The microbial and metazoan community associated with colonies of Trichodesmium spp.: a quantitative survey

Association with resource-rich particles may benefit a number of planktonic species in oligotrophic, open-ocean regimes. This study examined communities of microbes and zooplankton associated with colonies of the cyanobacterium Trichodesmium spp. in the Sargasso Sea. Trichodesmium colonies and seawater controls were collected near Bermuda using SCUBA during September 1995, and June, July and August 1996. Organisms associated with the colonies and those in the surrounding seawater were enumerated using light and fluorescence microscopy. We found that 85% of theTrichodesmium puff and tuft colonies examined harbored associated organisms. Associated organisms included bacteria (rod and coccoid), fungi, pennate diatoms, centric diatoms, heterotrophic and autotrophic dinoflagellates, chrysophytes, hypotrich ciliates, amoebae, hydroids, juveniles and nauplii of harpacticoid copepods, and juvenile decapods. The most common associates (in addition to bacteria) were dinoflagellates (present in 74% of the colonies examined), amoebae (50%), ciliates (24%), and diatoms (24%). Numbers of bacteria per colony volume averaged 8.2×108 bacteria ml−1 (range = 8.1×107 – 3.5 ×109 bacteria ml−1), and the density of associated microzooplankton and metazoans averaged 6.8×104 organisms ml−1 (range = 0 – 3.6 ×106 organisms ml−1). Associates of Trichodesmium colonies were enriched by two to five orders of magnitude over plankton in the surrounding water. This unique habitat allows for the association of primarily benthic ciliate, diatom and copepod species and could contribute significantly to plankton heterogeneity in the open-ocean. The distribution of associated organisms was affected by sample characteristics such as colony morphology, mucoid matrix structure and colony integrity. The influence of these factors indicates that succession or competition between heterotrophic microorganisms ultimately determines Trichodesmium microcommunity structure. Similar processes could regulate microbial and metazoan communities associated with other resource-rich microenvironments, such as marine snow particles.