Is chain length in phytoplankton regulated to evade predation?

Abstract

Many dominating phytoplankton form chains of attached cells. Chain length strongly influences how the organism interacts with its environment, but the factors driving the evolution of chain formation and chain length plasticity are not entirely clear. We tested the hypothesis that chain formation in diatoms is a grazer avoidance strategy. We modelled the effect of chain length plasticity on grazing mortality in Skeletonema marinoi over a temperate year, based on empirical data on grazer densities, induced chain length plasticity, and grazing rates. The predicted optimal chain length strategy was compared with field data of S. marinoi and copepod biomass. We found that low copepod densities, corresponding to spring conditions in the field, induced chain length reduction in S. marinoi. Modelled grazing risk over an annual cycle showed that fixed traits with either single cells or long chains have respectively 31 % and 36 % higher mortality than S. marinoi with grazer induced chain length plasticity. Field measurements of chain length and grazer abundances also agree well with chain length plasticity as a grazer defense strategy. We conclude that grazer regime could be a major driving force in the evolution of chain length plasticity in phytoplankton organisms.

Induction of defensive traits in marine plankton—new copepodamide structures

“…By grazing on microzooplankton and large phytoplankton, copepods may act as a switch between two alternate trophic cascades of different food chain lengths and with opposite selective pressure on phytoplankton size distribution (Stibor et al 2004). Measurements of chain length over time confirm that chain length is negatively correlated to copepod abundance in nature too (Bjaerke et al 2015). Based on the results reported here and earlier findings-a previous study shows that copepodamides induce bioluminescence in L. polyedra and Alexandrium tamarense (Lindström et al 2017)-we conclude that copepodamide presence induces a broad range of defensive traits in phylogenetically distant groups which may cause indirect cascading effects in plankton food webs.…”

“…Beside increased toxicity, copepod cues also trigger chain length shortening in Skeletonema , together with a massive transcriptional response (Bergkvist et al ; Amato et al ; E. Selander et al unpubl.). Copepods have an one order of magnitude lower grazing rate on single cells compared to longer chains, whereas microzooplankton, like ciliates, prefer the smaller units (Bjærke et al ). By grazing on microzooplankton and large phytoplankton, copepods may act as a switch between two alternate trophic cascades of different food chain lengths and with opposite selective pressure on phytoplankton size distribution (Stibor et al ).…”

Effects of predator lipids on dinoflagellate defence mechanisms - increased bioluminescence capacity

“…Other phytoplankters are known to reduce colony size and swimming speed in response to grazer cues 21–24 . This was not the case here, since L. polyedra does not form chains and consequently could not change colony size.…”

“…Both reduced colony size and swimming speed decrease encounters with grazers in these species and are attributed to reception of chemical cues from the copepods, rather than their physical presence 23,24 . The compounds that induce toxin formation in Alexandrium cells have now been identified as a group of eight polar lipids, copepodamides 20 .…”

“…Further responses of microalgae to copepod grazers include reduced colony size in Phaeocystis globosa , cell chain length in the chain-forming diatom Skeletonema marinoi and the dinoflagellate A. tamarense , as well as reduced swimming speed in A. tamarense 21–24 . Both reduced colony size and swimming speed decrease encounters with grazers in these species and are attributed to reception of chemical cues from the copepods, rather than their physical presence 23,24 .…”