Light inside sponges

Sponges are the most basal metazoan organisms. As sessile filter feeders in marine or freshwater habitats, they often live in close association with phototrophic microorganisms. Active photosynthesis by the associated microorganisms has been believed to be restricted to the outer tissue portion of the sponge hosts. However, phototrophic microorganisms have also been detected in deeper tissue regions. In many cases they are found around spicules, siliceous skelettal elements of demosponges and hexactinellids. The finding of phototrophic organisms seemingly assembled around spicules led to the hypothesis of a siliceous light transmission system in sponges. The principle ability to conduct light was already shown for sponge derived, explanted spicules. However it was not shown until now, that in deed sponges have a light transmission system, and can harbour photosynthetically active microorganisms in deeper tissue regions. Here we show for the first time, that, as hypothesized 13 year ago, sponge spicules in living specimens transmit light into deeper tissue regions. Our results demonstrate that in opposite to the actual opinion, photosynthetically active microorganisms can also live in deeper tissue regions, and not only directly beneath the surface, when a light transmission system (spicules) is present. Our results show the possibility of massive or globular sponges being supplied with photosynthetic products or pathways throughout their whole body, implying not only a more important role of these endobioses. Our findings also elucidate the in-situ function of a recently more and more interesting biomaterial, which is unique not only for its mechanical, electrical and optical properties. Biosilica is of special interest for the possibility to produce it enzymatically under environmental conditions.