The world's oceans are a major sink for atmospheric carbon dioxide (CO 2 ). The biological carbon pump plays a vital role in the net transfer of CO 2 from the atmosphere to the oceans and then to the sediments, subsequently maintaining atmospheric CO 2 at significantly lower levels than would be the case if it did not exist. The efficiency of the biological pump is a function of phytoplankton physiology and community structure, which are in turn governed by the physical and chemical conditions of the ocean. However, only a few studies have focused on the importance of phytoplankton community structure to the biological pump. Because global change is expected to influence carbon and nutrient availability, temperature and light (via stratification), an improved understanding of how phytoplankton community size structure will respond in the future is required to gain insight into the biological pump and the ability of the ocean to act as a long-term sink for atmospheric CO 2 . This review article aims to explore the potential impacts of predicted changes in global temperature and the carbonate system on phytoplankton cell size, species and elemental composition, so as to shed light on the ability of the biological pump to sequester carbon in the future ocean.

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Marine picocyanobacteria belonging to the genus Synechococcus are one of the most abundant photosynthetic organisms on Earth. They are often exposed to large fluctuations in temperature and CO2 concentrations in the ocean, which are expected to further change in the coming decades due to ocean acidification and warming resulting from rising atmospheric CO2 levels. To decipher the effect of changing temperature and CO2 levels on Synechococcus, six Synechococcus strains previously isolated from various coastal and open ocean sites were exposed to a matrix of three different temperatures (22 °C, 24 °C and 26 °C) and CO2 levels (400 ppm, 600 ppm and 800 ppm). Thereafter, the specific growth rates, photophysiological parameters (σPSII and Fv/Fm), C/N (mol/mol) ratios and the nitrogen stable isotopic composition (δ15N (‰)) of the strains were measured. Temperature was found to be a stronger driver of the changes in specific growth rates and photophysiology in the Synechococcus strains. Carbon-concentrating mechanisms (CCM) operational in these strains that shield the photosynthetic machinery from directly sensing ambient changes in CO2 possibly played a major role in causing minimal changes in the specific growth rates under the varying CO2 levels.

Monitoring and modelling of the growth profile of microalgae species should be an important tool for the hatchery industries before standardizing the best yielding and cost-effective protocol for their unit. Several factors are responsible in determining the nature of the growth profile. The most important regulator of such growth profile should be the volume of the initial inoculum. In addition, identification and determination of different phases (lag, log, stationary, etc.) of the growth curves of microalgae may be an essential part in the growth profile monitoring. Estimation of growth phases will also help the hatchery scientists in standardizing the commercial culture for industry. Moreover, the transition of different phases can be accurately identified through theoretical models, which are mostly overlooked in simple analysis. Summing up, we have two precise objectives: (1) to study the effects of choice of initial inocula levels on the time to maturity of the Chaetoceros sp., (2) to model the growth profile of the species from which we can theoretically determine its different phases, based on the optical density measurement as a proxy of the biomass. The estimated values of each phase are compared under two initial inocula levels through statistical tests. Using the conceptual approach of the proposed theoretical technique, there is scope for developing a similar model, which can be used in determining cost-effective culture protocol for commercial use.

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