Single species can enhance or inhibit their own growth via changing the pH.

<p>The curves show bacterial density over time, and the color shows the pH. (a) <i>C</i>. <i>ammoniagenes</i> increases the pH and also prefers these higher pH values, leading to a minimal viable cell density required for survival. Increasing the buffer concentration from 10 mM (−buffer) to 100 mM (+buffer) phosphate makes it more difficult for <i>C</i>. <i>ammoniagenes</i> to alkalize the environment and therefore increases the minimal viable cell density. (b) <i>P</i>. <i>veronii</i> also increases the pH yet prefers low pH values. Indeed, <i>P</i>. <i>veronii</i> populations can change the environment so drastically that it causes the population to go extinct. Adding buffer tempers the pH change and thus allows for the survival of <i>P</i>. <i>veronii</i>. An Allee effect can also be found in <i>L</i>. <i>plantarum</i> and ecological suicide in <i>S</i>. <i>marcescens</i> (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s008" target="_blank">S8 Fig</a>). Note that buffering often just slightly affects the final pH values (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s002" target="_blank">S2 Fig</a>) but saves the population by delaying the pH change (as shown in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s004" target="_blank">S4 Fig</a> and discussed in more detail in [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.ref035" target="_blank">35</a>]). Linlog scale is used for the y-axis. The data for this figure can be found in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004248#pbio.2004248.s022" target="_blank">S1 Data</a>. Ca, <i>Corynebacterium ammoniagenes</i>; CFU, colony-forming unit; Pv, <i>Pseudomonas veronii</i>.