Zakaria A. Mohamed

Growth inhibition of the cyanobacterium Microcystis aeruginosa and degradation of its microcystin toxins by the fungus Trichoderma citrinoviride

Harmful cyanobacterial blooms are recognized as a rapidly expanding global problem that threatens human and ecosystem health. Many bacterial strains have been reported as possible agents for inhibiting and controlling these blooms. However, such algicidal activity is largely unexplored for fungi. In this study, a fungal strain kkuf-0955, isolated from decayed cyanobacterial bloom was tested for its capability to inhibit phytoplankton species in batch cultures. The strain was identified as Trichoderma citrinoviride Based on its morphological characteristics and DNA sequence. Microcystis aeruginosa co-cultivated with living fungal mycelia rapidly decreased after one day of incubation, and all cells completely died and lysed after 2 days. The fungal filtrate of 5-day culture also exhibited an inhibitory effect on M. aeruginosa, and this inhibition increased with the amount of filtrate and incubation time. Conversely, green algae and diatoms have not been influenced by either living fungal mycelia or culture filtrate. Interestingly, the fungus was not only able to inhibit Microcystis growth but also degraded microcystin produced by this cyanobacterium. The toxins were completely degraded within 5 days of incubation with living fungal mycelia, but not significantly changed with fungal filtrate. This fungus could be a potential bioagent to selectively control Microcystis blooms and degrade microcystin toxins.

Grazing on Microcystis aeruginosa and degradation of microcystins by the heterotrophic flagellate Diphylleia rotans.

Cyanobacterial toxins can cause damage in aquatic ecosystems worldwide, as well as the poisoning of livestock, plants and humans when ingested in large amounts. Although many studies investigated grazing of harmful cyanobacteria by metazoan plankton, grazing of cyanobacteria by hetertophic flagellates is largely unexplored. This laboratory study investigated grazing of toxic Microcystis aeruginosa by the heterotrophic flagellate Diphylleia rotans isolated from a Saudi hypertrophic lake. D. rotans was able to feed on M. aeruginosa with estimated ingestion (10.2 to 16.5 prey flagellate (-1)d(-1)) and specific growth rates (0.71 to 0.99d(-1)) differed with the increase in the initial density of the flagellate. Grazing increased microcystin production within Microcystis cells during first two days of incubation, and afterwards induced the release of these toxins into the medium. The concentrations of releasing microcystin were strongly reduced in grazing-treated cultures compared to controls, indicating the degradation of microcystins by D. rotans growing under axenic conditions. Taken these results, D. rotans can play an important role in the reduction of Microcystis biomass and microcystin toxins, and thus could be used as a safe bioagent for the biocontrol of harmful algal blooms in aquatic environments.

Selective inhibition of toxic cyanobacteria by β-carboline-containing bacterium Bacillus flexus isolated from Saudi freshwaters.

A bacterial strain SSZ01 isolated from a eutrophic lake in Saudi Arabia dominated by cyanobacterial blooms, showed an antialgal activity against cyanobacteria species. Based on the analysis of the 16S rDNA gene sequence, the isolated strain (SSZ01) most likely belonged to the genus Bacillus with a 99% similarity to Bacillus flexus strain EMGA5. The thin layer chromatography (TLC) analysis of the ethyl acetate extract of this bacterium revealed that this strain can produce harmine and norharmane compared to different β-carboline analog standards. Harmine and norharmane were also detected in considerable amounts in bacterial growth medium, indicating a potential excretion of these compounds into the aquatic environment. The crude extract of Bacillus flexus as well as pure materials of harmine and norharmane inhibited the growth of tested species of cyanobacteria. However, the bacterial crude extract has a higher toxicity against tested species of cyanobacteria than harmine and norharmane. In addition, harmine was more toxic to cyanobacteria than norharmane. On the other hand, neither pure compounds of harmine and norharmane nor crude bacterial extract showed any antialgal activity against tested species of green algae. The results of the present study suggest that B. flexus SSZ01 or its crude extract containing harmine and norharmane could be a candidate for the selective control of cyanobacterial blooms without affecting other algal species.

Biodiversity and toxin production of cyanobacteria in mangrove swamps in the Red Sea off the southern coast of Saudi Arabia

Abstract This study investigated the diversity and toxin production of cyanobacteria forming microbial mats on the soil surface and mangrove pneumatophores in three different mangrove swamps in the Red Sea off the southern coast of Saudi Arabia. In total, 34 species belonging to 15 genera and five families were recorded. Benthic mats had higher numbers of species (25–31) than pneumatophore mats (10–12). Cyanobacterial biodiversity (H) and species evenness (E) were greater in benthic mats (H=2.52–2.83 and E=0.79–0.83) than in pneumatophore mats (H=1.7–1.82 and E=0.73–0.74). The results showed high similarity of species between different mangrove sites (67.6%–91.7%) and low similarity between benthic and pneumatophore mats at the same site (32.3%–35.75%). During this study, only nine cyanobacterial species were successfully isolated and tested for their toxicity. The extracts of these species exhibited toxicity to Artemia salina (LC50=0.3–5.1 mg l-1). Based on the results of ELISA and HPLC analyses, four species were found to produce microcystins (683–974 μg g-1) and three species produced saxitoxins (58.9–93.8 μg g-1). The presence of toxic cyanobacteria and their toxins in mangrove swamps may pose a risk for marine animals because these toxins can accumulate in aquatic food webs with a potential for transfer to higher trophic levels.

Allelopathic activity of the norharmane-producing cyanobacterium Synechocystis aquatilis against cyanobacteria and microalgae

The cyanobacterium Synechocystis aquatilis was observed growing as a monospecies in enriched phytoplankton samples in the laboratory, indicating its allelopathic activity on coexisting phytoplankton species. Therefore, the present study screened the culture medium of an axenic strain of this cyanobacterium for the presence of allelechemicals with algicidal properties by thin-layer chromatography (TLC). The allelopathic effect of S. aquatilis was evaluated by co-cultivation of target species of toxic cyanobacteria and green algae with this cyanobacterium, as well as by evaluation of norharmane (β-carboline 9H-pyrido(3,4-b) indole) crude extract prepared from the culture medium of Synechocystis. The growth of target algal species was measured as a cell density after 6 days incubation. The results showed that S. aquatilis produced the indole alkaloid norharmane with large quantities in the culture medium (86 μg l−1). In co-cultivation experiments, S. aquatilis inhibited the growth of all tested cyanobacteria and green algae. Norharmane crude extract exhibited stronger inhibition of cyanobacteria (EC50 = 4.6–4.8 μg ml−1) than green algae (EC50= 6.3–6.4 μg ml−1) in a concentration-dependent manner, indicating its apparent role in the allelopathic activity of S. aquatilis. The possible applicability of the allelochemical, norharmane, as an algicide to prevent the formation of harmful algal bloom was discussed.

Bioavailability of bound microcystins in mice orally fed with contaminated tilapia edible tissues: Implications to human health

ABSTRACT This study investigates for the first time in vivo potential release of bound microcystins (MCs) from edible fish tissues. Bound MCs were released in the mouse digestive system and reached the liver (2.1–14.3 ng g−1) and blood (1–5 ng ml−1). The released MCs were active and negatively affected liver functions of treated mice. The estimated daily intake (EDI) for bound MCs (93.5 &mgr;g kg−1 day−1) is about 2300 times higher than the WHO safe EDI (0.04 &mgr;g kg−1 day−1), compared to lower EDI for free MCs (0.002 &mgr;g kg−1 day−1). Therefore, bound MCs in fish tissues should be considered when fish are monitored for human consumption. HighlightsBound MCs can be released in the mouse digestive system.Released MCs are active and affect mouse liver functions.Bound MCs could increase the toxin burden in human upon fish consumption.

Grazing of the copepod Cyclops vicinus on toxic Microcystis aeruginosa: potential for controlling cyanobacterial blooms and transfer of toxins

Abstract Grazing of zooplankton on phytoplankton may contribute to a reduction of harmful cyanobacteria in eutrophic waters. However, the feeding capacity and interaction between zooplankton and toxic cyanobacteria vary among grazer species. In this study, laboratory feeding experiments were designed to measure the grazing rate of the copepod Cyclops vicinus on Microcystis aeruginosa and the potential microcystin (MC) accumulation in the grazer. Copepods were fed a mixed diet of the edible green alga Ankistrodesmus falcatus and toxic M. aeruginosa for 10 days. The results showed that C. vicinus efficiently ingested toxic Microcystis cells with high grazing rates, varying during the feeding period (68.9–606.3 Microcystis cells animal-1 d-1) along with Microcystis cell density. Microcystis cells exhibited a remarkable induction in MC production under grazing conditions with concentrations 1.67–12.5 times higher than those in control cultures. Furthermore, C. vicinus was found to accumulate MCs in its body with concentrations increasing during the experiment (0.05–3.21 μg MC animal-1). Further in situ studies are needed to investigate the ability of Cyclops and other copepods to assimilate and detoxify MCs at environmentally relevant concentrations before deciding on the biocontrol of Microcystis blooms by copepods.

Potentially harmful microalgae and algal blooms in the Red Sea: Current knowledge and research needs

Harmful algal blooms (HABs) have increased throughout the worlds coastal oceans during the last century mostly due to water eutrophication and climate change. These blooms are often accompanied by extreme extensive negative impacts to fisheries, coastal resources, public health and local economies. However, limited studies have reported HAB events in Red Sea coastal waters. This article reviews potentially harmful microalgae in the Red Sea, based on available published information during the last 3 decades. Five harmful algal blooms were recorded in the Red Sea; of which 3 blooms are formed by dinoflagellates (Noctiluca scintillans, Pyrodinuium bahamense, Protoperidinium quinquecorne), one by raphidophytes (Heterosigma akashiwo) and one by cyanobacteria (Trichodesmium erythraeum). Additionally, mangrove swamps in the Red Sea were occupied by cyanobacterial mats, which contain microcystin and saxitoxin-producing species. The existing data in this review could be a catalyst for the establishment of monitoring and management program for HABs and their toxins in Red Sea coastal waters. This review also identifies current research gaps and suggests future research directions.