Population Growth And Grazing Effects Of Marine Large Benthic Ciliates

Ciliate is a group of heterotrophs commonly found both in freshwater and marineenvironments, with high abundance and diversity. Their ecological role in microbialfood web has received a lot of concern since the concept of "micro-loop" was firstdiscussed in1980's. Ciliate has been recognized as an important nexus of the microbialfood web in pelagic water. On one hand, ciliates could effectively graze on the othermicroorganisms including bacteria, algae, flagellates and small ciliates. On the otherhand, ciliates could be largely grazed by metazoans (e.g. copepods). However, studiesaddressing the ecological role of ciliates in marine benthic environments are very scant.Marine sediments are different from the pelagic water not only in environmental factorsbut also in community structure of biota. Ciliates adapted to these habitats may havedifferent life strategies from the pelagic ones.Condylostoma has been documented as a very large ciliate frequently dominant invarious marine benthic microbial communities, but little is known about the effects oftemperature and food type and concentration on its growth and grazing. To reveal thepopulation dynamics of marine benthic ciliates and to assess the effects of temperatureand food supply on ciliate growth, we investigated the growth of Condylostomaspatiosum, a large and frequently dominant ciliate in intertidal zone. The laboratoryexperiments were conducted at three temperatures of15°C,20°C and25°C andsupplied with four individual algal species (Isochrysis galbana, Chaetoceros mulleri,Dunaliella salina and Phaeodactylum tricornutum) as prey. The growth rates ofCondylostoma spatiosum generally increased with increasing temperature, with those at15°C distinctly lower than at20°C and25°C. Meanwhile, food prey types markedlyaffected the growth rates of C. spatiosum. The growth rates and maximal populationsizes were the highest when supplying with the prey Dunaliella salina, while bothvalues were distinctly low when feeding with Isochrysis galbana, the lowest growthrates occurred at15°C with Chaetoceros mulleri as food prey. Statistical analysissuggested that the effects of temperature and food supply as well as their interactionswere significant (P <0.001). Our study indicates that the growth of benthic ciliatescould be enhanced by higher temperature and appropriate food supply. The diversity of benthic ciliates (e.g. size, feeding type) and the complexity of food supplies (e.g. preytype, quality) as well as the potential interactions between temperature and food supplymay result in deviation of growth rate when the value obtained from individualcircumstances is generalized. It is suggested that temperature and food supply should betaken into account simultaneously in the modeling of benthic microbial food webMeanwhile, using the heterotrophic dinoflagellate Oxyrrhis marina as prey, wedetermined the specific growth rate, cell volume, specific production and ingestion rateof Condylostoma spatiosum at different temperatures and prey concentrations. Thesegrowth and grazing parameters typically followed a hyperbolic response to preyconcentration. By applying iterative curve-fitting to the data at each temperature, wefound that, with increasing temperature, the maximum specific growth rate, maximumspecific production and maximum ingestion rate of C. spatiosum generally increased,while the maximum cell volume decreased. The gross growth efficiency of C. spatiosumgenerally decreased at saturated prey concentration from about45%to25%as thetemperature increased from12to24C. By fitting these data iteratively to multi-variablenonlinear models, we obtained predictive equations for the growth rate, cell volume andingestion rate with respect to temperature and prey concentration.In situ experiments were also conducted to investigate the potential effects ofCondylostoma spatiosum, a large-sized ciliate frequently dominated in various marinebenthic habitats, on benthic ciliate community in an intertidal flat. The manipulationsincluded the presence/absence of macro grazers and addition of C. spatiosum in theabsence of macro grazers. Ciliates responded to both manipulations of macro grazersand C. spatiosum. Total ciliate abundance tended to increase in the absence of macrograzers, while total ciliate biomass was remarkably constant. But this trend was likelyweakened by addition of C. spatiosum. Heterotrichea and Phyllopharyngea dominatedthe ciliate community in terms of biomass. Biomass percentage of Heterotricheadecreased from38%of total ciliate biomass to20%in the C. spatiosum added treatment,while it was relatively consistent in no C. spatiosum added treatment and control.Biomass percentage of Phyllopharyngea increased from27%to53%when C.spatiosum was added, while it decreased slightly for the first3days and showed anincrease in the following days in no C. spatiosum added treatment. ANOSIM alsoindicated that addition of C. spatiosum shifted the ciliate community. Zosterodasys agamalievi and Tunicothrix brachysticha generally contributed the most dissimilarity.However, neither tube effect nor addition of C. spatiosum changed the species richnessand diversity. The results are in consistent with previous work that ciliates are lesssensitive to changes at higher level.