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CIANOBACTÉRIAS, CARACTERÍSTICAS GERAIS

  Algas são aqueles organismos que possuem clorofila do tipo A e que possuem talo (corpo não diferenciado em raízes, caule e folhas). As algas azuis, como são chamadas as algas da divisão das Cyanophita, distinguem-se das outras por serem procariontes (não possuem núcleo organizado) e não terem plastos nas células (JOLY, 1975), daí serem também chamadas Cianobactérias.

Cianobactérias são um dos grupos de organismos mais antigos do mundo, e por isso, são também extremamente adaptáveis. Podem viver com pouquíssima luz, e suportar grandes oscilações de temperatura e salinidade. O seu mecanismo de fotossíntese incorpora um sistema que permite efetuar a oxidação da água, tendo como consequência a libertação de oxigênio - representam 25% da produção total de oxigênio de nosso planeta.

Dentro deste grande grupo, algumas das espécies fotossintetizantes são também, cumulativamente, heterotróficas, outras anoxigénicas, mantêm-se obrigatoriamente anaeróbias e ainda existem as aeróbias, que possuem todo o equipamento enzimático para efetuarem a respiração celular.

 

IMPORTÂNCIA ECOLÓGICA E ECONÔMICA

As cianofíceas são importantes ecologicamente como constituintes do plâncton, seja ele marinho ou dulcícola. Nas cadeias alimentares, ocupam a posição de produtores primários, geralmente servindo de alimento para microscustáceos; na formação de um novo ecossistema, são colonizadores primários (organismos pioneiros).

As toxinas liberadas por elas muitas vezes mantêm algum tipo de equilíbrio ecológico e as colocam numa posição em que isso é usado em relações de simbiose, juntamente com sua atividade autotrófica e a capacidade de fixar Nitrogênio.

Economicamente, mantêm a umidade e a estabilidade mecânica do solo e são utilizadas na alimentação em alguns países.

PRODUÇÃO, PROPRIEDADE E SIGNIFICÂNCIA PARA A SAÚDE PÚBLICA DAS TOXINAS CIANOBACTERIANAS

Em todos os gêneros de cianobactérias existem algumas espécies capazes de produzir potentes toxinas uma delas as microcistinas, que  são hepatotoxinas. Intoxicações ocasionais de animais selvagens e domésticos têm sido reportadas em todo o mundo. As toxinas das cianobactérias podem também representar risco a saúde humana, no Brasil os casos são ligadas à pacientes hemofílicos, onde foram encontradas as microcistinas na água de diálise.

Estudos estão sendo realizados para que se estabeleça fatos a respeito de efeitos ambientais, fisiológicos e regulação bioquímica da produção dessas toxinas.

Os animais aquáticos estão sujeitos a intoxicação pela microcistina em valores letais e subletais. As intoxicações subletais são importantes, porque dessa maneira as toxinas podem se acumular na cadeia alimentar sendo transferida entre os seus membros. Dentre os animais aquáticos susceptíveis estão os moluscos, crustáceos e os peixes. Essas toxinas podem também estar presentes em reservatórios utilizados para abastecer de água a população.  

Os moluscos apresentam tolerância à toxina. Nos crustáceos, que também apresentam tolerância à toxina o clearence, isto é a “depuração” da toxina ocorre lentamente.

Nos peixes, as mortes em sua maior parte não podem ser atribuídas única e exclusivamente às toxinas, uma vez que os sintomas de intoxicação são similares aos efeitos da anoxia, e produção de amônia e sulfetos. Os níveis e toxinas determinados não foram altos em peixes, mas de qualquer modo, pequenas concentrações da toxina foram achadas na musculatura, não apenas nas vísceras. Esses valores de concentração de toxina são sempre menores na musculatura e variam de espécime para espécime. A toxina em peixes, não é absorvida via guelras, mas são assimiladas quando injetadas. Pouco se sabe a respeito da dinâmica da toxina na musculatura dos peixes, mas acredita-se que deve haver uma flutuação dos seus níveis durante o período de clearence.

Esses níveis de toxina medidos nos crustáceos e peixes não causam efeitos adversos em humanos, que raramente consomem produtos com altos níveis de toxina, apesar de que esses números podem subir quando há concomitantemente consumo de água contaminada.

Considerando a concentração de 1 µg/l como limite tolerável , os alimentos que poderiam constituir maior risco à saúde humana são os moluscos bivalves.


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