COCCOLITHOPHORES

The coccolithophores are beautiful and most important phytoplankton (Division- haptophyta, class-prymnesiophyceae) live in the photic zone of the water column. During their life cycle, they produce organic carbon, carbonate and dimethyl sulfide.  After death, coccolithophores skeletons sink rapidly in the form of marine snow and form a part of marine sediment. These calcareous nannofossils/nannoplankton(less than 30 micron) (fossilized coccolithophores) formed about 230 million years record (up to late Triassic) which can be used to study the past climatic variations.

            In the present world ocean, more than 200 coccolithophores species are recorded so far. The distribution of coccolithophores in the oceanic ecosystem isdepending up on the light availability, temperature, salinity trace metals, vitamins and nutrient conditions. Major abundance of coccolithophores is found in the tropical and subpolar regions and at the poles their abundance and diversity decreases.

Why coccolithophores are important?

The coccolithophores have high turnover rate than any siliceous or carbonate phytoplankton in the ocean. Due to their short life cycle, they reproduce and die fast. As a result they help to drawdown maximum atmospheric carbon dioxide and store in the marine sediments. The coccolithophores don’t require much food for survival as other phytoplankton need so their abundance mostly occur in oligotrophic open oceans. In these regions, coccolithophores can form more than 90% of the phytoplankton population. The increasing carbon dioxide concentration in the atmosphere and in the ocean resulting into lowering of pH of the ocean, called as “Ocean acidification”. The increasing ocean acidification is expected to have an impact on phytoplankton, mostly on coccolithophores. It is assumed that, the increase of atmospheric carbon dioxide will lower the ocean pH and this will effect calcium carbonate forming capacity of coccolithophores and also their life cycle.

Ecology of coccolithophores

The phytoplankton needs both sunlight and nutrients for photosynthesis. The ideal place for growth of phytoplankton is on the surface of the ocean in an area where plenty of cold, nutrient rich water upwelled. In contrast to this, the coccolithophores prefer nutrient poor mild temperature regions that usually observe in the open oceans regions of world oceans. Among coccolithophores, there are numerous neritic species and one freshwater species in the group but all species are predominantly planktonic with maximum diversity and abundance as compare to any other phytoplankton species in the oligotrophic open ocean waters where they usually observed in abundance and form major portion of phytoplankton biomass. The coccolithophores often form blooms on the surface oceans. The most dominant coccolithorid Emiliania huxleyi blooms often occur during late spring and early summer that followed by onset of stable stratification, depletion of silicate and phosphate due to diatom blooms.

Coccolithophore life cycle

The coccolithophores possess coccoliths (calcified scales) at some stage in their life cycle. The coccolithophores are divided in to two major groups- the holococcolithophores and heterococcolithophores. The holococcolithophores are made up of minute euhedral calcite crystallites arranged in continuous arrays, calcify outside the cell and heterococcolithophores are by radial array of complex calcite (CaCO₃) crystals and produced in the cell.

Diagram taken from  Bown P (1998).

Paleoceanography

In the geological record, the first appearance of calcareous nannofossil was recorded in Late Triassic sediments (about 239 million years ago). The rapidly radiating nannofossil groups from Triassic to Holocene serves as an important biostratigraphic tool and also important organisms to decipher past oceanographic conditions including the physical and chemical characteristics of overlying water-masses. In the recent years calcareous nannofossils have become preferred tool for stratigraphic age determination in post-Palaeozoic calcareous sequences. The coccolithophores are also used as important biomarkers for age determination.

Coccolithophores from Indian sector of the Southern Ocean

(A: Calciosolenia brasiliensis, B: Calcidiscus leptoporus, C: Helicosphaera carteri, D: Polycalyptra aurisinae, E: Syracosphaera rotula, F: Coccolithus pelagicus, G: Gephyrocapsa sp., H: Emiliana huxleyi