Journalof J • AND B:BIOLOOY ELSEVIER Journal of Photochemistry and Photobiology B: Biology 30 (1995) 123-127 Temperature effects on motion parameters and the phototactic reaction of the marine ciliate Fabrea salina Roberto Marangoni *, Alfredo Batistini, Spartaco Puntoni, Giuliano Colombetti lstituto di Biofisica del C.N.R., Via S. Lorenzo 26, 56127 Pisa, Italy Received 26 October 1994; accepted 19 April 1995 Abstract Fabrea salina, a marine ciliate belonging to the order Heterotrichida, shows positive phototaxis and photophobic step-down reactions. In this work, we have investigated the effect of temperature on the motion parameters and the phototactic reaction. In the temperature range investigated, there is a quasi-linear increase in speed with temperature. In addition, the positive phototactic reaction is enhanced by an increase in temperature. These results are discussed on the basis of possible physical and physiological mechanisms. Keywords: Fabrea salina; Ciliates; Temperature; Phototaxis; Motion 1. Introduction Temperature has a large influence on microorganisms and is responsible for many different effects which are often very complex to understand. Since all biochemical reactions obey the Arrhenius equation, the metabolic activity of a microor- ganism depends on the temperature, and we can easily explain why some metabolic processes are accelerated at high tem- peratures. Such processes can be classified as "passive effects", since they do not consist of an active cell response to temperature. Many examples have been described in the literature. It has been found that cell excitability decreases as the temperature increases [ 1 ], which has been explained in terms of passive and active Ca 2÷ fluxes across the membrane. Similarly, the temperature-induced alterations of ciliary beat pattern and frequency can be classified as passive [ 2]. Some passive effects are long term: evidence reported in the liter- ature has shown that the culture temperature influences both the resting membrane potential [ 3 ] and the thermal responses themselves [4,5 ]. Other effects may be classified as active. Many microor- ganisms, for instance, can sense temperature, and alter their motile behaviour in response to a thermal stimulus. Blephar- ismajaponicum cells, exposed to a temperature gradient, tend to accumulate in an optimal temperature zone; under these conditions, two different thermal responses can be detected: ( 1 ) an increase in thermophobic responses when cells swim * Corresponding author. 1011-1344/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDllOI 1-1344(95)07160-1 away from the optimal zone; (2) a speed increase when cells move towards the optimal zone [6]. This indicates that a single microorganism can show different reactions to tem- perature, interacting with each other in a manner not easy to define. Starting from these considerations, it is of great interest to explore the effect of temperature on cell motility and on other sensorial functions, such as the photoperception. In this paper, we report the results of an investigation aimed at describing and understanding the effect of temperature on the motility and photomotility of the marine ciliate Fabrea salina, which is an active swimmer and shows positive phototaxis and a photophobic step-down response [7-9]. 2. Materials and methods Fabrea salina was grown in artificial seawater (details in Table 1) at room temperature (23 °C) under weak diffuse illumination (4 W m -2) in a 15 h : 9 h light--dark cycle. The green marine alga Dunaliella salina, used to feed the ciliates, was grown in Johnson's medium [ 10]. The algae were cen- trifuged and washed twice with the protozoa medium before being fed to the ciliates. Fabrea salina cells used in the experiments were starved for 4 days, and collected by suction from the culture batch. The experimental apparatus for the measurement of pho- totaxis has been described previously [9]. Here, we recall briefly its most important parts. The cells swim in a controlled