Community Structure And Ecological Characteristics Of Microbenthos With Special Reference To Benthic Ciliate Diversity In The Yellow Sea And East China Sea

Microbenthos mainly composed of bacteria, cyanobacteria, diatoms, flagellates,amoebae and ciliates are important components in benthic microbial food webs.These organisms serve as remineralizers, primary producers, and energy and mattertransmitters in ecosystems. Basic information on abundance, biomass, speciescomposition, community structure and seasonal and spatial distribution ofmicrobenthos and their interactions with biotic and abiotic environments areprerequisites for the evaluation of their role in the microbial food web. Benthicciliates are important component of microbenthos. Previous data showed that marinesediments harbor very diverse, abundant and productive benthic ciliates which mayplay important role in regulating bacterial and primary production. However, thestudies of microbenthos have long been hampered by the methodological difficultyin extraction micro-organisms from sediments. Therefore, limited resources areavailable for the study of benthic ciliates and microbenthos. Moreover, these datawere mainly focused on estuaries, intertidal zones, and shallow water areas. Fewstudies concern the offshore sediments.We carried out the research on benthic ciliate diversity and ecologicalcharacteristics of microbenthos community structure. Environmental factorsincluding water depth, bottom water temperature, bottom water salinity, sedimentorganic carbon content, Chl a, Ph a, and sediment granularity were analyzed andtheir possible relationships with the community parameters and distribution ofmicrobenthos were also discussed.We applied the Flow Cytometry technique to count benthic bacteria obtainedfrom38stations in the Changjiang River estuary and its adjacent area of the EastChina Sea. The results showed that the average bacteria abundances were0.95±0.5510⁸cells cm^–3in the0–2cm sediments and0.90±0.5010⁸cells cm^–3in the2–5cm sediments, and the average biomasses were4.19±2.43μg C cm^–3in the0–2cm sediments and3.98±2.21μg C cm^–3in the2–5cm sediments. There was nosignificant difference between the two layers in terms of bacteria abundance andbiomass, which were positively correlated to the Chl a content. Generally, thecounting by Flow Cytometry is comparably lower than that by epifluorescencemicroscope method (10⁸–10⁹cells cm^–3). The protocol for counting benthic bacteriausing Flow Cytometry needs further improvement. The abundance and biomass of microbenthos assemblages of28stations inspring and33stations in autumn from the Yellow Sea and39stations in spring and19stations in autumn from East China Sea were investigated. The average bacteriaabundance ranged from9.110⁷to8.110⁸cells cm^–3, biomass ranged from4to35μg C cm^–3; average cyanobacteria abundance ranged from1.910⁴to2.710⁶cells cm^–3, biomass ranged from1to28μg C cm^–3; average PNF abundance rangedfrom7.110⁵to1.410⁷cells cm^–3, biomass ranged from4to318μg C cm^–3;average diatom abundance ranged from0.910²to1.5103cells cm^–3, biomassranged from0.02to0.37μg C cm^–3. Generally, the abundances and biomasses ofbacteria, PNF and HNF were comparably higher in the Yellow Sea than in theadjacent sea area of Changjiang River estuary; the cyanobacteria abundances andbiomasses were comparable in the two areas, however the values were high in EastChina Sea during autumn cruise; the diatom abundances and biomasses wereobviously higher in East China Sea than in the Yellow Sea due to the high bottomwater temperature. Vertically, more diatoms, flagellates and ciliates distributed insurface layers, while the bacteria and cyanobacteria were not different in0–2cm and2–5cm sediments. In comparison with intertidal area, the bottom environmentfactors in the the offshore area were comparably stable, therefore no single factordecisively regulate the microbenthos.We identified329ciliate species/taxa from the whole sediment samples. Theinvestigations were based on sediment samples collected from11stations from theYellow Sea and39stations from the East China Sea during the spring cruise and12stations from the Yellow Sea and18stations from the East China Sea during theautumn cruise in2009. In the Yellow Sea the maximum average abundance was28±28cells cm^–3with average biomass of0.08±0.09μg C cm^–3from the0–8cmsediments in spring, and he abundance was9±6cells cm^–3and biomass0.02±0.01μg C cm^–3in autumn. In the East China Sea, the abundance was9±5cells cm^–3andbiomass0.01±0.01μg C cm^–3in spring, and the abundance was13±8cells cm^–3and biomass0.02±0.01μg C cm^–3in autumn. Prostomatea were the most abundantciliate group, accounting for on average58%of total abundance in spring and72.7%in autumn, while accounting for more than70%of total biomass in the two seasonsin the Yellow Sea; in the East China Sea, the ratios were comparably lower.Vertically, more than60%of the total ciliate abundance and biomass weredistributed in the0–2cm sediments. Algivores and bacterivores were dominant inabundance, while carnivores and algivores were dominant in biomass. The estimated ciliate bacterivory and herbirory indicate that ciliate ingestion had little impact onbacterial abundance but possibly had an evident influence on diatoms. Jaccardsimilarity analyses showed that the ciliate species compositions were different atdifferent sea areas and in different seasons. No single environmental factor wasfound to be decisive in shaping the ciliate community structure. However, theBIOENV analyses showed that bottom water temperature, salinity and sedimentgranularity usually combined to shape ciliate community. It is worth to note that, inthe Yellow Sea, the lower bottom water temperature supported higher ciliateabundance and biomass in spring, while in autumn the high bottom watertemperature supported only about1/4of the ciliate abundance and biomass obtainedin spring. This is likely due to the higher primary production that supported a highersecondary production in spring. The results imply that the planktonic system shouldbe taken into consideration to understand the microbenthos.Moreover, we described two species of Tunicothrix from intertidal sediments, T.brachysticha and T. multinucleata, and established the new family Parabirojimidae.Based on morphology, molecular phylogeny and physiological experiment, wesuggest treating Euplotes parawoodruffi as a junior synonym of E. woodruffi, awidely distributed ciliate species able to survive in both fresh and brackish water.