Biochemical prey recognition by planktonic protozoa

Abstract

Planktonic flagellates and ciliates are the major consumers of phytoplankton and bacterioplankton in aquatic environments, playing a pivotal role in carbon cycling and nutrient regeneration. Despite certain unicellular predators using chemosensory responses to locate and select their prey, the biochemical mechanisms behind prey reception and selection have not been elucidated. Here we identify a Ca(2+)-dependent, mannose-binding lectin on the marine dinoflagellate Oxyrrhis marina, which is used as a feeding receptor for recognizing prey. Blocking the receptor using 20 microM mannose-BSA inhibited ingestion of phytoplankton prey, Isochrysis galbana, by 60%. In prey selection studies, O. marina ingested twice as many 6 mum diameter beads coated with mannose-BSA as those coated with galNac-BSA. When pre-incubated with mannose-BSA, O. marina was no longer able to discriminate between different sugar-coated beads. Thus, these findings reveal molecular mechanisms of protozoan prey recognition. Our results also indicate the functional similarity between cellular recognition used by planktonic protozoa to discriminate between different prey items, and those used by metazoan phagocytic blood cells to recognize invading microorganisms.

Trait changes induced by species interactions in two phenotypically distinct strains of a marine dinoflagellate

“…Prior investigations of endocytosis in marine heterotrophic protists revealed that protein kinases (Hartz et al, 2008) and lectins (Wootton et al, 2007) can be directly linked to phagocytosis. Genes that clustered into category D in Alex2 included those for lectins (C-type and Ricin-B-type), which were significantly enriched in the expression analysis after 48 and 96 h ( Figure 5).…”

Nitrogen-deficient microalgae are rich in cell-surface mannose: potential implications for prey biorecognition by phagotrophic protozoa

“…Some 16 years ago it was proposed that certain protozoa might use 'contact chemoreceptors' to discern the profitability of different prey items on the basis of their cell-surface biochemical composition (28). However, exploration of molecular-level signalling and recognition processes between predatory and prey cells has only been pursued more recently (23,26,27,32). Of significance are the recent findings of Wootton et al (32) who identify a mannose-specific feeding receptor on the phagotrophic protozoan Oxyrrhis marina -a marine species in which selective feeding is well-documented (7,10,16,29).…”

“…However, exploration of molecular-level signalling and recognition processes between predatory and prey cells has only been pursued more recently (23,26,27,32). Of significance are the recent findings of Wootton et al (32) who identify a mannose-specific feeding receptor on the phagotrophic protozoan Oxyrrhis marina -a marine species in which selective feeding is well-documented (7,10,16,29). Wootton and colleagues demonstrate that ingestion of the microalga Isochrysis galbana by O. marina can be inhibited by saturating receptor binding domains with mannose-BSA (32).…”

“…The aim of this study was simply to determine whether-or-not nitrogen stress affects the abundance of mannose residues displayed at the cell-surface of I. galbana. Given that mannose-specific feeding receptors are implicated in the process of prey recognition by phagotrophic protozoa (32), it was hypothesised that nitrogen-sufficient (good-quality) I. galbana might produce more cell-surface mannose than nitrogen-limited (poor-quality) I. galbana. As a consequence good-quality I. galbana might be more readily adhered to and ingested.…”

“…Alternatively, it may be that mannose-rich, poorquality microalgae are actually too sticky for protozoan predators to adhere to. Excess cell-surface mannose residues on nitrogendeficient microalgae, may shear off and inhibit the binding efficiency of feeding receptors (as demonstrated by sugar inhibition experiments conducted by Wootton et al [32]). Furthermore, it is possible that the enhanced production of preysurface glycoconjugates under conditions of nitrogen stress could accentuate the influence of repulsive (negative-negative) electrostatic forces between predatory and prey cells (see Ryter & deChastellier [20]).…”