注意，thus 是副词，不是连词，如果你的句子要用 thus

threatened 也可以（注意此时的 threatened 是过去式，不是过去分词），则前面需加上并列连词 and，即：

The oil spill has

killed off billions of microscopic sea plants, thus threatening marine life further up the food chain.

= The oil spill

has killed off billions of microscopic sea plants, and thus threatened marine life further up the food chain.

也可以补全省略的主语写成：The oil spill has

killed off billions of microscopic sea plants, and thus it threaten marine life further up the food chain.

副词 thus 也可以放在主语的后面写成：The oil spill has

killed off billions of microscopic sea plants, and it thus threaten marine life further up the food chain.

原句意思：

The oil spill has killed off billions of microscopic sea plants, thus threatening marine life further up the food chain.

石油的溢出使得无数微小的海洋植物死亡，从而威胁着食物链中上层的海洋生命。