Benefits
Both the phyla Bacillariophyta and Dinoflagellata can benefit mankind and other creatures. For example, dinoflagellates can create symbiotic relationships in which each organism benefits. This is mutualism. Dinoflagellates can occupy organisms such as sponges, corals, jellyfish, and flatworms. "The host provides a protective environment and the growth of the dinoflagellates impart carbohydrates to the host" (http://www.encyclopedia.com/topic/Dinoflagellata.aspx). Diatoms can also be beneficial. For example, because of the large quantity of diatoms, many organisms rely on diatoms as a significant food source. These organisms can also be very important to mankind. When diatoms die, their fossils accumulate to form "diatom-rich sediments" that are minded to make abrasives, which can include soaps, detergents, and toothpaste; these sediments can also be mined for the purpose of making sieve-like filters that can separate tiny particles from solutions (http://www.encyclopedia.com/topic/Diatoms.aspx).
http://periclean.com/abrasive-toothpaste http://www.thatpetplace.com/205461g.jpg
Disadvantages
Bacillariophyta and Dinoflagellata can also be harmful. For example, an excessive accumulation of algae can result in an algal bloom. These blooms can take oxygen and other nutrients from the ocean and after they die, rot in the ocean, leaving an unsafe environment in which other marine organisms must live (http://www.npr.org/). These blooms can result in a "red tide," which is the discoloration of parts of the ocean due to an excessive amount of nutrients from plankton. Red tides can be harmful because they spread toxins to other organisms, which can cause problems in the food chain. Dinoflagellates in particular are known for having caused these toxic problems. Dinoflagellates' toxins can spread to organisms such as shellfish, which filter water for consumption and are commonly eaten by humans. By consumption of a shellfish harmed by these toxins, humans can be poisoned. Toxins of dinoflagellates are known to cause "gastrointestinal illness, permanent neurological damage, or even death" (http://www.mnh.si.edu/highlight/sem/dinoflagellates.html).
Red Tide