns for the health of the clams or the humans who eat them, given other dietary exposures. These findings support the potential of MCII for HAB mitigation while underscoring the need for further evaluation of exposure risks to all benthic species.

Keywords: HABs; Karenia brevis; brevetoxin; floc; flocculation; harmful algal blooms; modified clay; red tide.

Figure 3

Sampling timeline for YSI water quality, cell counts, water toxins, sediment toxins and clam tissue toxins. Yellow arrows indicate MCII treatment time point and time at which water was swapped in each tank followed by addition of clams.

Figure 5

(A) Mean ± standard deviation of total brevetoxin concentrations (ng/L) measured in water over time in Control (−Floc) and MCII Treatment (+floc) tanks (n = 3 tanks per group). Vertical black dashed lines indicate the time points when water was exchanged, and clams were introduced. Total brevetoxin levels represent the sum of individual toxin congeners detected in each tank. (B) Mean ± standard deviation of K. brevis cell concentrations (cells/L) over time in Control (−Floc) and MCII Treatment (+floc) tanks (n = 3 tanks per group). Vertical black dashed lines denote the timing of water exchange and clam addition. The level of significance is indicated for cellular concentrations (* t < 0.01) using Welch’s t-test, two-tailed, two-sample unequal variance.

Figure 1

Chemical structures of Type A brevetoxin backbone congener BTX-1 and Type B brevetoxin backbone congeners BTX-2, BTX-3, and BTX-B5.

Figure 2

Experimental setup of 80 L tanks to assess the effects of MCII. Each setup included three replicate columns and illuminated with overhead lighting. The treatment group (left) was exposed to both K. brevis and MCII (0.2 g/L), while the control group (right) was exposed to K. brevis only. The bottom of each column contained 3 plastic cups (each with 3 clams) and 5 glass jars filled with sand to simulate sediment conditions.

Figure 4

Mean ± standard deviation of YSI water quality measurements of (A) temperature (°C), (B) pH, (C) dissolved oxygen (mg/L), (D) salinity (ppt), and (E) oxidation reduction potential (mV) taken over time from control (−Floc) and MCII treatment (+Floc) tanks (3 tanks averaged per control and treatment). Red line represents control (−Floc). Blue line represents MCII treatment (+Floc). Vertical black dashed line represents time at which water was exchanged in each tank and clams were added. The level of significance is indicated for cellular concentrations (* t < 0.01, *** t < 0.0001) using Welch’s t-test, two-tailed, two-sample unequal variance.

Figure 6

Mean ± standard deviation of total brevetoxin concentrations (ng/g) measured in sediment over time in Control (−Floc) and MCII Treatment (+Floc) tanks after water exchanges (n = 3 tanks per group). Total brevetoxin levels represent the sum of individual toxin congeners detected in each tank.

Figure 7

Mean ± standard deviation of total brevetoxin concentrations (ng/g) measured in clam tissue over time in Control (−Floc) and MCII Treatment (+Floc) tanks after water exchanges (n = 3 tanks per group). Total brevetoxin levels represent the sum of individual toxin congeners detected in each tank.

Figure 8

Mean ± standard deviation of trace aluminum (Al) concentrations within tissue of MCII Treatment (+Floc) and Control (−Floc) groups (n = 3 clams per group). Tissue samples were collected at 24-, 48-, and 72 h post treatment and analyzed by ICP-MS. Concentrations of target analyte are expressed as milligrams of target analyte per kilogram of clam tissue (mg/kg). The level of significance is indicated for aluminum concentration (* t < 0.01) using Welch’s t-test, two-tailed, two-sample unequal variance.