SUMMARY The responses of relative growth rate (% day –1 ) and pigment content (chlorophyll a, phycocyanin and phyco- erythrin) to temperature, irradiance and photoperiod were analyzed in culture in seven freshwater red algae: Audouinella hermannii (Roth) Duby, Audouinella pygmaea (Kützing) Weber-van Bosse, Batrachospermum ambiguum Montagne, Batrachospermum delicatulum (Skuja) Necchi et Entwisle, ‘Chantransia’ stages of B. delicatulum and Batrachospermum macrosporum Montagne and Compsopogon coeruleus (C. Agardh) Montagne. Experimental conditions included tempera- tures of 10, 15, 20 and 25°C and low and high irradi- ances (65 and 300 μmol photons m –2 s –1 , respectively). Long and short day lengths (16:8 and 8:16 LD cycles) were also applied at the two irradiances. Growth effects of temperature and irradiance were evident in most algae tested, and there were significant interactions among treatments. Most freshwater red algae had the best growth under low irradiance, confirming the pref- erence of freshwater red algae for low light regimens. In general there was highest growth rate in long days and low irradiance. Growth optima in relation to tempera- ture were species-specific and also varied between low and high irradiances for the same alga. The most significant differences in pigment content were related to temperature, whereas few significant differences could be attributed to variation in irradiance and photo- period or interactions among the three parameters. The responses were species-specific and also differed for pigments in distinct temperatures, irradiances and photoperiods in the same alga. Phycocyanin was gener- ally more concentrated than phycoerythrin and phyco- biliproteins were more concentrated than chlorophyll a. The highest total pigment contents were found in two species typical of shaded habitats: A. hermannii and C. coeruleus. The expected inverse relationship of pig- ment with irradiance was observed only in C. coeruleus. In general, the most favorable conditions for growth were not coincident with those with highest pigment contents. Key words: chlorophyll, ecophysiology, freshwater red algae, growth, irradiance, photoperiod, phycocyanin, phycoerythrin, temperature. INTRODUCTION Light regimen, including fluctuations in intensity, quality and photoperiod, is one of the major factors affecting the distribution and seasonality of freshwater red algae (Sheath 1984). Temperature also influences latitude, altitude and drainage basin distribution pat- terns, as well as seasonality, of freshwater red algae (Sheath and Hambrook 1990). Despite the importance of these environmental factors, few experimental studies have been conducted to evaluate their eco- physiological role in freshwater red algae. The effects of light and temperature on pigment content have been little studied in freshwater red algae. Rider and Wagner (1972) observed that the chlorophyll content of Batrachospermum turfosum [as B. vagum (Roth) C. Agardh] decreased two- to threefold when plants were exposed for 1 month to green or far red light, in comparison to white, blue or red wavelengths. They also found a greater chlorophyll production in low- intensity ranges (< 55 μmol photons m –2 s –1 ). Thirb and Benson-Evans (1983) reported alterations in pigment content in Lemanea sp. in response to changes in both light quantity and quality. The highest concentrations of chlorophyll and phycoerythrin were obtained at low irradiance (ca 25 μmol m –2 s –1 ) and in red light, whereas minimum values were found at high irradiance (ca 94 μmol m –2 s –1 ) and in yellow light. The effect of temperature and suspended solids on the pigment concentration in Lemanea sp. was tested by Thirb and Benson-Evans (1984, 1985). They reported a trend of decreasing chlorophyll a and phycoerythrin content with Phycological Research 2001; 49: 103–114 Effects of temperature, irradiance and photoperiod on growth and pigment content in some freshwater red algae in culture Marcelo Ribeiro Zucchi and Orlando Necchi Jr* São Paulo State University, Zoology and Botany Department, Rua Cristóvão Colombo, 2265 – 15054-000, São José do Rio Preto, SP, Brazil *To whom correspondence should be addressed. Email: orlando@bot.ibilce.unesp.br Communicating editor: D. J. Garbary. Received 11 July 2000; accepted 11 December 2000.