Temperature-Dependent Mixotrophy in Natural Populations of the Toxic Dinoflagellate <i>Karenia brevis</i>

Abstract

Previous studies have revealed that mixotrophs can become more heterotrophic as the temperature rises, although these studies were primarily conducted under laboratory conditions with temperature-acclimated grazers. This study investigated the short-term thermal regulation of grazing and photosynthetic performance, measured in terms of the maximum relative electron transport rate (rETRmax), of natural Karenia brevis populations on cultured Synechococcus. Bloom waters were collected in Sarasota, Florida, during the fall of 2022. Synechococcus were inoculated into K. brevis bloom waters in varying ratios and incubated at an ambient temperature and an ambient temperature ±5 °C (19, 24, and 29 °C). In general, the grazing coefficient, clearance, and ingestion rates were higher in warmer waters, although ingestion rates were significantly regulated by the prey-to-grazer ratios and, to a lesser degree, by temperatures (22 to 204 Synechococcus K. brevis−1 d−1). Overall, the rETRmax of Synechococcus controls generally increased over time with a more substantial increase at warmer temperatures, but, in the presence of grazers, the rETRmax of Synechococcus did not increase, and, remarkably, even decreased in some cases. These findings suggest that grazing on Synechococcus could directly regulate Synechococcus concentrations and indirectly reduce the photosynthetic performance of prey. Furthermore, this study demonstrates that the thermal regulation of grazing and photosynthetic performance can occur on a short-term basis.

Keywords

• <i>Karenia brevis</i>
• <i>Synechococcus</i>
• temperature
• maximum relative electron transport rate
• ingestion rate
• prey-to-grazer ratio