Crustacean zooplankton communities, mixotrophic cililates, and optical properties in North Patagonian lakes, investigated by remote sensing

North Patagonian lakes are characterized by their oligotrophic or oligo-mesotrophic status. These conditions bring with them, respectively, the presence of abundant mixotrophic ciliates and a low species number of crustacean zooplankton under oligotrophic status, and low numbers of mixotrophic ciliates and a high species number of zooplankton under oligo-mesotrophic status. The aims of the present study are, (1) to use remote sensing techniques for determining abundances of mixotrophic ciliates and crustacean zooplankton, and (2) to characterize these mixotrophic and zooplankton communities by using null models. The sensing was accomplished from a satellite, i.e., by measuring the reflectance of the sunlight on a waterbody, which result will vary according to the contents of the water column. The results of Principal Component Analysis (PCA) revealed that sites with low reflectance of all bands have a high abundance of Stentor accompanied by low zooplankton absolute abundance, whereas a markedly opposite situation was observed under high reflectance, where Stentor has low abundance in conjunction with high zooplankton absolute abundances. The null models revealed that the communities in the studied sites do not have structured species associations, and that there is an overlap of niches. These results obtained agree with similar observations for Argentinean Patagonian lakes.

Micractinium tetrahymenae (Trebouxiophyceae, Chlorophyta), a New Endosymbiont Isolated from Ciliates

Endosymbiosis between coccoid green algae and ciliates are widely distributed and occur in various phylogenetic lineages among the Ciliophora. Most mixotrophic ciliates live in symbiosis with different species and genera of the so-called Chlorella clade (Trebouxiophyceae). The mixotrophic ciliates can be differentiated into two groups: (i) obligate, which always live in symbiosis with such green algae and are rarely algae-free and (ii) facultative, which formed under certain circumstances such as in anoxic environments an association with algae. A case of the facultative endosymbiosis is found in the recently described species of Tetrahymena, T. utriculariae, which lives in the bladder traps of the carnivorous aquatic plant Utricularia reflexa. The green endosymbiont of this ciliate belonged to the genus Micractinium. We characterized the isolated algal strain using an integrative approach and compared it to all described species of this genus. The phylogenetic analyses using complex evolutionary secondary structure-based models revealed that this endosymbiont represents a new species of Micractinium, M. tetrahymenae sp. nov., which was further confirmed by the ITS2/CBC approach.

Does Mixotrophy in ciliates compensate for poor-quality prey? Experiments with heterotrophic–mixotrophic species pairs

Fatty acid and sterol profiles play a large role in determining the food quality of prey in aquatic food webs. Ciliates have limited ability to synthesize essential polyunsaturated fatty acids (PUFA) and sterols, and must obtain these from their food. Many mixotrophic ciliates, however, are in a symbiosis with Chlorella, and this may compensate for prey that is low in PUFA and sterols. Moreover, when PUFA-rich prey are P-limited, mixotrophs may be less dependent than heterotrophs on having the optimal mixture of PUFA-rich and P-rich prey. To test these hypotheses, experiments were conducted using heterotrophic-mixotrophic species pairs and prey mixes with varying proportions of good and poor quality prey. Prey proportions had clear effects on ciliate growth rates, often with a clear threshold. In only one species, however, did mixotrophy appeared to be advantageous, with growth independent of the proportion of PUFA-rich prey. In the other two species pairs, mixotrophy gave at best a small advantage over heterotrophy when there was a large proportion of PUFA-poor prey, independent of whether the PUFA-rich prey was P-limited or P-replete. While PUFA- and sterol-rich prey are important for heterotrophic ciliates, mixotrophy cannot be universally employed as an alternate source of these required nutrients.

Experimental evidence for crustacean zooplankton grazing activity on mixotrophic ciliates

With their ultraoligotrophic status, the Chilean North Patagonian lakes have mixotrophic ciliates in their pelagic environments as producers, whereas the primary consumers are crustaceans that are low in abundance and species numbers. The aim of the present study was to determine the potential grazer role of mixotrophic ciliates on crustacean zooplankton collected in a lake without mixotrophic ciliates. Three experiments were conducted; one had a control with mixotrophic ciliates and an experimental treatment including the copepod Boeckella gracilipes obtained from Caburgua lake, whereas the second and third experiments had a control without zooplankton, and three treatments with the addition of Daphnia pulex and Mesocyclops araucanus, and a third treatment with equal amounts of both species. The results revealed grazing effects on the mixotrophic ciliates in the experimental treatments. This finding supports the evidence from the field and experiments suggesting that, in the transition from oligotrophy to mesotrophy with consequent changes in zooplankton, species number and abundance of mixotrophic ciliates decrease in their abundance.

Use of null models to explain crustacean zooplankton assemblages in North Patagonian lakes with presence or absence of mixotrophic ciliates (38°S, Chile)

Zooplankton assemblages in Northern Patagonian lakes are characterized by a low species richness and a marked dominance of calanoid copepods. The aim of the present study was to do a null model analysis to explain the composition of the zooplankton assemblages in two North Patagonian lakes. The first aspect to which null model analysis was applied was species co-occurrence, and there the model indicates that the species associations found are not random. The second aspect was niche sharing, and that analysis denotes that the species do not share niches. The third aspect tested was size overlap, and the model there denotes that the species have no size overlap. The results obtained indicate that the sites are relatively homogeneous, which would explain the random effect in species co-ocurrence, whereas the absence of niche sharing is due to the presence of different energetic resources at both sites investigated, which would separate the niche of each species from those of other faunistic elements. Finally, the third application of the null model analysis demonstrates that there is no competition among the species reported upon herein.

Use of Null Models to Explain Crustacean Zooplankton Assemblages in Northern Patagonian Lakes (Chile) with and Without Mixotrophic Ciliates

The zooplankton assemblages in deep, mostly oligotrophic northern Patagonian lakes are characterized by a low species number and a marked dominance of calanoid copepods, as well as often by the presence of large mixotrophic ciliates (Stentor). In the present study, data collected in field work on four northern Patagonian lakes were analysed using co-occurrence of null models, with the aim to determine potential regulating factors for the observed crustacean and mixotrophic ciliate assemblages, using a presence-absence matrix. The null model uses species co-occurrence, and the basis of this model is that the species associations observed are random. The results revealed the absence of regulating factors and indicate that the sites are relatively homogeneous, with low species numbers, and similar plankton assemblages at all study sites. This finding coincides with similar descriptions for zooplankton assemblages in other Chilean Patagonian lakes.