The following project outcomes are highlighted:

1. Predator-prey interactions of planktonic protists are fundamental to plankton dynamics and include prey selection, detection, and capture as well as predator detection and avoidance. Propulsive, morphology-specific behaviors modulate these interactions and therefore bloom dynamics. We used a high-speed microscale imaging system (HSMIS) to demonstrate these fundamental processes by investigating the predator-prey interaction between the mixotrophic, harmful algal bloom dinoflagellate Dinophysis acuminata and its ciliate prey Mesodinium rubrum.  D. acuminata was shown to detect its M. rubrum prey via chemoreception while M. rubrum was alerted to D. acuminata via mechanoreception at much shorter distances (89+-39 μm versus 41+-32 μm). On detection, D. acuminata approached M. rubrum with reduced speed. M. rubrum responded through escape jumps that were long enough to detach its chemical trail from its surface, thereby disorienting the predator. To prevail, D. acuminata used capture filaments and/or released mucus to slow and eventually immobilized M. rubrum cells for easier capture. Mechanistically, our results support the notion that the desmokont flagellar arrangement of D. acuminata lends itself to phagotrophy. In particular, the longitudinal flagellum plays a dominant role in generating thrust for the cell to swim forward, while at other times, it beats to supply a tethering or anchoring force to aid the generation of a posteriorly-directed, cone-shaped scanning current by the transverse flagellum. The latter is strategically positioned to generate flow for enhanced chemoreception and hydrodynamic camouflage, such that D. acuminata can detect and stealthily approach resting M. rubrum cells in the water column.

2. The mixotrophic ciliate Mesodinium rubrum is an ambush feeder relying on cryptophyte prey motility for prey encounter and perception; therefore, cryptophyte species-specific swimming behaviors affect M. rubrum's prey preference. We used the HSMIS to quantify the swimming behaviors of three cryptophyte species (Teleaulax amphioxeia, Storeatula major, and Guillardia theta) and to conduct quantitative microvideography of M. rubrum-T. amphioxeia predator-prey interaction. T. amphioxeia, a preferred prey of M. rubrum, swam at path-averaged speeds of 155+-73 μm s^-1 along rather straight paths. In contrast, S. major regularly tumbled slowly downward or upward at 64+-16 μm s^-1, while G. theta moved slowly in looped/curved trajectories at 57+-15 μm s^-1; neither supports M. rubrum growth. Only while motionlessly sinking passively did M. rubrum detect and initiate an attack on swimming T. amphioxeia at reaction distances of 8.2+-8.2 μm. It seemed that M. rubrum needed to use oral tentacles to initially poke T. amphioxeia's ventral posterior part and subsequently poke the prey multiple times in a short duration to compromise the prey's escape ability, presumably by discharging extrusomes into the prey. T. amphioxeia also responded to nearby predators by switching to tumbling similar to S. major in normal swimming, suggesting an effective anti-predator defense behavior that prevents M. rubrum from accurately poking the prey's ventral posterior part. T. amphioxeia swimming at significantly higher speeds leads to sufficiently high prey encounters and hydrodynamic signals for M. rubrum, thereby partially explaining M. rubrum's ability to select T. amphioxeia prey.

Outcomes 1 and 2 form the basis to build a comprehensive behavioral repertoire for the marine protistan food chain: cryptophyte→Mesodinium→Dinophysis. Our published papers and videos are a first of this kind in the current literature.

3. Studies of multiple flagellated phytoplankton species showed that five of six cultures (four of five species tested) swam faster during nutrient stress. Likewise, five of the six cultures (four of five species tested) had reduced cell size. In addition, we noted modulation of flagellar beating under these conditions translates to time-varying swimming speed at frequencies of 50-70 Hz. The observations suggest that this time variance in swimming speed may alter the diffusive boundary layers around cells and thereby enhance uptake of nutrients and other dissolved chemicals and/or signaling compounds. Results also suggest that time series of swimming behavior can indicate changes in the nutrient physiology of natural bloom populations.

4. Substantial progress was made in engineering the imaging platforms and instruments used in this study.  Past analyses were complicated by substantial convective flows and vibrations within tissue culture flasks used for image capture.  This was a result of temperature differences between our culture incubators and the primary HSMIS recording device used for our experiments. A new HSMIS system was thus set up within a renovated walk-in temperature-controlled room for imaging. An active vibration isolation table can effectively isolate samples and recording equipment from vibrations produced by the room's air handling system, and the constant room temperature reduces convection.  Another change involved adaptation and testing of a new Linux-based version of the Imaging FlowCytobot (IFCB) phytoplankton sensor.  The project team worked with McLane engineers to automate IFCB-based maintenance of cultures through application and development of the sensor's new API. They successfully maintained a replete culture of Alexandrium catenella for three months as a test of the system.

Last Modified: 05/06/2021
Modified by: Houshuo Jiang