Carbon Flow within the Colonial Radiolarian Microcosm

Colonial radiolaria (CR) are planktonic sarcodines generally found in the upper 30 m of oligotrophic oceanic waters. The CR feed on microplankters such as copepods and mollusc larvae which become entangled in the feeding rhizopodia of the floating colonies. In addition to their carnivorous nutrition, CR receive photosynthates from algal endosymbionts located within the colonies and even ingest some of these endosymbionts. This nutritional versatility may be the key to the exceptional success of colonial radiolaria in highly oligotrophic environments such as the central gyres of the Atlantic and Pacific Oceans. Although most of the small plankters that collide with CR are rapidly consumed, there are several small organisms associated with the colonies which apparently possess the means to avoid predation. These include hyperiid amphipods, dysterid ciliates, gyrodinioid dinoflagellates and Cryptobia-like bodonid flagellates. Colonial radiolaria also harbor large populations of bacteria on their outer surfaces. The flux of carbon through the colonial radiolarian microcosm was summarized in a model which includes estimates of the carbon flow attributed to the various associated organisms. In this model, the major sources of carbon entering the CR microcosm (all rates are normalized to a 24 hr day) are predation (estimated rate = 239 ng carbon/hr) and algal photosynthesis (measured rate = 9.53 ng carbon/hr), assuming an average colony has 100 cells and a biomass of 48-mu-g carbon. A colony of these proportions respires approximately 110 ng carbon/hr. In this model, associated bacteria take up between 0.04 and 10 ng carbon/hr (estimated by a "bacterial carbon demand equation"), juvenile amphipods consume 4.6-22 ng carbon/hr and adult amphipods consume 40-200 ng carbon/hr, while free-swimming gyrodinioid dinoflagellates incorporate between 0.002-30 ng inorganic carbon/hr. The carbon uptake rates of dysterid ciliates and Cryptobia-like bodonids were not determined and their trophic role in the CR microcosm is not clear. One of the unique characteristics of the radiolarian microcommunity is the fact that it is a self-supporting system, similar in many respects to coral reef communities. Carbon is incorporated by the radiolarian, either by predation or through endosymbiont activity, and some of this carbon is then utilized by associated organisms. Furthermore, these associated organisms may play an important role by recycling nutrients within the microcosm and possibly to the surrounding waters as well.