Impact of an Organic Rich Environment on Mixotrophic Diatom Phototaxis

Abstract

Mixotrophy, when an organism can perform both autotrophy and heterotrophy, can occur in some species of pennate diatoms. However, processing organic carbon through heterotrophy is energetically expensive, and diatoms are observed to mainly perform it under limiting light. Previous research has investigated mixotrophic diatom growth in light and dark conditions, but which feeding method diatoms use when autotrophy requires investment in the form of phototaxis is unknown. Cultures of Navicula and apochlorotic Nitzschia were grown in agar plates with F/2 media or with added organic carbon media in a control or limited light condition. Daily measurements of the culture’s maximum diameter were taken, and an elliptical ratio was calculated to quantify for any phototaxis. No statistically significant results were found, but there were strong trends and observable results. When in an organic rich environment, Navicula did not display phototaxis compared to strong phototaxis in the normal F/2 media. Nitzschia did not exhibit any phototactic behavior in either medium, but outcompeted Navicula in both media types. In both species, the diatoms had a higher growth rate when grown in an organic rich environment. The trend in results indicate the possibility of underestimating the role that heterotrophy plays in important phytoplankton processes such as vertical migration and carbon cycling.