Areas like the Caribbean reefs are a biodiversity hotspot. Deeper waters are shown to be of importance for the structure and composition of biological marine communities, but until now only a view studies have been performed on mesophotic reefs (30-100m depth). During three exploratory dives in a submarine along the coast of Bonaire, widespread fields of benthic cyanobacterial mats (BCMs) were found from 45m till 90m depth. These mats are dense structures consisting of different microbial organisms, dominated by cyanobacteria. No previous studies are available of such mats at these depths. Therefore the first aim of this study was to map the distribution of deep BCMs along the west coast of Bonaire, including Klein Bonaire, and to gather more bathymetric data. Almost 30% of the Bonairean west coast contained BCMs. Thereof 44% were found in front of Kralendijk or its suburbs along the coast. BCMs were only found on relative flat and sandy bottoms. Therefore it is thought that the presence of flat and sandy bottoms play a key role in the development of BCMs, but pollution associated with populated areas might play an important role as well. More research is needed to investigate how big the effect of pollution on BCMs formation is and if this can be minimized.
Subsequently a BCMs characterisation study was performed. Hereby, the focus was on the light availability in the mesophotic waters and the light-harvesting pigments of BCMs. At 14.5m depth light of 600nm and higher frequencies was almost completely filtered out and at a depth of 61.4m the 5% light intensity left ranged between 460nm and 500nm. While analyzing the phycobilisome pigments, clear PUB and PEB absorption peaks were found, but no clear PC and APC peaks were found, which is unexpected since these pigments are thought to be always present. Therefore it would be interesting to perform more research on the presence of the phycobilisomes in these BCMs, to find an explanation for these results. Twelve hydrophobic pigments were found, including zeaxanthin, which originates from cyanobacteria. Deep samples contained more pigments than shallow samples (p=0.005). Moreover, deep samples compared to shallow samples contained on average 47% more of the light absorbing pigment chlorophyll c3 (p < 2.20-16) and 62% less of the light protecting pigment zeaxanthin (p = 4.62-8). These results are according to expectations and indicate that these BCMs are adapted to the life on the ‘dark’ mesophotic reefs.