Limnol. Oceanogr., 44(3), 1999, 608–617

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 nm 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 O2 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. Trichodesmium spp. is a genus of nonheterocystous, marine, diazotrophic (i.e., nitrogen fixing) cyanobacteria that form extensive blooms in oligotrophic tropical and subtropical seas. These organisms possess phycobilipigments with absorption peaks centered at 495, 545, and 565 nm (Fujita and Shimura 1974). The latter two peaks correspond to PEB, while the first corresponds to PUB, which is a modified form of PEB (Sidler 1994). The combination of all three optical 1 Present address: Chesapeake Biological Laboratory, P.O. Box 38, Solomons, Maryland 20688.