Influence of mycosporine-like amino acids (MAAs) on UV absorption by particulate and dissolved organic matter in La Jolla Bay

Experimental work with cultures of the red tide dinoflagellate Lingulodiniu m polyedrum suggested mycosporine-like amino acids (MAAs) are a component o f the organic matter excreted by cells. MAAs in dissolved organic matter (D OM) also may have it large influence on absorption of ultraviolet (UV) ligh t through the water column. To test these hypotheses in a natural setting, data were collected from March 1995 through April 1996 in coastal waters of f California, U.S.A. During this time, a large red tide of L. polyedrum occ urred in March and April 1995. In this field study, we show that MAAs are a quantifiable component of DOM, particularly during the period following th e red tide events. Absorption spectra of particulate organic matter (POM) a nd DOM samples revealed disproportionately high levels of UV absorption rel ative to visible wavelengths. UV absorption by POM at 330 nm was linearly r elated to phytoplankton biomass (between 0-10 mug chlorophyll [Chl] a L-1), measured as Chi a, accounting for 71% of the variability in absorption. Ch romatographic analyses revealed the presence of various MAAs in both POM an d DOM pools. MAAs were observed in 83% (n = 53) and 47% (n = 13) of the sam ples analyzed with concentrations ranging from 0 to 2.75 muM (0-696.00 mu m ol mug(-1) Chi a) in the POM and 0 to 111.40 nM in the DOM fraction. Absorp tion by dissolved MAAs, as calculated from the measured concentrations, cor related with DOM UV absorption (r(2) = 0.77) and accounted for up to 10% of the total DOM absorption at 330 nm. Thus, MAAs are a small but quantifiabl e component of the DOM pool in the field and contribute to UV absorption.