Figure 1. Citrate-AgNPs are more toxic to cultures in earlier stages of growth than in later stages and our dynamic model captures the algal dynamics.

5/L citrate-coated AgNPs were introduced to C. reinhardii batch cultures at three different stages of growth: fast growth (solid lines), slowing growth (large dashes), and stationary growth (small dashes). The dynamic model developed through analyses of these data captures the algal dynamics well with a single parameter set (lines). Batch cultures in slowing and stationary growth phases had grown for one and two weeks, respectively, prior to the start of the experiment and before the introduction of AgNPs. Time in this figure is represented as the absolute day of growth of the culture – all cultures were exposed on the same day of the experiment but on different days of growth (cultures in fast growth phase were dosed with AgNPs on day 1 of growth, cultures in slowing growth phase were dosed on day 6 of growth, and cultures in stationary growth phase were dosed on day 13 of growth). AgNPs caused complete mortality of cultures in fast growth phase within two days of introduction. For these cultures, chlorophyll measurements were below detectable limits (denoted by x) by day 3 but the culture was sampled through day 6. We measured concentrations of chlorophyll a to indicate algal cell viability and response to AgNPs because we empirically confirmed that chlorophyll a/cell ratios remain constant after day 5 of growth in algal cultures grown in the light and temperature environments used in this experiment. However, AgNPs had an initial toxic effect on cultures in slowing and stationary growth phases from which the cultures were able to recover until they declined again on days 8 and 10 of the experiment (days 13 and 15 of growth for cultures in slowing phase and days 20 and 22 of growth for cultures in stationary growth phase). The data points are averages from three replicate cultures and the error bars reflect their standard error.