Bio-optical properties of the marine diazotrophic cyanobacteria Trichodesmium spp. I. Absorption and photosynthetic action spectra

Publication Type  Journal Article
Year of Publication  1999
Authors  Subramaniam, A.; Carpenter, E. J.; Karentz, D.; Falkowski, P. G.
Journal Title  Limnology and Oceanography
Volume  44
Issue  3
Pages  608-617
Journal Date  May
ISBN Number  0024-3590
Accession Number  ISI:000080326300012
Key Words  oscillatoria trichodesmium; buoyancy regulation; synechococcus; phycoerythrin; thiebautii; pigments; nitrogen; prochlorococcus; phytoplankton; fluorescence
Abstract  

The optical absorption, fluorescence excitation and emission, and photosynthetic action spectra were measured in vivo on intact colonies of Trichodesmium from the Caribbean Sea. The optical cross-sections were dominated by ultraviolet-A (UVA) absorption, which was a consequence of massive accumulations of mycosporinelike amino acids. The visible region of the spectrum was decomposed into several bands, among which chlorophyll a (Chl a), carotenoids, and individual phycobilipigments could be discerned. There was a clear diel periodicity in the ratio of the optical absorption cross-sections of phycourobilin (PUB) to phycoerythrobilin (PEB), increasing from around 1.7 at night to 2.1 at midmorning. The diel cycle in PUB/PEB is consistent with a reversible interconversion of the two pigments. The ratio of PUB/PEB was inversely correlated with the transfer of excitation energy to photosystem II (PSII). Light absorbed by PUB was not transferred to PSII with a high efficiency, but rather, a significant fraction was reemitted at 565 Mn as fluorescence. These observations suggest that the PUBs and PEBs in Trichodesmium act as a dynamic biophysical energy valve that modify the rate of excitation energy delivered to PSII in response to changes in ambient Light regime. The low-temperature (77 K) fluorescence emission spectra reveal an extremely weak 685-nm emission signal in relation to that at 730 nm. Based on a simple model, these data suggest that the ratio of PSI/PSII reaction centers in Trichodesmium is about 24:1. Such an extraordinary bias against PSII may help minimize damage to nitrogenase from O-2 production in PSII, but it also reduces the photosynthesis-enhanced growth and makes Trichodesmium virtually undetectable by chlorophyll fluorescence. The unique bio-optical properties of Trichodesmium can be used to develop algorithms to study its temporal and spatial distributions from remotely sensed information.

Notes  

196UQTimes Cited:43Cited References Count:40

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