Data from: Allelopathy as an emergent, exploitable public good in the bloom-forming microalga Prymnesium parvum

Many microbes cooperatively secrete extracellular products that favorably modify their environment. Consistent with social evolution theory, structured habitats play a role in maintaining these traits in microbial model systems, by localizing the benefits and separating strains that invest in these products from ‘cheater’ strains that benefit without paying the cost. It is thus surprising that many unicellular, well-mixed microalgal populations invest in extracellular toxins that confer ecological benefits upon the entire population, for example, by eliminating nutrient competitors (allelopathy). Here we test the hypotheses that microalgal exotoxins are (1) exploitable public goods that benefit all cells, regardless of investment, or (2) non-exploitable private goods involved in cell-level functions. We test these hypotheses with high-toxicity (TOX+) and low-toxicity (TOX-) strains of the damaging, mixotrophic microalga Prymnesium parvum and two common competitors: green algae and diatoms. TOX+ actually benefits from dense populations of competing green algae, which can also be prey for P. parvum, yielding a relative fitness advantage over coexisting TOX-. However, with non-prey competitors (diatoms), TOX- increases in frequency over TOX+, despite benefiting from the exclusion of diatoms by TOX+. An evolutionary unstable, ecologically devastating public good may emerge from traits selected at lower levels expressed in novel environments.

许多微生物会协同分泌胞外产物（extracellular products），以对自身生存环境进行有益改造。契合社会演化理论的预测，在微生物模式系统中，结构化生境可通过将收益本地化，并将投入生产该产物的菌株与不付出成本却能获益的“欺骗者”菌株加以区分，从而维持这类性状的存续。

然而令人意外的是，诸多处于均匀混合环境的单细胞微藻种群，会分泌胞外毒素（extracellular toxins）为整个种群带来生态收益——例如通过清除营养竞争者实现化感作用（allelopathy）。本研究针对两项假说展开验证：其一，微藻胞外毒素属于可被利用的公共物品（public goods），无论细胞是否参与分泌，均可为所有细胞带来收益；其二，此类毒素属于不可被利用的私人物品（private goods），仅参与细胞层面的功能活动。

我们选用兼具高毒性（TOX+）与低毒性（TOX-）的混合营养型有害微藻*Prymnesium parvum*，以及两种常见竞争者：绿藻与硅藻（diatoms），对上述假说进行验证。实验结果显示，高毒性菌株（TOX+）可从高密度的竞争性绿藻种群中获益——绿藻同时也可作为*P. parvum*的捕食对象，这使得TOX+相较于共存的TOX-菌株具备相对适合度优势。但当竞争者为非捕食对象的硅藻时，尽管TOX+会通过清除硅藻使TOX-间接获益，TOX-菌株的频率仍会高于TOX+菌株。

这类在演化层面不稳定、却能造成毁灭性生态影响的公共物品，或许起源于在全新环境中以较低选择层级表达的性状。