Journal of Aquatic Science, 2015, Vol. 3, No. 1, 14-18 Available online at http://pubs.sciepub.com/jas/3/1/3 © Science and Education Publishing DOI:10.12691/jas-3-1-3 The Daily Light-dark Cycle of Photosynthetic Oxygen Evolution in Three Species of Tropical Calcareous Algae Alvin P. Monotilla * , Serafin M. Geson, Paul Isaac O. Dizon, Danilo T. Dy Marine Biology Section, University of San Carlos, Cebu City, Philippines *Corresponding author: alvinmonotilla@yahoo.co.jp Abstract Endogenous coordination between light, temperature and other factors by different species of algae would be vital in the production of several proteins needed for growth and adaptations, and therefore will affect their productivity. Among plant activities that are governed by daily light-dark cycles (i.e. 12L:12D and 12L:12L) among calcareous algae (Halimeda simulans, Mastophora rosea and Padina australis) were conducted by monitoring the dissolved oxygen (DO) concentration inside incubating bottles with algal samples for 24h and the features of the resulting DO curves were determined by measuring the number of pixel under the curve through image analysis. There was no significant difference among algae during the initial 12L:12D cycle suggesting their normal response to light-dark cycles. However, after six days under continuous light, M. rosea showed a significant decrease in the DO curve (lesser number of pixel under the curve) compared to DO curves during the initial 12L:12D cycle. The decrease in the DO concentration during the continuous L:L treatments might be attributed to the photoinhibitory effect of the red alga being less adoptive to subsequent high intensities. Although an increase in DO concentrations is expected with continuous light, not all algae responded to it. Only Padina exhibited circadian rhythm in our 24h observation under continuous light. Keywords: Coral reef, Padina, Halimeda, Mastophora, circadian rhythm Cite This Article: Alvin P. Monotilla, Serafin M. Geson, Paul Isaac O. Dizon, and Danilo T. Dy, “The Daily Light-dark Cycle of Photosynthetic Oxygen Evolution in Three Species of Tropical Calcareous Algae.” Journal of Aquatic Science, vol. 3, no. 1 (2015): 14-18. doi: 10.12691/jas-3-1-3. 1. Introduction There are behaviors and processes in organisms that are observed to be repetitive and therefore predictable during the course of their existence. The adaptation of living cells to daily fluctuations of light, over a period of about a day, is reflected by endogenous rhythms (circa diem) [3]. These rhythms are thought to be regulated by internal oscillators (circadian clocks) or are endogenous biological clock that governs biochemical phenomenon or behavior which allows organisms to predict environmental changes, such as day/night and seasons that result from the rotation of the earth [19]. Among the many environmental cues, light affects virtually all aspects of plant growth and development like gravitropism, phototropism, floral initiation, germination and vegetative circadian rhythm to daily changes in environmental conditions over a 24h light dark (L:D) cycles (Kendrick and Kroneberg, 1994 in [2]). It is for this reason that studies on the effects of light on circadian rhythm and daily L:D cycles in plant are needed to maximize their productivity potential or their sustainability toother geographic locations. Similar to land plants, circadian rhythms are also expressed in marine algae under various light regimes and irradiances. The role of circadian rhythm has been very evident on the high productivity of some algae in the initiation and formation of proteins vital in the synthesis of organic compounds needed for growth and other functions. Various activities tested in rhodophytes Kappaphycus alvarezii [10], Gracilaria tenuistipitata [14] and Gracilaria chilensis [5] on nitrate assimilation characteristics revealed that nitrate reductase activity is under circadian control, exhibiting daily rhythm under L:D cycles and constant light. On the other hand, Drew and Abel [7] observed that the segments of Halimeda rapidly become very pale after dark, but are green enough by dawn to permit rapid photosynthesis as soon as light is available suggesting that the re-emergence of the chloroplast before dawn and their subsequent withdrawal appears to be controlled by an endogenous rhythm which is independent of light and reportedly work at temperatures ranging between 20°C and 30°C [9]. Photosynthesis and other processes (i.e., calcification in calcareous algae) appear to be under circadian control in some of the algae studied. Larkum et al., [12] concluded that in Halimeda macroloba, alkalization of the internal utricular space triggers calcification during photosynthesis and exhibited an endogenous rhythm during a 12:12 hrs L:D treatment but not in continuous light. Unlike Halimeda, another green alga, Acetabularia exhibited endogenous rhythm even in continuous light [11]. This contradiction indicates that further studies are needed to verify the effects of continuous light on other calcareous or coralline algae. The polarographic dissolved oxygen (DO) meter can be used to monitor oxygen evolution and consumption of an incubated algal sample. With appropriate methodology