Microzooplankton are a group of heterotrophic and mixotrophic planktonic organisms between 20 and 200 µm in size. Important contributors to the group are phagotrophic protists such as flagellates, dinoflagellates, ciliates, acantharids, radiolarians, foraminiferans, etc, and metazoans such as copepod nauplii, rotiferans and meroplanktonic larvae, among others.
Tintinnid ciliate from NW Mediterranean coastal waters. Ciliates have been traditionally recognized as the main group of microzooplankton, because their abundance and trophic role in the food web. However, present research is questioning this preponderance in the planktonic ecosystem, highlighting other groups such as the dinoflagellates.
The role of microzooplankton in marine ecosystems
Traditionally, this group has been relegated to a marginal role when describing the dynamics of marine ecosystems, especially those of productive waters (e.g. upwellings) where herbivorous food chains are considered as the most important via of energy and matter circulation. However, recent research has revealed that microzooplankton play a key role in shaping the structure of most marine ecosystems (including productive ones), as primary grazers of marine phytoplankton, as major secondary producers, and as intermediaries between primary producers and copepods. Actually, microzooplankton circulates most of the particulate primary production that is not lost by sedimentation or advection. For instance, in tropical and subtropical oligotrophic ecosystems microzooplankton are the main grazers of the primary production, consuming, on average, 75% of the production, and about half of the phytoplankton biomass per day. In temperate waters the grazing activity of microzooplankton, on average, daily accounts for ca. 60% of the primary production (half of the phytoplanktonic biomass per day). This picture further extends to Antarctic waters where microzooplankton daily grazing during the austral summer also accounts for 60% of the production of algae, although only 20% of their biomass.
Protoperidinium sp. pallium-feeding on a chain-forming diatom. Dinoflagellates capture prey (at times even larger than their own size) by 3 different raptorial mechanisms: pallium feeding, tube feeding, and direct engulfment. They can be active predators during diatom blooms.
Nauplius of the copepod Acartia grani. Nauplii are the larval form of copepods and other pelagic and benthic crustaceans. They are the most abundant metazoans in the oceans, and probably on Earth. They usually prey upon moving cells of medium size, and their role in the ecosystem is similar to that of copepods, but scaled to their size and biomass.
It is quite straightforward to understand the key role of microzooplankton (and nanograzers) in oligotrophic communities, where the primary producers tend to be smaller in size, usually escaping the grazing of most mesozooplankton groups (e.g., copepods). However, it is more difficult to picture microzooplankton consuming large diatom chains, typical from upwelling systems. Even though the mechanisms behind these elevated impacts (up to 60% of primary production consumed daily by microzooplankton of upwellings and productive coastal sites) are still under debate, evidences point to heterotrophic dinoflagellates and large tintinnid ciliates as the main responsible for these grazing activity.
Rotifers are typical components of coastal communities. Rotifers are small metazoans characterized by a ciliated corona around the mouth, which give a vivid impression of rapidly rotating wheels. The current created by the corona helps to propel the rotifer and also brings food particles to the mouth.
Microzooplankton as recyclers of inorganic nutrients in the ocean
Using proper C/N ratios and estimates of microzooplankton excretion rates we estimate microzooplankton recycles from 40 to 63% of the nitrogen requirements for new phytoplanktonic production. For comparison, the sum of copepod excretion and remineralization of non-assimilated nitrogen could only explain 14% of the nitrogen requirements for primary production. Nevertheless, these figures will be of relative importance depending on the allochthonous inputs of nitrogen. That is, in eutrophic and very productive ecosystems, even if the contribution of nutrients by microzooplankton is high, it is likely it will not be relevant compared to the nutrient concentrations available in the euphotic zone. On the other hand, it is expected that in oligotrophic systems algal growth and production largely rely on microzooplankton recycling.
The concept of animal and vegetal gets tricky when relating to unicellular organisms since many unicellular organisms, even if conducting photosynthesis, can be voracious predators. In the sequence of pictures we can see the mixotrophic dinoflagellate Karlodinium veneficum, species responsible for many harmful algal blooms, feeding on the algae Rhodomonas salina.
A greater relevance of microzooplankton versus mesozooplankton as factor of control of phytoplanktonic populations by grazing and by recycling of inorganic salts does not imply that large zooplankters are not important components of marine food webs. Even if their grazing impact is generally lower than that of microzooplankton, they remind as important agents in structuring pelagic food webs and are a crucial link between autotrophs and fish. Moreover, episodes of high mesozooplankton grazing impact on phytoplankton are not rare, and indirect effects on phytoplankton by predation of microzooplanktonic grazers seems to be a general feature in most marine systems.
Foraminiferans, radiolarians and acantharids are common, albeit greatly unknown, components of microzooplankton in the ocean. They can feed on a variety of prey, including bacteria, other protozoans and algae, and even small metazoan zooplankton. Furthermore, many species also retain symbiotic algae.
The movies show two heterotrophic dinoflagellates pallium-feeding on diatom chains. In the movie on the right you can see in detail how the pallium retracts after feeding.