Identification of Harmful Algal Blooms (HABs) Toxins in Seawater and Shellfish along the Coast of Ghana

Abstract

Algal toxins can accumulate in fish and shellfish in the marine environment and thus cause these essential seafood products to be poisoned. The toxins and the algae containing them are routinely surveyed in Europe, North America, Japan and other developing countries. Interestingly, there is scarce or no information on the occurrence of toxin-causing algae in most African nations, including Ghana, apart from the North African countries and South Africa. The study identified and evaluated the distribution of harmful and potentially toxic phytoplankton causing harmful blooms of algae; the distribution of harmful phytoplankton along the coast of Ghana was evaluated. This was achieved through: (i) identification and characterization of harmful and potentially toxic phytoplankton species along the Central Coastline of Ghana; (ii) assessment of the distribution of the harmful phytoplankton; (iii) evaluation of the influence of the environmental driving factors {(Temperature, DO, Nutrients [PO43-, NO3-, SiO4] and Trace Elements [Zn & Fe]} on the distribution of HABs in Ghanaian Waters; (v) evaluation of the levels of iron (Fe) and zinc (Zn) in seawater and Hg, Mn, Cu, Zn, Fe and Cd in algae and establish the correlation between the levels of the essential elements and the presence of algae; (vi) isolation of the potentially Harmful Algal Bloom(HABs) species and assessment the level of toxicity of successful cultures and; (vii) investigation of the toxin effects of HABs in selected shellfish (Oysters, Bloody cockle and Clams) [Profiling of Toxins]. Seawater samples from five beaches (Tema, Accra Lighthouse, Bortianor-Tsokomey, Gomoa Nyanyanor and Ekumfi-Narkwa) were collected monthly between January 2014 to February 2018. Diatoms generally dominated the phytoplankton community; however, twelve (12) harmful species which are mostly thecate dinoflagellates. Five of the identified species (Lingulodinium polyedra, Gonyaulax spinifera, Dinophysis caudata, Dinophysis fortii, Alexandrium spp) were toxin producers (potentially causing diarrheic and paralytic shellfish poisoning). With the aid of the Ocean Data View and SPSS software, it was observed that temperature, pH, dissolved oxygen, phosphate, nitrate, zinc and Fe were the environmental driving forces influencing the availabilities of the harmful algal species. The densities of species varied seasonally (wet and dry). Critical densities (6200 cells/L) of Lingulodinium polyedra were recorded in December 2016 at Gomoa Nyanyanor; indicating its abundance in the five study areas. Preliminary observations from the study revealed the possible presence of Alexandrium spp, one of the most toxic genera among the HABs species. Principal Component Analysis (PCA) used to establish possible associations between algae and the physicochemical parameters revealed two distinct clusters corresponding to the sampling seasons (wet and dry seasons). The wet season (upwelling), established proliferation of diverse species whilst in the dry season less diverse species proliferated, but with high densities. Successful isolation and cultures of Alexandrium sp, Prorocentrum sp, Prorocentrum micans, Levanderina fissa and Coolia sp were achieved. Prorocentrum micans, Levanderina fissa and Coolia canarensis were successfully identified to species level by DNA sequencing. Toxin profile of successful cultures of Alexandrium sp, Prorocentrum sp, and Coolia sp, as well as of shellfish (Oysters, Bloody cockle and Clams) were ascertained. Three toxins [two (2) Paralytic Shellfish Poisoning) toxins, i.e. GTX2,3 GTX1,4; and one (1) Diarrheic Shellfish Poisoning toxin (an okadaic acid)] were identified in algae. A strain of Alexandrium spp was found to contain two gonyautoxins (GTX2,3) with 1.875×10-6 ng/cell and GTX1,4 with 1.502×10-4 ng/cell; while Prorocentrum spp were found to have toxicity of 2.65×10-4 ng/cell okadaic acids. Seven (7) bloody cockle samples tested positive to lipophilic toxins, Okadaic acids (OA) and Dinophysistoxins (DTX2). The levels of okadaic acids and dinophysistoxins ranges between 16 to 19 μgOA/kg and 3-6 μg OA/kg respectively. Three Bloody Cockle samples tested positive for traces of Paralytic Shellfish Poisoning toxins, Gonyautoxins (GTX2,3) and decarbamoylsaxitoxin (dcSTX). Interestingly, lipophilic toxins were identified in Ghanaian coastal waters. Okadaic acid and Dinophysistoxin (DTX2) quantified in bloody cockles may be attributed to Dinophysis species, although a partial contribution by Prorocentrum species cannot be ignored. The average concentration of dissolved Zn and Fe in studied coastal waters ranged from 0.021 to 0.23 mg/L and 0.05 to 0.204 mg/L, respectively. The highest concentrations were recorded at Gomoa Nyanyanor and Tema, while Accra Light House had the lowest levels. Studied coastal waters with high Zn and Fe concentrations showed corresponding high concentrations of phytoplankton species. The coastal water of Accra Light House with low levels of Zn and Fe showed low counts of phytoplankton. From the results, among the metals analysed for, the concentration of Cd was found to be below the detection limit (Cd<0.006). Mercury concentration detected from the total microalgae ranged from 10.05 to 18.29 (μg/kg). Other metals concentrations in the microalgae were in the ranges of 162.00 to 4418.90 (μg/kg) for Zn, 4533 to 24567 (μg/kg) for Fe and 225.10 to 2121.46 (μg/kg) for Mn. and 154.50 to 497.90 (μg/kg) respectively. Overall, the study identified the presence species of harmful algal blooms and biotoxins in the studied portion of the Ghanaian coastal waters. Apart from Accra Light House were just minute traces of harmful algal blooms species were present, the rest of the studied coastal waters had HABs well distributed. Generally, biotoxins were not identified in shellfish from the studied waters, except Ekumfi Narkwa were biotoxins were identified in only bloody cockles.