Sexual biochemistry in the deep sea – the link between phytoplankton and abyssal holothurians

Abstract

Holothurians play an important role in carbon cycling. They dominate the abyssal oceanic megabenthos, reworking large amounts of organic matter. Holothurians require essential organic nutrients, such as carotenoids for their reproduction. Enhanced carotenoid concentration in the ovaries of echinoderms increases reproductive output and larval survival. Carotenoids cannot be synthesised de novo by holothurians, only by phytoplankton. To examine the link between diet and reproduction in deep-sea holothurians, the pigment biochemistry of holothurians, sediment and particulate organic matter from three abyssal sites as investigated. A temporal comparison at the Porcupine Abyssal Plain (PAP), NE Atlantic, has shown 1) the supply of organic material (OM) can affect the diet of holothurians, depending on their feeding adaptations and 2) holothurian reproductive biochemistry can be affected by compositional differences in the OM reaching the seafloor, although the extent of this influence appears to differ between species. Two abyssal sites around the Crozet Islands, Southern Ocean, were investigated to compare contrasting OM supply on the diet and reproductive biochemistry of holothurians. The sites are only 460 km apart, with no topographic boundary to separate them. However, they are subject to differing overlying primary productivity regimes and therefore biochemical differences can be ascribed to the composition and amount of organic matter reaching the sea floor at each site. The results showed that 1) the quantity of OM reaching the seafloor at each site differed, mirroring the overlying primary productivity regimes. This was also reflected in the diet of some holothurian species, depending on their ability to take advantage of the fresh material. 2) The reproductive biochemistry of the holothurians sampled at both sites showed quantitative differences, mirroring the supply of OM to each benthic site.The present study has shown that changes in the composition and quantity of the supply of OM to the deep-sea floor can affect holothurian diet and ovarian biochemistry. This may lead to large community changes as seen at the PAP in the NE Atlantic, which alters the reworking rate of the sediment, ultimately affecting the sequestration of carbon.

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