The microbial ecology of colonial radiolaria

Colonial radiolaria (CR) are distributed throughout most of the world's oceans and it is likely that specific CR assemblages are associated with the different water masses. In this study, cluster analysis of radiolarian assemblages collected in the eastern waters of the North Atlantic Ocean revealed that they were substantially different in composition from CR collected in the waters on the western side of the ocean. Radiolarian colonies are macroscopic and often harbor communities of small organisms including heterotrophic bacteria, a Cryptobia-like bodonid flagellate, Gyrodinium sp. (dinoflagellate), dysterid ciliates and parasitic hyperiid amphipods. Other planktonic organisms have been observed in association with the colonies, however this study was devoted to the description of the five groups mentioned above and to the flow of carbon within the radiolarian microcosm. Gyrodinium sp. and the Cryptobia-like bodonid flagellate were the most common microorganisms associated with the colonies and often occurred in very high abundances, e.g. 10{dollar}sp2{dollar} to 10{dollar}sp4{dollar}/CR; the dysterid ciliates and hyperiid amphipods were less frequently associated with the colonies and their abundances were much lower, generally between 1 and 40/CR. Bacteria were observed on all of the radiolaria that were examined and their abundances ranged from 1.75 {dollar}times{dollar} 10{dollar}sp4{dollar} to 5.00 {dollar}times{dollar} 10{dollar}sp6{dollar} cells/CR. Carbon flow within the radiolarian microcommunity was examined by calculating the production of bacteria, Gyrodinium sp. and hyperiid amphipods. Both bacteria and Gyrodinium sp. may reach production rates equivalent to the primary production rates of the radiolarian endosymbiotic algae. Production rate estimates for bacteria associated with CR ranged from 0.04 to 10 ng carbon/hr and Gyrodinium sp. associated with CR fixed between 0.002 and 30 ng inorganic carbon/hr. It is not clear, however, whether the bacteria and the various associated protozoa are beneficial or detrimental to the host colonies. Parasitic hyperiid amphipods often feed on the radiolarian host and may even lead to the demise of the colony. Although the contribution of colonial radiolaria to total planktonic production is relatively small, the dynamic microbial communities they support may play an essential role in nutrient recycling and regeneration in the epipelagic oceanic waters that they occupy.