Phylogenetic analysis of photosynthesis related proteins in Chromera velia and study of its photosynthetic response to iron limitation

Abstract

Chromera velia (C.velia) is a newly discovered algal species in Australia (Moore et al., 2008). It possesses photosynthetic characteristics similar to photosynthetic dinoflagellates, but has physiological and molecular features of non-photosynthetic apicomplexan parasites. Hence, it has been proposed that C.velia may be the missing link between photo-autotrophic dinoflatellates and heterotrophic apicomplexans. This project aimed to: (1.) analyse the light harvesting complexes (LHC) and enzymes involved in the Calvin-Benson cycle in C.velia using a phylogenetic method to obtain a better understanding of the evolutionary development of light and dark reactions in photosynthesis; and (2.) characterize the photosynthetic apparautus in C.velia under normal and iron-stress conditions using a set of biochemical analysis methods as well as bioinformatics. LHC comprise proteins that absorb light energy and transfer it to photosynthetic reaction centres. Sequencing of the LHC in C.velia identified three typical membrane spanning regions (MSR). Phylogenetic analysis indicates that one group is closely related to diatoms, another to red algae, and a third one containing a LHC peptide closely related to the LI818/LHCSR group in charge of photosynthesis regulation. This is the first time LHC in C.velia have been analysed using phylogenetic methods. Our results clearly support the hypothesis that there is a connection between C.velia and diatoms (Moore et al., 2008). This relationship was also confirmed by protein sequencing of isolated LHC from C.velia. The Calvin-Benson cycle is an important part of the dark reaction of photosynthesis. It fixes CO2 and converts inorganic carbon into organic chemicals using adenosine triphosphate synthesized from light reactions. Phylogenetic analysis of the enzymes involved in the Calvin-Benson cycle in C.velia revealed a complicated evolutionary pathway. The majority of enzymes in C.velia originate from proteobacteria, with two exceptions (Fructose-bisphosphate aldolase and Phosphoribulokinase). Different enzymes in C.velia share close relationships with green algae, red algae, diatoms, photosynthetic dinoflagellates, and Apicomplexa. We carried out our phylognetic analysis of the enzymes involved in the Calvin-Benson cycle as a whole for the first time. Our results clearly showed that these enzymes have a mosaic pattern of evolutionary relationships with other groups, supporting the “shopping bag” theory proposed by Larkum et al. (2007). Iron is an essential element for photosynthesis, vital for assembling photosystem I and other photosynthetic proteins. Given that the oxidized form of iron does not dissolve in water, iron is often limited in the marine environment. Therefore, oxygenic photosynthetic organisms have developed different strategies to cope with iron limitation during their evolutionary process. A study of iron-stress response in the primitive C.velia can help improve our understanding of its photosynthetic system. Our study revealed that iron-stress conditions led to decreased growth rate (with a doubling time of 7.54 days in the iron-stress culture, compared with 2.82 days in a normal culture), decreased oxygen evolution rate, decreased chlorophyll concentration per cell, shifted carotenoid composition, and shifted protein expression pattern. As a potential evolutionary intermediate between photoautotrophs and heterotrophs, C.velia offers an excellent opportunity to explore the link the evolutionary development of photosynthesis in apicomplexan parasites. Thus, phylogenetic analysis and study of its photosynthetic apparatus contributes to a better understanding of its evolutionary development as well as its inherent molecular mechanism.