Photoacoustic Spectroscopy Studies of Biofilm-Covered Solid Substrata Submerged in the Sea

Biofouling on solid substrata was studied in the coastal waters of the Baltic with a photoacoustic spectroscopy (PA) technique. Several substrata biotic and abiotic of varying surface energy and bioaccumulation efficiency were deployed, for a certain length of time corresponding to the characteristic stages of biofilm development. Periphyton consists of algae, cyanobacteria, and heterotrophic microbes which stand for a photosynthetic system with a mixture of pigments. PA signal amplitude and phase were evaluated using the phase-sensitive method with a novel closed-cell type photoacoustic spectroscopy (PAS) system. Photosynthetic apparatus properties (photosynthetic energy storage (ES), PA amplitude and phase spectra) exhibited a seasonal variability. ES values were higher, for a season of high primary production. In addition, ES values were found to be higher by a factor of 1.5-2, for biofilm settled on biotic macroalgae. The peak values in PA signal amplitude spectra maximum at ~ 680 nm were highest for biotic substrata, lower for filtered planktonic phase, and lowest for abiotic surfaces. The photoacoustic parameters (ES and PA signal amplitude) turned out to be unequivocally related to the biofilm structural signatures (thickness, volume, number of cells, fractal dimension etc.) as learned from confocal microscopy and wettability techniques.