A dinoflagellate adaptive behavior model: response to internal biochemical cues
Introduction
Observational studies on single-cell phytoplankton demonstrate adaptive behaviors according to changes in the physical environment (Kamykowski, 1981a, Cullen and Horrigan, 1981, MacIntyre et al., 1997). Many phytoplankton are not mere passive particles carried by the turbulence or currents in the ocean, but move autonomously from the surface to the deeper section of the ocean to accommodate themselves to various environmental conditions (Jennings, 1976). Although phytoplankton do not have neurons, they function as though they have sense organs for light and gravity (Levandowsky and Kaneta, 1987). Their adaptivity, however, may come not only from a sensitivity to external cues but also from mechanical or perceived changes in their internal state by mechanisms that are poorly defined at present (Jones, 1993, Kamykowski, 1995). Since marine phytoplankton live by depending on photosynthesis and the nutrients in the ocean, their decision to swim up or sink/swim down should be associated with the relation between plankton’s internal condition and the outside physical state.
In this study we have focused on the behavior of planktonic algae called dinoflagellates, which typically are single-cell organisms that autonomously migrate in the ocean. They gain energy and carbon/hydrogen/oxygen through photosynthesis and absorb other necessary nutrients from the surrounding seawater. We built models for an individual dinoflagellate’s vertical swimming behavior and formulated these models to identify the key elements in its adaptation to the environment. Most individual-based models for plankton describe relationships between changes in the external environment and an organisms' behavior, but not the organism's internal state (Murdoch, 1992). We, however, here considered the fluxes and changes of its internal biochemicals which are influenced by changes in the external environment. To construct models, we translated the external environmental changes into a plankter’s internal changes, viewing its behavior as an emergent result of interacting processes inside the organism.
Section snippets
Dinoflagellates and their environment
Dinoflagellates can migrate between the ocean surface and deeper water of more than 12 m, using their flagella. Many dinoflagellate species obtain energy and carbon/hydrogen/oxygen from photosynthesis and absorb other necessary nutrients like nitrogen from the seawater (Levandowsky and Kaneta, 1987). These elements are eventually combined to produce organic material including significant amounts of protein for cell growth. Many observations have noted that dinoflagellates migrate with a 24 h
Model description
Two models were constructed to examine if it is advantageous for a cell to determine its movement according to its biochemical fluxes. The first model, which we call Clock-Driven Model, represents the idea that the internal clock of the cell related to the diel light cycle governs its vertical movements. The other model, which we call Decision-Making Model, was developed based on our assumption that the cell is monitoring its internal biochemical changes to determine its next move. In both
Simulation representation, external conditions and results
We have implemented the models to simulate different external conditions, and made graphs to represent the simulation results. The vertical movement of the dinoflagellate cell is plotted along with the light intensity at the surface in a graphical window, while another window shows the amounts of internal nitrate, photosynthate and protein relative to their predefined maximum values. The value of accumulated protein is reset to half when the amount reaches its predefined maximum value, so that
Discussion
Although the control of dinoflagellate DVM is attributed mainly to the light intensity or to its internal clock, the simulation results suggested that the migration pattern also can profitably vary according to internal changes in photosynthate and nitrate. These results indicate that the periodicity of dinoflagellate diel movement may be regulated by its response to environmental cycles or to its internal clock but that its irregularity may be affected by nitrate availability. The migration
Acknowledgements
We would like to thank Dr Hiroyuki Matsuda at Ocean Research Institute of Tokyo University for his useful advice on the adaptive aspects of our models and Dr Hidekatsu Yamazaki at Tokyo University of Fisheries for providing advice in the development of our models. This work was supported by NSF OCE-9503253.
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