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

The optical absorption, fluorescence excitation and emission, and photosynt hetic action spectra were measured in vivo on intact colonies of Trichodesm ium from the Caribbean Sea. The optical cross-sections were dominated by ul traviolet-A (UVA) absorption, which was a consequence of massive accumulati ons of mycosporinelike amino acids. The visible region of the spectrum was decomposed into several bands, among which chlorophyll a (Chl a), carotenoi ds, and individual phycobilipigments could be discerned. There was a clear diel periodicity in the ratio of the optical absorption cross-sections of p hycourobilin (PUB) to phycoerythrobilin (PEB), increasing from around 1.7 a t 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 ef ficiency, but rather, a significant fraction was reemitted at 565 Mn as flu orescence. These observations suggest that the PUBs and PEBs in Trichodesmi um act as a dynamic biophysical energy valve that modify the rate of excita tion energy delivered to PSII in response to changes in ambient Light regim e. The low-temperature (77 K) fluorescence emission spectra reveal an extre mely 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 center s in Trichodesmium is about 24:1. Such an extraordinary bias against PSII m ay help minimize damage to nitrogenase from O-2 production in PSII, but it also reduces the photosynthesis-enhanced growth and makes Trichodesmium vir tually undetectable by chlorophyll fluorescence. The unique bio-optical pro perties of Trichodesmium can be used to develop algorithms to study its tem poral and spatial distributions from remotely sensed information.